Refurbishing a CIR Astro 200 HF amateur band transceiver

The Astro 200

The CIR Industries Astro 200 is a compact, synthesized 100 watt HF transceiver from the mid-late 1970s that covers the 80, 40, 20, 15 and 10 meter bands.  Intended for both home and mobile use, it is quite small - 9.75" wide, 12.5" deep and 3" tall (24.8x31.8x7.6cm) - including the rear heat sink.  Back in 1977 - when this unit was made - it seems to have cost around $995 for the version without the CW filter - about $5000 in 2025 dollars!

Figure 1:
The front panel of the CIR Astro 200.  While advanced for
its day, the radio is pretty simple by today's standard.
The lack of a tuning knob seems a bit odd.
Click on the image for a larger version.

I don't know too much about CIR Industries, except that it was around only for a few years, apparently absorbed by Cubic-Swan in about 1978 where it was rebadged with the name of the new company and - with very minor changes - became the "200A".  The history of Cubic-Swan becomes a bit muddy after the early 1980s and appears to have fizzled entirely by the mid-late 1990s.  Much of the design of the Astro 200 - and other Cubic-Swan radios - was apparently done by Don Stoner, W6TNS (who was also the "S" in SGC).

The later version of this radio, the Astro 200A, sported a 6 pin round microphone connector, black knobs, slightly different switches, a lighted meter and very slightly modified scales on the meter itself:  I suspect that the electrical differences - some of which are noted below - may have evolved during the production of the original Astro 200.

The radio's history

This unit was purchased new in 1977, with the extra-cost CW filter option, and owned by a friend of mine, having first resided in his International Scout II - and then his Jeep CJ-7 - until about 2020 (when it was removed during vehicle maintenance) seeing many hours and miles bouncing around rough, 4WD roads.  Despite having banged around for about 40 years in a vehicle, it's in remarkably good physical shape, the case having only a few minor scratches.  Unfortunately, my friend became a silent key in 2022 and the radio ended up in my hands.

A "unique" radio

The advertisements for this radio tout it as being the very first completely synthesized amateur transceiver:  Whether or not it's actually the "first", I can't be sure, and this can vary depending on what you mean by "synthesized" - but in this case the local local oscillators are referenced from a single crystal while the BFOs were independent - a common practice even into the early 2000s.  Being an early synthesized radio, it does have a few interesting quirks:

• There's no tuning knob.  Tuning is accomplished by a pair of "up/down" momentary toggle switches.  At first, this seems awkward, but one can quickly become adept to tuning a radio this way.  My friend (the one who'd owned this radio) noted that this tuning method was more convenient when bouncing about on a bumpy Jeep road than trying to use a conventional knob.
  
• Operating the "fast" switch moves the frequency up/down by about 20 kHz/second after a brief pause.
• Operating the "slow" switch moves the frequency up/down about 400 Hz/second after a brief pause.
• A brief up or down push-and-release of either switch moves the frequency by 100 Hz.
  

• 100 Hz tuning steps + Fine Tuning.  The radio tunes in 100 Hz steps, but it has a "Fine" tuning knob that moves the frequency up/down by a bit more than +/-65 Hz to allow one to get the frequency as close as you wish.  With the tendency for most amateurs these days to set their radios to an integer number of kHz (and occasionally to "0.5", 100 Hz steps are just fine and this control can be left centered most of the time.
  

• The synthesizers are a bit slow to lock.  As one tunes the radio - particularly in the "fast" mode - the synthesizers may take a second or so to catch up as it "swoops" in onto the correct frequency.  This also means that after power-up, the radio is unusable for about 30 seconds, or for up to 15 seconds after changing bands.  As the synthesizers "land" within about a second during normal tuning with the up/down switches, the radio is on frequency by the time normal human reaction time has "locked in" to what is on frequency.
  

• The "WWV" mode.  You'll note that the mode switch includes a "WWV" position.  This is actually a completely separate, direct-conversion receiver - with no AGC - that is tuned only to 10 MHz. Since it uses the (doubled) 5 MHz reference as its local oscillator, it provides an easy way to check/set the radio precisely on-frequency.
  

Despite having a digital readout and a synthesizer, it does not have a computer of any sort.  "Programming" is done using PROMs (Programmable Read-Only Memory)  to look up the synthesizer tuning information and "LS" type logic as counters for the frequency dividers and tuning - but this also means that when it's first powered up, it always defaults to the bottom edge of the band to which it is tuned.  This is a bit of an inconvenience - but in the mid 1970's, prior to inexpensive single-chip microcontrollers with onboard program memory - there was no real way around this without adding significantly to complexity and cost.  I'm looking into a simple way for the radio to "remember" the last-tuned frequency on each band - perhaps the topic of a later article.

About this radio

Figure 2:
The radio's tag - Serial #8, apparently!
Click on the image for a larger version.

This radio is apparently a very early production unit - somewhat different from that depicted in the manual:

• The Microphone connector is a standard 1/4" TRS (headphone) jack rather than a 6-pin round connector apparently used later in the production run and in a later revision, the 200A.  The additional pins on the 6 pin connector provide up/down tuning and 11 volts, allowing one to do fine-tuning via the microphone.
  

• It was lacking the "ANL Board".  This is a very simple circuit circuit (two pairs of back-to-back diodes and an electrolytic capacitor) that reduces, according to the manual, "excessive popping or AGC pumping".  As this circuit is very simple, it was trivial to add to this radio and this somewhat reduced the tendency for the receiver to be momentarily deafened when changing modes or bands.
  

• Upon inspection of the PA (Power Amplifier) module I noted that the driver transistors were Motorola, marked with "604/438 Sample" which further implies an early production radio.  The PA transistors themselves - which are shown as being of type MRF454 in the service manual - were CD3435 made by CTC. 
  

• There are a number of doubly-balanced diode-ring mixers used throughout.  Based on the manual and photos of other units, these seem to be implemented with some sort of module.  On this unit the, modules are not used as the corresponding circuit on the PC board is populated with a pair of trifilar transformers and individual diodes comprising the mixer.
  

• The serial number of this radio is "706008".  Based on other photos that I've seen online, this is apparently serial number 8 - likely having been made in June of 1977.  The date codes on internal components are consistent with the possible June 1977 assembly date.
  

Figure 3:
The inner synthesizer board - a bunch of counters.  LS-TTL
circuitry is used extensively, along with a few diode-type
PROMs for frequency/display lookup and counter set-up.
Click on the image for a larger version.

Evaluation

As I had other projects in the queue, it was only recently that I pulled this radio off the shelf.   Prior to setting it on my workbench, I blew the dust off it and carefully cleaned the front panel and around controls, throwing the knobs into an ultrasonic cleaner.

Powering it up, the unit worked - sort of:  I could hear noise, but it seemed a bit deaf - but the sensitivity changed wildly with a bit of thumping on the case, an indication of a dirty transmit/receive relay.  Even with a massively strong signal into the antenna connector - which produced a deafeningly-loud tone in the (external) speaker - I got no S-meter reading.  Many years ago, my friend and I used this radio (when it was still in his Jeep) and noticed this same problem and that it was also mitigated by a "percussive repair" and/or clicking the PTT several times, indicating that the Transmit/Receive relay may have problems.

Figure 4:
The main RF/AF board, post repair.  The layout is a bit
crowded, but pretty clean on a two-sided, glass-epoxy board.
This radio includes the optional 400 Hz CW filter.
Click on the image for a larger version.

Popping the top cover I could see that I had some work to do.  While it was remarkably clean inside for having been in a dusty Jeep for decades, I could see evidence of a few problems:  I saw at least one "blowed-up" capacitor near the audio amplifier.

Fortunately, the synthesizer itself seemed to be OK:  The tuning controls did their jobs properly, the tone in the speaker indicating that the radio was landing on the same frequency as the display.  The only "digital" problem seems to be that one of the segments of each digit on the display was constantly illuminated, weakly, possibly indicative of a problem with a segment driver.

Refurbishing

The first order of business was to replace the electrolytic capacitors.  As a few of them had clearly failed as evidenced by inspection, they all had to go - particularly since the radio had spent many summers in a closed vehicle during hot, Utah summers - plus, this radio is nearly a half-century old (which seems amazing when you consider that it's "digitally synthesized") so time would saved to simply "shotgun" them all.  Furthermore, many of the boards are "tethered" with soldered cables:  There is just enough slack to pull them out and work on the boards without unsoldering anything, but doing so many, many times would not only be tedious, but risk fatiguing and breaking them - another reason to replace the capacitors in just one session.

Figure 5:
VCO/Synthesizer board.  There are two synthesizers - one
for them provides the 100Hz tuning steps.  Again, LS-TTL
logic is used, along with a few op-amps.
Click on the image for a larger version.

Capacitors, and more capacitors!

I took inventory, inspecting the entire radio and come up with the following list of capacitors - including those found in the PA module and places other than on the PC boards:

• (5) 470uf, 10 volt
  
• (2) 330uF, 16 volt (axial) 
  
• (2) 220uF, 16 volt
  
• (14) 100uF, 16 volt
• (13) 33uF, 16 volt
• (2) 10uF, 25 volt
• (4) 10uF, 16 volt
  
• (8) 4.7uF, 25 volt
• 1) 4.7uF, 16 volt
  
• (9) 1uF, 25 volt
• (4) 1uF, 50 volt
• (3) 1uF, 50 volt (axial)
• There are several dipped tantalum capacitors in low-level voltage and signal filtering lines that seem to be OK for now.   As none of these are on power rails there's no chance of a catastrophic failure (e.g. flames) should one short out.  These capacitors will be replaced in the future.
  

I suspect that the differing voltage ratings of some of the same-value capacitors was likely to save space (lower-voltage capacitors are generally smaller) and allow the use of less-expensive capacitors, but these days, capacitors are much smaller (and cheaper, in equivalent money) than their decades-old counterparts.  When ordering replacement capacitors I simply got same value rated for at least the voltage of the highest in the list above, but the new capacitors also had a temperature rating of 105C rather than the 85C of the original.  Since the electrolytic capacitors were pretty inexpensive - typically less than US$0.10/each for the smaller values - I ordered more than just the number above (in some cases, many more) in the event that I missed something.

Figure 6:
The pile of electrolytics removed from the radio.
Replacing every capacitor was the right choice!
Click on the image for a larger version.

Removing capacitors en masse is best done with the appropriate tools - particularly on an older circuit board.  Fortunately, I have a Hakko FR-300 desoldering iron/pump which made removal much easier and I was able to avoid damaging any traces on the board.

When replacing a bunch of capacitors, I prefer to do so methodically, moving from section to section on the circuit board - noting the polarity orientation of the capacitor before removing it and if there was any doubt as to which way it went, referring to the board layout diagram in the service manual - particularly since the circuit boards have neither solder mask or silkscreen as a visual reference.  Once a capacitor is replaced, I typically mark the top of the can with a colored marker to help make sure that I don't miss any.

One possible "gotcha" was that unlike modern electrolytic capacitors which are typically marked only on the negative lead, many (but not all) of the original capacitors in this radio had only their positive side marked - which was the custom of some manufacturers of the day - so I had to be particularly careful to identify the polarity correctly as I replaced each capacitor.

When I was done, the receiver seemed to be more "alive" than before, but it was still a bit deaf - and the synthesizer seemed to be a bit "wobbly", being very sensitive to slightly changes in power supply voltage.  The biggest change was the WWV receiver which was profoundly deaf prior to the capacitor change-out, but "normal" afterwards.

Capacitor brand implies longevity

After replacing the capacitors I went through the pile and found that most of them were "OK" - or at good enough that their respective circuits would have worked.  The brand seemed to be a pretty good indicator of which was likely bad:  The Japanese blue-label Nichicon and gray "Sun" and "Elna" brands were generally OK, the silver and gray Taiwanese "T.I." brand were all over the map, the lone "Sam Hwa" and "Towa" capacitors were marginal, but  all of the "Temple" branded capacitors (which seemed to have 1970 date codes, apparently already a few years old when the radio was made) were extremely bad.

After doing this I still believe that replacing all of the electrolytics was, in fact, the correct choice as I would have probably spent more time finding and diagnosing capacitors individually - and possibly suffered near-term failures - than simply swapping them all out.

A wobbly power supply

With all of the electrolytic capacitors replaced, I systematically went through the adjustment steps found in the user and service manual (which can be found online) - more or less.  Knowing that before you make ANY adjustments that you must make sure that the power supply is correct, I probed about with a volt meter noticing that the 11 volt supply was actually just below eight volts, likely accounting for its seeming deafness.  Locating the 11 volt regulator on the synthesizer board, I noted that the act of slightly adjusting the potentiometer resulted the voltage jumping, indicating that it was somewhat "stratchy", with the wiper likely not making good contact.  A bit of cleaning spray and exercising of this control resolved the issue and I reset the voltage to precisely 11.0 volts.

Figure 7:
Original S-meter coil.  It would seem that the coil winding
was broken in several places - hence, unsalvagable.
Click on the image for a larger version.

With the correct voltages now applied to the circuits in the radio, its sensitivity seemed to be much better and the synthesizer was no longer sensitive to fluctuations in the power supply, being able to tolerate a drop to about 11.25 volts at the radio's DC input before the synthesizer "wobbled".  

No S-meter!

Going through the alignment steps, I applied a signal from my generator and noted that while the sensitivity seemed to be about right - and the AGC was now working as it should - the S-meter did not move.  It's worth noting that the S-meter on this radio works ONLY when the meter switch is set to the "ALC" position - but I was getting no reading on any setting.  Using a voltmeter, I could see that the voltage across the S-meter's movement was increasing with the signal strength indicating that the AGC was working (which was also obvious by listening to off-air signals) but a quick check with an ohmmeter - after disconnecting one of the meter's leads - indicated that it was open circuit.

This was bad news, particularly since it was likely that I would never find a meter of the same, exact physical size - and even if I did find a replacement, I'd probably have to re-create the scale in the meter.  This wasn't impossible to do, but I took another path.

Figure 8:
The meter with its rewound meter coil using #30 wire.
As many turns were wound as would fit - the coil shaped to
prevent mechanical interference and then covered with
varnish to hold it in place.
Click on the image for a larger version.

Carefully disassembling the meter and inspecting it I noted that it was of the inexpensive "moving vane" type, the coil wound with very fine wire - probably around 46 AWG.  In probing very carefully I noted that one of these hair-thin wires was disconnected at the base of the coil.  Further probing showed that the wire itself was frayed where it was wound onto the phenolic paper stator - probably a victim of both temperature cycling and (possibly) some corrosion.  A bit of later inspection of the wire showed that it seemed "brittle" - something that I've seen on older gear:  I don't know if it's the copper hardening in some way or some sort of reaction between the wire, enamel and its environment that causes this.

Since the meter's coil was a total loss I decided to do something a bit drastic:  Rewind it.  Rather than trying to use #46 wire, I chose, instead, to use less-fragile wire - #30, which is about 10 times larger diameter:  I'd have used a smaller - but not overly fragile - wire (likely #36) if I'd had it on hand to get more turns and better sensitivity.  Of course, I was not going to get nearly as many turns on the stator as the original - which meant that it wasn't going to be as sensitive as it had been originally and would be unlikely to work properly in the circuit - but I had a plan for this.

Carefully winding the #30 wire into the phenolic stator until it was "full", I scrunched the coil down to reduce its height and then pushed it sideways to clear both the meter's axle and the moving magnets on the rotor before covering all of the windings with urethane varnish.  With the varnish dry, I reassembled the meter and found that it operated nonlinearly, particularly near the upper and lower ends of meter travel.

I quickly realized that the screwdriver that I'd used was slightly magnetic - and the two screws used to hold down the phenolic stator had become magnetized as well from using that screwdriver.  Using a TV degaussing coil (I could have used a soldering gun's magnetic field instead) I demagnetized the two screws and the screwdriver, solving this nonlinearity problem.

Re-zeroing the meter and using a series 470 ohm resistor and a variable bench power supply I found that the meter's full-scale sensitivity was about 23 milliamps - very much higher than the 500-ish microamp sensitivity that I'd calculated it to be originally.  In looking at the circuitry I noted that the negative side of the meter was grounded in all three of the front panel meter switch settings which meant that all I needed was to come up with a circuit to multiply the current linearly - and with one end of the meter being connected to circuit ground simplified that task:  Here's the circuit:

Figure 9:
Schematic of the circuit used to drive the re-wound meter on the CIR Astro 200.

The circuit shown in Figure 9 is the classic "precision current source" using an op  amp to drive a transistor and then the meter.  The input voltage is scaled with the trimmer potentiometer (R3) and applied to the non-inverting (+) input of the op amp with R4 in parallel to set an input resistance of about 250 ohms - which is my guess of the resistance of the original meter movement.  By its nature, the op amp will attempt to adjust its output to make the voltage on the inverting (-) input the same as the non-inverting (+) input and to do this, it turns on the transistor, causing current to flow through the meter and the current sense resistor, R2.  Resistor R1 is there to limit the maximum current to a "sane" value to prevent the meter from being slammed too hard in the case of an "oops".

Figure 10:
The as-built circuit from Figure 9 constructed on some
prototyping board.  This circuit is adhered to the top of the
meter itself.
Click on the image for a larger version.

The result of this is that this circuit will happily convert the voltage through R2 into a proportional current, the magnitude set by the adjustment of R3, allowing our now-rebuilt meter movement of arbitrary sensitivity to be used.

As the schematic shows, this circuit was built using the venerable LM324.  This device was chosen mainly because I have plenty of them, and it's one of the most common op amps that has an input and output voltage range that includes "ground" (V-):  Many "standard" op amps don't work near one or the other power supply rail and will work incorrectly if the input voltage is the same as the "V-" lead (ground, in our case) and about as many cannot output voltage down to the negative rail, either.

Since I needed only one of the four LM324's op amps, the other three were simply strapped to the power supply to keep them from floating and possibly causing noise issues:  It's possible that I could have used one or more of the op amp sections to directly drive the meter, but the single transistor was cheap and easy.  The circuit was built onto a small piece of glass-epoxy perfboard and attached to the top of the meter movement - the power supply from this circuit stolen from a trace containing the +11 volt supply found on the front-panel circuit board - but even the 13 volt, unregulated supply would have been fine.

Setting up the "new" meter

While the actual sensitivity of the original meter - which is believed to be around 500 microamps - is not known for certain, there is one step in the manual that is revealing in that it has no actual circuit adjustment, relying on the sensitivity of the meter itself for accuracy.  Because of this, we must do this step first and calibrate the sensitivity of our new meter circuit.

In the section of the manual about "Power Meter, Reflected Power Meter Adjustment" it describes connecting a 2:1 VSWR load (25 ohms using two 50 ohm dummy loads in parallel) and using an external power meter connected between the radio and the load:  The radio should be set for 40 meters for this step.  Switching to "CWW" (CW Wide - using the SSB filter) mode, set the Mic Gain to maximum (fully clockwise), key the radio and then increase the power (turning the Mic gain counter-clockwise to increase power) and adjusting R312 to limit the maximum power to 90 watts even when the Mic gain control is fully counter-clockwise (maximum power): These adjustments should be done quickly to avoid overheating the power amplifier.  The manual notes that with the meter set to the "REF" position, the meter should read "2" (for 2:1 VSWR) - and we quickly adjust R3 in Figure 9 for a reading of "2" on the meter.  Again, the key point here is that the REF meter gets its output from the reverse power detector amplifier - but since its threshold is fixed, when the power is being reduced by this circuit, it will always output the correct voltage/current to make the meter read "2".  In other words, this is fixed reference and we can use it to calibrate the meter for all other modes.

After this, the procedures for adjusting the S-meter, ALC and forward power readings outlined in the manual should be applied without further adjustment of R3, the 10 turn trimmer potentiometer.

It's worth reiterating the point that as the AGC, ALC, FWD and REF signals feeding the meter are ground-referenced, the circuit design was simple.  If the meter was driven by a "floating" circuit - one in which the negative side of the meter was at some potential other than ground - I would likely have used several sections of the LM324 configured as an "instrumentation amplifier" - one that measured the voltage drop across a fixed resistor (in lieu of current through the meter coil) regardless of the actual voltages.  This circuit would be somewhat more complex, but not overly so.

Radio alignment

With the capacitors replaced and the meter working, I went through the alignment steps outlined in the manual.  Fortunately, I had reviewed the manual in its entirety and noted a few "inconsistencies", notably:

• The listing of the carrier oscillator frequencies in the alignment steps shows the same frequency for LSB and USB.  The correct frequencies are shown on the previous page.
  

• When adjusting the ALC using potentiometer R296, the manual says to do so at mid-rotation in one place and and fully CW (clockwise) in another:  I presume that they meant fully CW.

Additionally, I would suggest the following additions to the procedure at the beginning of the procedure.

• Verify/adjust the setting of the 11.0 volt regulator on the synthesizer board (R92).
  

• Verify/adjust the 5.0 MHz oscillator on the synthesizer board using C52.
  

• If you had to re-wind the meter and add the circuit described above, I would do the reverse power meter calibration (described above) before the other meter calibration steps:  This is noted in the procedure at the end of this article.
  

After this, proceed with the alignment/calibration as described in the manual.  There is a revised/annotated alignment procedure at the end of this article.

Power cable

As I was unable to find the original power cable (it may still be in the Jeep) I needed to find the mating power connector.  Recognizing it as a "Jones" connector, I did a bit of research and found that I needed to get a Cinch-Jones S-306-CCT, which is a 6 pin female connector.  Unfortunately, this line of connectors was discontinued by the manufacturer several years ago, but EvilBay came to the rescue and I found a "new" one with the inline cable shroud and strain relief.

Using 12 AWG wire and an inline holder with a 30 amp blade fuse I put together a power cable with an Anderson power pole connector on the far end.  This allowed me to connect it to a high-current power supply so that I could get on with testing the radio's final power amplifier.

"Final" problems

With the radio otherwise aligned, I noted that I was unable to get anywhere near full power out of the power amplifier - about 35 watts on 80 meters, nearly 50 watts on 40 meters and 10-15 watts on 10 meters.  Checking the output on the main RF/AF board, I noted that the voltages were equal to or higher than noted in the manual so I removed the PA module from the back via its ten screws.

I immediately noticed something that further indicated that this was an "early" unit:  The PA driver transistors were Motorola, but marked as "604/438 Sample" and rather than using MRF454 outputs, they were CTC CD3435.  In poking around with an oscilloscope with about 10 watts of output on 40 meters I noticed that the waveforms on the collectors of the driver transistors were not equal - and neither were the corresponding waveforms on the output transistors:  This indicated that in each stage, at least one of the transistors had failed - or was badly degraded.

While annoying (the transistors aren't cheap!) it didn't surprise me.  It is (apparently) common for RF transistors from the 70s and, perhaps, into the early 80s to fail - even when not being used - due to internal defects that seem to "grow" over time.

Figure 11:
The repaired PA board with the new driver and output
transistors.
Click on the image for a larger version.

For the driver transistors, the originals were 2N6367, but the equivalent is the MRF433 or the 2SC2395 - but the MRF455 may work OK.  Rummaging around my bin of RF transistors I found a pair of pulled 2SC2395s (I don't recall where I got them) and put them in, saving me from spending about $100 for them.  Greeted with onlyabout 80 watts on 40 meters - and much lower power than that on 10 meters - I could still see from the waveform on the 'scope - probing the collector leads - that one of the output transistors was still an issue.

While I could get a pair of MRF454 transistors from RF Parts, I noted that they were available from Mouser Electronics for a lower price (about $55 each at the time of writing) and when they arrived, I saw that they sported a recent date code.  Plopping them in I saw that the PA was now capable of well over 125 watts on 80 and 40 meters - working as it should - allowing me to complete the adjustment procedures related to the ALC and power metering.

In testing the two original PA transistors out of circuit, I noted that both their beta and "diode drop" voltage were radically different.  I suspect that at least one of these devices had lost some "emitter sites" or tiny bond wires on the die, making it "less of a transistor" than it once had been.

With the final board now repaired, the radio met the specifications outlined in the manual:  100+ watts on all bands except 10 meters where the output was a bit over 85 watts.

A few loose ends...

The "stuck" LED segment

I also noted that the "stuck" segment on the LED display seemed to have fixed itself during a toggle of the "bright/dim" switch:  In looking at a YouTube video reviewing this radio I noted that it, too, had this exact problem - but I have no idea if it's common (e.g. happened on at least two different radios) or why it fixed itself - nor is there an obvious clue from the schematic diagram why that one particular segment would be affected on my radio and the one in the video.

Adding the clipper/limiter

As for the receiver, the sensitivity is good - but I decided to make a modification that apparently became standard in production just after this unit was produced.  I noted that when changing modes and bands, the S-meter would "pin" with the very loud "pop" that occurred, the AGC taking 5-10 seconds to recover

Figure 12:
The clipper circuit in tubing, installed in the radio.  One end
is connected to a leg of R290 - the other end to ground.
Click on the image for a larger version.

Noting that the manual included the description of a "Limiter" board - and that the radio in the YouTube channel - which had a serial number of about a dozen units higher - also had this board, I figured that this might be one of the reasons why it was added.

The circuit itself is simple:  Two pairs of diodes - one silicon and one germanium in series (for a clipping voltage of about 0.9 volts) - were placed in anti-parallel configuration and coupled with a 10uF capacitor.  This circuit was placed between ground and input of the AGC detector.  Rather than make a small circuit board as was done in the production units I simply wired the components in free space and covered them with PTFE and heat-shrink tubing, connecting the assembly between the AGC circuit and a handy ground pin as can be seen in Figure 12.

My suspicion about its later addition was confirmed:  While there is still a loud "click" when changing modes, the AGC now recovers much more quickly and the radio's AGC is also very much less prone to being badly deflected with a long recovery time when there is a loud static crash.

The T/R relay and filter module

Mentioned briefly, there was the problem with the intermittent T/R relay.  This is contained within a module that sits along the right edge, inside the radio that extends from the front panel to the back of the radio along with the band switch.

This module - in addition to the T/R relay - contains the receiver pre-selector filters, the transmit mixer filters and the transmit low-pass filters on a compact, shielded assembly.  To pull this assembly out of the radio would be quite a job, requiring the partial removal of the front panel, disconnecting (mostly unsoldering!) a number of wires, connectors and signal cables and pulling it out of the radio - something that I have not attempted to do.

Fortunately, the designers provided an access hole near the back panel of the radio (on the bottom side) that is covered with tape where one can burnish the relay's contacts and apply contact cleaner.  After both burnishing and the application of cleaner, the T/R relay is now working perfectly.

Using the radio

Tuning with switches

With the use of toggle switches instead of a round, "spinny" tuning knob, operating the Astro 200 is decidedly different than using a conventional radio.  As mentioned before, the previous owner told me that he thought using toggle switches was a bit better for tuning while bouncing along bumpy roads than a large knob - and in the days of analog radios, this was likely the case.

In perusing online references to this same radio, the users also noted that one quickly becomes accustomed to this method of tuning - but everyone had the same comment:  It's slow to tune across the band.  When powered up, this radio always starts at the bottom of the selected amateur band - and on 10 meters, this particular radio starts at 27.0000 MHz (transmit is inhibited below 28 MHz) which means that it takes about a minute to even get into the 10 meter band!

The AGC

The radio's AGC is not adjustable and the time constant is fine for CW, but a bit fast for SSB in my opinion.  As is common with many analog radios, the apparent AGC time constant gets shorter with more AGC action (e.g. higher S-meter reading).  This is a result of the "dB per Volts" curve getting steeper with many gain reduction schemes (e.g. more dB gain reduction per volt of change) effectively shortening the time constants.

Since this radio has a front panel RF attenuator control, switching this in to reduce the signal level helps with this effect somewhat.

Noise blanker

The noise blanker (enabled by pulling on the "Squelch" control) seems to work pretty well, operating in the wideband IF prior to the crystal filters.  As is typical with noise blankers in analog receivers - and some modern digital radios - its efficacy is somewhat affected by very strong, adjacent signals which "desense" the noise detector - a difficult problem to overcome.

CW usage

As is common for radios of that era, the sidetone frequency in the CW mode has little to do with the frequency offset.  This radio uses USB and a positive transmit frequency shift when in CW which means that neither the display or the tuned frequency changes when going from USB to CW mode.  This was pretty common in the era (many makers - including Drake - did it this way) which meant that if the operator wanted to know the actual frequency of their transmitted signal that they would have to do some mental math.

One "quirk" that I need to investigate is that if this radio's heterodyne oscillator is set precisely according to the manual, the receive (and transmit) frequencies do not match the display, being offset by a bit more than 100 Hz.  This is easily corrected by setting the display to a known frequency, inputting a signal 1 kHz above and below (for USB and LSB, respectively) and adjusting for an audio tone of 1 kHz, but doing so shifts the passband of the crystal filters audibly - and in CW mode, it puts the center of the passband at about 1200 Hz.  This slight shift does not result in either "tinny" or "muffled" audio when using SSB on either sideband, and the radio sounds quite good on air!

As this offset - which is mentioned in the manual as being around 1000 Hz - appears to be programmed into PROMs, it does not seem possible to shift the local oscillator to overcome this issue - and while there's a difference between the USB and LSB passband, it is not a "show stopper" but a 1200 Hz-centered passband for CW is too high in my opinion.  I suspect that this being a very early production radio may have something to do with this issue and I'll have to think about possible ways to address it.

The Mic Gain:  When a "Mic Gain" control isn't really "Mic Gain"

Another unusual design feature of this radio is the transmit audio path.  From the microphone input, the signal path goes directly to the amplifier (there's no level adjustment preceding it) and into the clipper/compressor stage.  Interestingly, the clipper/compressor takes the form of a logarithmic amplifier which has less of a sharp "knee" than a typical clipper, making it quite effective in functioning very much like a compressor-type speech processor.

The designers made an interesting design choice here:  The control marked "Mic Gain" is placed in the signal path after the clipper/compressor - but this has some important implications.  In testing, I used an old Sure 440SL high impedance dynamic microphone which has a fairly high output level, but this caused the clipper/compressor to be "hit" very hard:  On-air reports indicated that that I was readable, but that my speech processing was very "heavy" and off-air recordings from a remote WebSDR verified this.  Since the "Mic Gain" control is between the clipper/compressor and the radio's balanced modulator, it affects only the RF output power and how hard one is "hitting" the ALC and doesn't affect the amount of audio compression at all.

What should really be done was to include a means of adjusting the microphone level into the clipper/compressor stage and this could take the form of having a level control on the microphone itself or in a box between the microphone and the radio, or, if the same microphone will always be used with the radio, put such a control inside the radio.

To accommodate this need, I rummaged around my parts box and found a 500k vertical chassis-mount trimmer potentiometer. This potentiometer was wired such that the "CCW" (counter-clockwise) end was grounded and the opposite end connected to the microphone jack with the audio to the radio on the wiper.

Figure 13:
A 500k potentiometer - reachable using a long, thin blade
screwdriver is accessible through the 1/4" TRS MIC/KEY
connector on this radio.  See text for more details.
Click on the image.

As depicted in Figure 13, behind the 1/4" MIC/KEY jack is a 5 volt regulator in a TO-3 case - but this doesn't line up with the connector, so I glued the potentiometer to a small piece of circuit board to allow it to be offset.  When I glued the pot to this board, I took care to avoid fouling the adjustment knob and after curing.

I then glued the small piece of circuit board to the top of the 5 volt regulator, taking care to offset it so that the potentiometer was aligned such that a long, thin blade screwdriver through the MIC connector could be used to adjust the level from the microphone being applied to the MIC amplifier.  The adhesive that I used was "Shoe Goo" which remains flexible:  I would not recommend epoxy, cyanoacrylate ("super") glue or hot-melt glue as none of these are a good choice in this application (e.g. the bonds will fail with temperature cycling and/or mechanical stress.)

Adjusting this new "MIC Level" control is an iterative process:  Plug in the mic - check the AGC deflection and output power, unplugging, and then making the necessary adjustments.  The goal here is to have enough audio to activate the compressor, but not so much that it sounds very "heavy" on-air.

As noted earlier, THIS radio uses a 1/4" TRS connector rather than the round, multi-pin connector used on later production models:  If this radio had this latter connector, blocking access to an adjustment behind it, I would have mounted the potentiometer facing down and drilled an access hole in the bottom of the chassis, probably making a right-angle bracket on which it was mounted.

Carrier balance

One interesting omission by the designers is the lack of a "carrier balance" control.  When SSB is generated, the "balanced modulator" - which literally mixes the audio with RF - this carrier is nulled on most radios via one or two adjustments to minimize the amplitude of the original carrier - but not on this radio.  This radio uses a diode-ring type of doubly-balanced mixer and by themselves these typically have a "bleedthrough" of between 25 and 35 dB- much less than the 40-50dB of a typical balanced modulator in an analog SSB transmitter after it has been carefully nulled.

What this means is that on 40 meters there is a carrier bleedthrough of about 200 milliwatts (which varies with band and operating temperature) when keyed down with no transmit audio.  Compared with a 100 watt output level, this represents a level that is 25-30 dB below peak power that cannot be adjusted.  This is nowhere near enough to impair efficiency of the transmitter by "wasting" power in the carrier but it is enough to be easily visible to the "waterfall police" using a modern digital radio if the conditions are good.

Frequency (in)stability

When it came out, this radio was remarkable compared to its contemporaries in that it didn't really drift:  You set the frequency and it just stayed there, within a few Hz.  Unlike most radios of the day, it moved only a few Hertz from the instant that it was turned on while most others at the time would change by hundreds of Hz in the first half hour or so - particularly if operated in a cold environment.

Compared to today's radios, the synthesizer is a bit crude - it has large (100 Hz) tuning steps and a bit slow to lock.  As the radio uses rather low reference frequencies (100 and 163 Hz) for its two synthesizers, their oscillators are rather slow to respond - but this also means that they are easily disturbed by slight changes in power supply voltage, mechanical vibration and just the physics of electronic circuits.

What this means is that the frequency can easily "wobble" a few Hz - or even 10s of Hz - around the nominal frequency.  This is generally unnoticeable for SSB usage or even RTTY - and most people will likely not even notice this when running CW - but it does make this radio unsuitable for some of the very narrow digital modes that are seen today, like FT-4, FT-8, WSPR or similar - a trait that it shares with its non-synthesized (VFO-only) predecessors.  These modern digital modes require that the radio be stable within 1-2 Hz over the duration of the transmission/reception window - and this radio simply may not be able to do that.

Mechanical work

If you look very closely at Figure 1, you'll see aluminum brackets on either side of the front panel that were used to screw it to the underside of the dash on the CJ-7 in which it was mounted.  During my refurbishment, I drilled out the pop rivets on these brackets and filled the holes - and a few scratches - with metal-filled epoxy and sanded them down.

Even though the exterior of the case was in reasonable shape, it did show a bit of the wear of having been in a vehicle or two for over 40 years, so I decided to repaint it.  Having been in the vehicle for so long, the original light blue color was varied, depending on how much sun had faded it, but inside the top cover - out of sight - was a "virgin" section of paint to which I was able to find a very close match at the store:  Rustoleum satin "French Blue".  Just in case I - or someone else - wanted to match the original color, exactly, I masked off and left a patch of the original paint inside the lid.

Aside from a bit of wear on the knobs and slight yellowing of the panel meter's clear plastic - most of which was removed with the application of a bit of elbow grease and Novus plastic polish - the radio looks almost brand new.

On the air

I've made several contacts on the air with this radio and and have gotten good reports.  Even with the prevalence of waterfall displays these days, few people mention the slight carrier leakage - but I also wonder how many people actually look at their waterfall not to mention how many others would immediately recognize carrier leakage, anyway?

The addition of the "MIC Level" potentiometer was a good one.  When properly adjusted, the radio now sounds "normal" rather than very heavily "compressed" as before.

I haven't used the radio enough to become very adept at quickly tuning across the band using the UP/DOWN toggle switches, constantly overshooting signals - but I'd guess that this would be a skill that could be readily acquired.  At the risk of sacrilege, I'm considering the addition of a small, PIC-based microcontroller board that will track the button presses and the current band selection to "pre-set" the frequency when the unit is powered up and band is changed, making it a bit more convenient to use:  Such a modification would be completely reversible

Final comments

One should treat this radio in a way similar to "vintage" radios of decades gone by.  It's remarkable in its capability and design considering that it's nearly a half-century old and it needed relatively little in repair - and even more remarkable in that most of the parts that it uses are still available from electronics suppliers at the end of the first quarter of the 21^st century.

As a general-purpose radio for SSB, CW and even RTTY operation, it's still very usable:  It's small size belies its capabilities, particularly in context with its vintage.  Being made prior to 1980, it obviously lacks the WARC bands (30, 17 and 12 meters) - but so do other radios of that time period.  Once the radio was restored - mostly a matter of replacing electrolytic capacitors - it operates pretty much as it did when it was new and it would not seem out of place on the air among modern radios on the air.

Given its quirks (no tuning knob being the most obvious) it is a bit of curiosity, reminding the user of a time just before completely analog radios gave way to synthesized radios becoming the norm - a revolution not too dissimilar to the more recent trend of "analog" radios giving way to those that are almost entirely digital from the antenna port to the speaker.

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Alignment notes

Here are notes related to aligning the Astro 200 (Non "A" version) - although they should be generally correct for the "A" version as well.  These should be used to augment the instructions noted in the operation/maintenance manual.

Power supply check - IMPORTANT!

• Verify 11.0 volt power supply - adjust R92 on synthesizer board as appropriate.
• Verify that 11.0 volt supply will remain stable down to a supply voltage of at least 11.5 volts as measured on the radio's voltage input.
  
• Verify 8.0 and 5.0 volt supplies (each being +/- 0.25 volts of nominal).  Note that there are two separate 5.0 volt regulated supplies.
  

Reference (Master) oscillator:

• Frequency counter to set to 5.000000 MHz or use WWV setting (which listens to 10 MHz via a direct-conversion receiver) and listen for zero beat
  
• Set C52 for 5 MHz, exactly.  This is accessible via a small hole in the bottom cover.
  

Carrier oscillator:

• USB/LSB
• MIC Gain CCW
• RIT and FINE at 12:00 position
• MODE to USB
  
• Key radio and adjust C180 for 5.601650 MHz
• MODE to LSB
• Key radio and adjust C174 for 5.598350 MHz
• MODE to CWN
• MIC Gain fully CW (for minimum CW TX power) and connect radio to dummy load.
  
• Key radio and adjust C204 for 5.60060 MHz

RX Delay adjustment - used to delay time between release of PTT/VOX and RX activation

• Adjust R239 for desired delay time preference in switching from TX back to RX when PTT is released.
  

VOX Trip and Anti-Trip

• Turn on VOX and set volume to desired level using your typical ham shack speaker/audio environment.
  
• Adjust R181 for VOX activation level with normal speaking voice.
  
• Adjust R158 for anti-VOX level with signals/static present to prevent unwanted triggering.
  

Meter adjustments.  Be sure to view meter "straight on" and consistently to minimize parallax for the readings below.

• VSWR shutdown/reflected power:  R312 calibrates the VSWR shutdown of power.  DO THIS STEP AS QUICKLY AS POSSIBLE.  Be sure to view the meter "straight on" to avoid parallax in the following steps.
• NOTE:  As mentioned earlier in this article, I had to "repair" the meter by re-winding its coil and using an external driver circuit.  If you restore the meter in this manner, do THIS step before the other "Meter adjustment" steps.
  
• Connect two 50 ohm loads in parallel for 2:1 VSWR (25 ohms) - use the shortest length coaxial cable possible.
• Set to a mid-band frequency on 20 meters.
• Set meter switch to REF
  
• In CWW mode, turn MIC gain fully CW, key transmitter.
• Increase power.  Quickly adjust R312 so that the forward power can not be increased to more than 90 watts on the forward meter and unkey.
  
• In VSWR mode, the meter should read about 2.

• Forward power:  R306 calibrates forward power reading.
  
• Connect 50 ohm dummy load and power meter.
• Set the radio to a mid-band 40 meter frequency and pre-set the MIC gain control fully CW to set minimum power.
  
• In CWW mode, turn MIC gain CCW, key transmitter and adjust for 100 watts on the power meter.
  
• Adjust R306 for full-scale indication indication (to the "Set" marking) on meter.
  

• ALC Setting.  Be sure to view the meter "straight on" to avoid parallax in the following steps.
• Connect 50 ohm dummy load and power meter.
• Set MIC gain to 12:00 position, meter mode to FWD. (CONFLICT:  Manual says says fully CW in earlier section about adjustment)
• Note:  Since the transceiver has no actual "Microphone Gain" adjustment prior to the clipper, the fully-CW adjustment setting would make sense as it will maximally drive the ALC (worst-case).
  
• Key transmitter and whistle or produce tone into the microphone.
  
• Adjust R296 for a reading of an average of 40 watts on the power meter.  This should correspond roughly with a reading of "30 over" on the meter.
  

• ALC Meter setting.  Be sure to view the meter "straight on" to avoid parallax in the following steps.
• Connect to 50 ohm dummy load.
  
• Set mode to CWW, meter to ALC and set MIC Gain fully CLOCKWISE
• Key transmitter:  There should be low/no power.
• Adjust R291 for FULL SCALE ALC meter deflection.

AGC set-up.  Be sure to view the meter "straight on" to avoid parallax in the following steps.

• Connect signal generator to antenna input and mode to CWW.
• Set front attenuator switch to OFF (down)
  
• Set for 20 meters and tune to a frequency mid-band and adjust the signal generator so that there is a tone of about 1 kHz
• Set the signal generator for an output of 1.5 microvolts (-103.4dBm)
• Adjust R280 for an S-meter reading of S3.
  
• Increase the signal to 50 microvolts (-73dBm)
• Adjust R272 for an S-9 meter reading
  
• Re-check the steps above for 1.5 and 50 microvolts and adjust as necessary.

Sidetone Level set

• Connect to 50 ohm dummy load, set to CWN and adjust MIC gain control fully CLOCKWISE (minimum power)
• Key transmitter and adjust R257 for desired sidetone level in speaker.

In-depth alignment:

Carrier oscillator peaking

• Using and oscilloscope or high-impedance RF voltmeter, measure the amplitude at the base of Q60
• Adjust L11 for maximum amplitude.  Use only a plastic adjustment tool to avoid breaking the core.
  
• Check carrier oscillator frequencies as noted above - adjust as appropriate.

TX mixer and ALC attenuator

• Connect 50 ohm dummy load.
• Set to CWN and adjust fully CCW (max power)
• Key down and adjust L6 for maximum signal on collector of Q20 using an oscilloscope or RF voltmeter.  Use only a plastic adjustment tool to avoid breaking the core.

WWV receiver adjustment

• Set MODE switch to WWV and turn AF gain all of the way down.
  
• Apply signal generator at 50uV (e.g. -73dBm - equivalent to S9) to antenna, offset from 10 MHz by about 1 kHz so that a tone will be heard.
  
• Connect AC voltmeter to speaker and adjust level to indicate on meter, but keep it well below clipping.
  
• Tune L15 for maximum speaker output.  Use only a plastic adjustment tool to avoid breaking the core.
• Remove input signal.
  
• Using a high-impedance RF voltmeter or oscilloscope, adjust L16 for maximum 10 MHz at collector of Q77.  Use only a plastic adjustment tool to avoid breaking the core.
• A signal of 5uV (-93dBm) should be audible.

Noise blanker adjustment

• Connect a signal generator to the antenna input.
• Adjust receiver and signal generator for a mid-band 20 meter frequency and adjust for a level of 100uV (-67dBm) and an approx. 1 kHz tone in the speaker.
• Adjust L9 and L10 for maximum voltage on D30.  Use only a plastic adjustment tool to avoid breaking the core.

SWR Bridge adjustment

• Connect two 50 ohm loads in parallel for 2:1 VSWR (25 ohms) - use the shortest length coaxial cable possible.
  
• Set MODE switch to CWW and set MIC Gain control fully CW (minimum power) and set to mid-band on 20 meters.
• NOTE:  Do the following measurements as quickly as possible to minimize stress on power amplifier.
• Key down.  Increase power (MIC gain turned CCW) and note that SWR protection limits to 90 watts as adjusted in SWR protection steps noted above.
• Note power reading on front panel meter and external wattmeter (if used) and then un-key.
  
• In the same manner, check the maximum power into the same 2:1 VSWR on 80, 40 and 15 meters.
• Adjust C3 as necessary for flattest (most consistent) power reduction on all bands:  Power should be between 80 and 105 watts.
• On 10 meters, power into a 2:1 VSWR may be in the 70-80 watt range.

RF Tuning assembly

This is the unit inline with the BAND switch.  The coils noted below correspond with the frequency range and should be adjust for best response across that noted below.

NOTE:  As the receive and transmit filter inductors are not normally accessible, it is necessary to remove the band switch module to perform these adjustments - a laborious task which requires unsoldering a lot of different cables and removal of the front panel.  It should be done ONLY if problems are suspected.  These adjustments should only be done with a spectrum analyzer and tracking generator OR a VNA/SNA.  If the sensitivity of the receiver is adequate and the transmit drive is within specifications, there is probably little need to even touch these adjustments.  As my radio was "up to spec" in terms of sensitivity and TX drive, I did not pull the module and make any adjustments.

Use only plastic adjustment tools to avoid breaking the cores!

Receive filters

• 80 Meters:  L101, L102 - 3.5-4.5 MHz
• 40 Meters:  L103, L104 - 7.0-7.5 MHz
• 20 Meters:  L105, L106 - 14.0-14.5 MHz
• 15 Meters:  L107, L108 - 21.0-21.5 MHz
• 10 Meters:  L109, L110 - 28.0-30 MHz
• WWV:  L111, peaked at 10.0 MHz.

Transmit mixer band-pass filters

• 80 Meters:  L201, L202 - 3.5-4.5 MHz
• 40 Meters:  L203, L204 - 7.0-7.5 MHz
• 20 Meters:  L205, L206 - 14.0-14.5 MHz
• 15 Meters:  L207, L208 - 21.0-21.5 MHz
• 10 Meters:  L209, L210 - 28.0-30.0 MHz

Synthesizer adjustments

Unless the synthesizer has difficulty locking - particularly at the upper or lower edge of one or more bands - there's probably no need to make these adjustments.

Major Loop VCO

• Adjustments should be made at low edge of the respective band.
• Coil should be set for a voltage of 2.5 +/- 0.25 volts on R18
• Exception:  For units that can tune to 27.0 MHz, the voltage should be 3.0 +/- 0.25 volts when tuned to 28.0 MHz.
• Start with the highest band first.  For the progressively-lower bands, the following inductors are in series meaning that a higher-band coil's adjustment will affect all lower bands.
  
• 10M:  L9
• 15M:  L8
• 20M:  L7
• 40M:  L6
• 80M:  L12
  
• Notch filter:  Adjust R11 and R15 for minimum amplitude of 100 Hz signal on the output (pin 6) of IC21 (0.035Vpp or lower)

Minor Loop VCO

• Adjustments should be made on low edge of the respective band.  (Manual isn't clear about this)
  
• If adjustment is needed, it will be necessary to remove the brass shield by unsoldering its three corners.
• Coils should be set for a voltage of 1.6 +/- 0.2 volts as measured on R21.
• Start with the highest band first.  For the progressively-lower bands, the following inductors are in series meaning that a higher-band coil's adjustment will affect all lower bands.
• 10M:  L5
• 15M:  L4
• 20M:  L3
• 40M:  L2
• 80M:  L1
  
• Notch filter:  Adjust R25 and R27 for minimum 165 Hz signal (0.025Vpp or lower) on the output (pin 6) of IC20.
  

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This page stolen from ka7oei.blogspot.com

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