Repairing a ("smoked") MFJ-998 1.5kW automatic antenna tuner

Figure 1:
The (repaired) MFJ-998 front, now working, sitting
atop my Heathkit SA-2060 manual tuner and underneath
my homebrew neon bar-graph VSWR/Power meter.
Click on the image for a larger version.

To inspect or not inspect 

When I buy some types of ham gear second-hand, I'll treat it like as I would if I were to buy used Heathkit gear that had been put together by someone with "average" (or unknown) kit building experience:  I take the cover off, tighten the screws, reflow suspicious solder joints and do a visual inspection.  Regardless of brand, it's probably a good idea to pop the covers and take a look at gear that you buy second hand before using it.

For some reason - when I recently bought a second-hand MFJ-998 1.5kW automatic antenna tuner on EvilBay - I didn't do that.

When this '998 arrived I did a cursory look at the case, connected it, and tried it out at 100 watts - and it seemed to work OK - but over the course of a few weeks being used at just 100 watts I noticed something odd:  It would occasionally start re-tuning during a transmission for no obvious reason and, perhaps, there was a faint whiff of "something" in the air - but I never connected the two and didn't investigate.  I'd previously operated using my old Heathkit SA-2060 (non-"A" version) antenna tuner for several years on this same antenna and hadn't noticed a randomly changing VSWR that might explain this tendency for the '998 to "hunt".

Figure 2:
A of relay K2 from the top:  Evidence of
damaged glass-epoxy PC board material
is very clearly evident!
Click on the image for a larger version.

I checked for the usual suspects external to the tuner:  Loose coaxial or wire connections, a branch touching the antenna (an 80 meter horizontal loop) somewhere - but there was nothing obvious.  This "problem" wasn't consistent, either - not happening frequently enough to cause me to expend more effort to track it down.

Magic smoke escapes!

Several weeks ago I had occasion to run a bit more power and threw about 800 watts through it and things seemed OK for a while, but then it started retuning itself again - this time, accompanied by a very distinct burning smell.

I immediately pulled the tuner out of the circuit, going back to the Heathkit SA-2060 manual tuner, and things were fine once again, further indicating that the "instability" was related to the auto-tuner and not the antenna itself.  Later, when I had time to do so, I pulled the cover off the MFJ-998 and immediately saw the problem:  As can be seen in Figures 2 and 3, the PC board was carbonized in the vicinity of relay K2 which is used to select antenna #1 or antenna #2.

Figure 3:
The damage to the PC board as seen from the
bottom side.  Sections of the board have become
carbonized, offering current paths to RF, causing
the degradation to accelerate.
Click on the image for a larger version.

Upon seeing this, I ordered some replacement relays - ten of them, as they weren't particularly expensive - and tuner sat around for several weeks until they arrived.  I did briefly consider just omitting this relay, "hardwiring" it for just one of the antenna outputs, but decided to proceed with the repair:  If this happens again I'll reconsider doing this - or perhaps changing the way the relay(s) are configured.

What happened?

Clearly, the PC board material had "carbon tracked" at some point:  A bit of leakage between the traces had obviously occurred and with the higher RF voltage resulting from my running higher power, this "slight" leakage had gotten very much worse, heating the board material, decomposing the epoxy base of the PC board material and causing it to become conductive - and the it gets worse and worse from there.

I have the suspicion that a carbon track was present before I owned this tuner and it likely occurred due to attempting to tune an antenna that wasn't connected (possibly resulting in very high voltage), an intermittent antenna fault, or perhaps even lightning.  At the time it failed,  I'd been using antenna #1 and had nothing connected to the connector or post of antenna #2 so I'm not sure why it so readily burned across the traces between the antenna connections - but it did.

Figure 4:
Damage to the original relay.  I don't think that
the relay itself initially failed, but rather that it
was damaged by the intense heat of the glowing,
carbonized board material.
Click on the image for a larger version.

Analysis of the damage

With the "carbon tracking" between the tuner and the antenna "#1" and "#2" connections, the circuit board was nearly burned-through in a few places and the relay was destroyed - but it looked as though the damage of the relay was caused by the heat from the (burning!) PC board.  Despite looking really bad, the damage was very localized - and it provided an opportunity for improvement.

The fact that the damage was worse on the bottom of the board than the top also indicated that it was likely on the bottom side that the issue first started.

What to do?

In many cases - when high voltages are present across a section of PC board - manufacturers will place an "anti-tracking slot" between the two points:  Rather than rely on the surface of the PC board to withstand high voltages - the ability decreasing if moisture, dust or other contaminants are present - a physical slot is cut in the PC board material between those connections, greatly increasing the path length and high voltage stand-off ability.

Figure 5:
Using a rotary tool to remove ALL carbonized PC board.
All potentially-conductive board material must be removed
or else the same thing will happen again!
Click on the image for a larger version.

This "slotting" technique is frequently found on mains-powered devices that have human contact - such as phone chargers and other power supplies.  This is done where excess leakage between the high voltage from the wall plug and the low voltage output could result in injury or death if someone touched an accessible metal contact.  

While this application isn't a "life-safety" issue like a power supply, it would have made sense, given the high voltages that are possible, to implement such measures here.  For "reasons", MFJ did not choose to manufacture the PC board with such "anti-tracking" slots in this particular location - on that is very likely to have the highest voltages present across adjacent contacts to be found in the tuner.

Using a small routing bit in a rotary tool I ground away ALL of the carbonized (conductive!) material:  NOT doing so would have risked additional "tracking" in the future where it could have found a new conduction path.  The result of this work can be seen in the photos - a bit of "Swiss-cheese" of the circuit board - but now, there was only air between the contacts across which there was likely to be high voltage.

Installing the new relay

With all of the carbonized material removed, I vacuumed up the debris and cleaned the area around the relay on both the top and bottom side with alcohol, removing deposits from the soot of the burning PC board material.  I repeated this process after the new relay was soldered in.

Figure 6:
After cleaning the board with alcohol, the new.
relay was installed and the connections made
using 16AWG wire.  The "air gap" between
pins should make it more resistant in the future.
Click on the image for a larger version.

On this relay, the "common" (armature) pins are in the center with the normally-open and normally-closed pins on either side - but since I removed most of the PC board material around these pins, under the relay the "common" connection was completely missing.  This problem was easily solved using a piece of tinned wire as can be seen in Figure 6:  The air gap between the other relay contacts was maintained.

I was fortunate that the PC board connections for the coil (the two solder pads near the top of Figure 6) and both the "Normally Open" and "Normally Closed" terminals were still intact (e.g. the board wasn't burned in those areas) and this provided a solid mounting for the relay.   It was only the "common" relay contacts - those that connected back to the tuner itself - that were no longer extant so I folded a piece of 16AWG copper wire and made the connection back to the remaining PC board trace as seen in Figure 6.  (Note:  This relay, K2, is a DPDT relay of the same type as all of the other relays and both of its sections are connected in parallel.)

Testing and comments

It worked!

As there was no damage to any other circuitry, the relay properly selected between antennas 1 and 2 as designed and the memory pre-sets (on the bands other than where I was operating when the failure occurred) were just as they were indicating that the matching conditions were identical to before.

In doing research on the relays used in this tuner (all of the relays are identical - for my tuner, they are Hui Ke HK14FH-DC12V-SH - which is the same as the American Zettler AZ576-1C-12DE, the Songle SMIH-12VDC-SL-C and many others ) I noted a few things about their specifications.  As expected, they have 12 volt, non-latching coils (e.g. power must be applied for it to hold the tuning configuration) and their contacts are rated for 16 amps (resistive) for DC and mains-frequency AC, and they also have good isolation between the contacts and the coil (rated for 5kVAC at mains frequency).

What did concern me a bit was the fact that they have only a 1kVAC rating between "open" contacts - and a quick check with a "Hi-Pot" tester verified that it did, in fact, break down (arc internally between the armature and the normally-open contacts) at about 1.7KVDC.  It would seem that the designers of this tuner considered this aspect of the relays' limitations to an extent as all of the capacitors in the tuner are switched in/out using two relays in series to accommodate higher voltages - but this technique was not applied to K2, the antenna switching relay.

A quick check of the ratings of this tuner indicate that it is rated for up to 1600 Ohms at full power (1500 watts).  Knowing the power and impedance - and presuming a resistive (e.g. resonant - neither capacitor of inductive) load - we can use the following formula:

V = √(P x R) 

Where:

V = Voltage

P = Power in Watts

R = Resistance in Ohms

and based on this, at 1500 watts and 1600 Ohms, the voltage in a purely resistive load would be about 1550 Volts RMS (or about 2200 peak volts) - well above the known breakdown voltage of the contacts of the relay.  It's worth noting that it's often the case that at radio frequencies, insulation and breakdown ratings are lower than they might be at mains frequencies and DC - something else to consider!

What this implies is that under such conditions there would be enough voltage for the armature of the relay to arc to those of the unused antenna and, perhaps, the designers of this tuner should have put two relays in series for antenna switching, too:  If the "new" relay breaks down with voltage, it wouldn't be too difficult to wire a second one in series to increase the stand-off voltage, driven by the same coil driver.

What about the other extreme, where the impedance is low?  The tuner is rated for as low as 12 Ohms where the voltage would be lower and current higher and here we use a different formula:

I = √(P / R) 

Where:

V = Voltage

P = Power in Watts

R = Resistance in Ohms

 

In this case 1500 watts and 12 Ohms (resistive) yields a current of about 11.2 amps RMS - somewhat lower than the relay's contact rating of 16 amps, plus there's the fact that both sets of contacts of this relay are in parallel which further increases their durability.  At such high currents, the concerns aren't necessarily with the relays, then, but rather potential I*R heating of the inductors and capacitors whenever an extreme match condition is encountered.

 

In short:  If you are running anywhere near 1500 watts and have a high impedance being presented to the tuner by your antenna (e.g. using something like a directly-fed half-wave end-fed antenna with no matching device) and using both antenna ports,  you should probably reconsider your arrangement!

 

* * * * *

 

This page stolen from ka7oei.blogspot.com

 

[END]

 

Adding ALC and overdrive protection to the MFJ ALS-500M "500 watt" amplifier

Figure 1:
The ALS-500M front panel.  Because this
this unit is equipped with the 10 meter low-pass
filter, the "AUX" position on the front panel
switch is used to select 10/12 meters.
Click on the image for a larger version.

The MFJ ALS-500M is a (nominally) 500 watt amplifier that was produced by MFJ, capable of covering from 160 meters through 15 meters - and 12/10 meters if so-equipped.

If you own an ALS-500M, you may have realized that it is a bit awkward to use:  If you are using it with a 100 watt radio, driving it with this much power will not only cause it to be badly overdriven - causing terrible on-air distortion on an SSB signal - but it will likely cause damage to the amplifier itself by throwing about twice as much power at it as it needs to work properly.  For this reason one must use a 50 watt radio (does one even exist?), one that puts out much less power (not taking advantage of the full-power output of the amplifier) or, more likely, always remember to turn down the output of a 100 watt radio and hope that it doesn't have a problem with "overshoot" (an issue described later).

To understand the problem, a friend's ALS-500M was powered from a variable-voltage supply with known-accurate wattmeters - one on the input to measure drive power and another to measure the output power, the amplifier itself terminated in a known-good 50 ohm load.  This same test set-up included a known-accurate DC ammeter as it was determined that the ALS-500M's own ammeter wasn't particularly accurate.

With this set-up, the characteristics of a friend's ALS-500M were measured on most amateur bands in terms of input and output power, at both 14.5 and 12.5 volts from the power supply.  (The voltage at the amplifier was lower than this due to resistance of the factory-supplied DC power cable.)

Freq (kHz)	PWR In	PWR Out	DC Voltage	DC Current
1825	5	65	14.5	18
	8	180	14.5	31
	22	300	14.5	43
	30	410	14.5	52
	45	525	14.5	60
	60	600	14.5	63.5
	6	82	12.5	21
	22	300	12.5	43
	30	450	12.5	56
	45	480	12.5	59
	65	500	12.5	63
1975	6	90	14.5	23
	20	350		45
	33	480		56
	45	560		63
	60	600		66.5
	33	425	12.5	53
3650	6	80	14.5	23
	20	290		47
	36	425		60
	50	500		66
	62	500		70
3850	6	78		22
	19	300		45
	33	410		57
	50	450		61
	63	490		67
	33	370	12.5	54
5371	6	95	14.5	27
	18	340		52
	34	480		64
	50	525		71
	34	380	12.5	60
7050	4	85	14.5	22
	18	350		47
	30	400		53
	52	460		59
	33	350	12.5	50
7250	5	87	14.5	23
	18	350		46
	33	410		53
	52	480		58
	33	350	12.5	49
14225	4	95	14.5	26
	18	310		47
	34	380		53
	52	400		56
	33	290	12.5	47.5
18100	5	75	14.5	22
	20	210		37
	35	260		41
	55	300		44
	35	200	12.5	36
21250	6	95	14.5	23
	24	210		35
	37	275		38
	56	280		39
	37	200	12.5	33
28345	5	68	14.5	25
	22	275		50
	34	325		55
	52	390		62
	34	300	12.5	53

From this data we can determine several things:

• There are no instances where more than 50 watts drive is useful.  If you were to graph the input versus output power in the above chart you would see that the curve "flattens" by the time you get to about 50 watts drive meaning that further increases of input power do not result in the same proportion of increase in output power.  It is at this point that the amplifier is becoming very non-linear and severe distortion of SSB and AM signals will result if one attempts to drive it to still-higher output power.
  
• While described as a "500 watt" amplifier, this is clearly optimistic. While barely capable of about 600 watts at 160 meters, the maximum usable "clean" (non-distorted) output drops to about 400 watts at the highest band, 10 meters.  This effect is due to physics:  The transistors in the amplifier are simply less capable at higher frequencies.
• The power output is lower with a 12.5 volt supply than a 14.5 volt supply.  This is also due to physics and clearly specified in the manual:  You'll get 25-100 watts less output at the lower voltage, depending on the frequency and drive power.

Too much power is NOT a good thing!

The ALS-500M manual clearly warns against driving with too much power for the reasons mentioned above, but in addition to producing a bad-sounding signal on the air, feeding too much power to the amplifier (more than about 60 watts) is significantly exceeding the specifications of the (expensive!) transistors and it will dramatically increase heating of the components:  On this test amplifier, even briefly driving it at 65 watts caused the input power resistors to overheat slightly, resulting in an obvious smell, not to mention completely "flat-topping" (severely over-driving) it.

Why no ALC?

Since at least the 1960s both amateur transmitters and amplifiers have included an ALC (Automatic Level Control) circuits.  In a typical amateur transmitter, this circuit monitors the transmitter's output power and if it exceeds the pre-set threshold (e.g. 100 watts for a radio rated at 100 watts) it will send a signal back to reduce the output power.  RF amplifiers have a similar circuit:  It detects the amount of RF being output and sends a negative voltage back to the transmitter driving it.  If the output of the amplifier gets too high, this voltage causes the transmitter to reduce its drive power.

In both cases this circuit does two important things:

1. Prevents excess drive to the amplifier(s), which prevents distortion of the transmitted signal.
2. Preventing damage.  All amplifiers have electrical and thermal limits above which they may be damaged and/or their operational lifetime may be dramatically shorted. 

Despite most commercially-produced amplifiers made for the past 60 years having a circuit to produce an ALC voltage to feed a radio the ALS-500M does not - which is all the more confusing as this circuit is not complicated at all:  Having this circuit would help in the prevention of grossly overdriving the ALS-500M and having bad signals on-air and it may have saved many ALS-500M's from damage.

Adding an ALC circuit

As it made sense to do so, an ALC circuit was added to my friend's ALS-500M:

Figure 2:
This is the schematic of the ALC circuit.  It develops a negative voltage related to the RF output
power that is fed into the transmitter driving it.  When properly adjusted, this feedback loop will
limit the maximum drive to the amplifier, reducing the probability of distortion and damage.
Click on the image for a larger version.

 The circuit is quite simple - consisting of just TEN components including the output jack.  Here's how it works:

• J1 is the existing "RF Out" jack on the ALS-500M, an SO-239.
  
• Resistor R1, attached to the RF Out connector, samples the transmit power.
• Resistor R2 - with R1 - form a voltage divider:  At 500 watts into 50 ohms with R1 being 12k, there would be 447 peak-to-peak volts on the RF Output, but it is divided to 34 volts peak-to-peak at the junction.
• Capacitor C2 couples the RF to diodes D1 and D2, blocking DC.
• Diode D1 clips the positive-going voltage and together with D2, forms a voltage doubler circuit.
• Capacitor C3 filters the output of diode D2 - a negative voltage -  removing residual RF.
• Potentiometer R3 allows adjustment of the produced ALC voltage so that the proper threshold may be set for the transmitter being used to drive it.
  
• Capacitor C4 further filters any RF from the ALC line.
• J2 is a phono ("RCA") jack used to connect the ALC voltage to the driving transmitter.

Here are links to a few of the more difficult-to-get parts:

• 1N5711 shottky diode with wire leads:  https://www.digikey.com/en/products/detail/stmicroelectronics/1N5711/603524 
• 10K pot with 1/4" shaft:  https://www.digikey.com/en/products/detail/bourns-inc/3310Y-001-103L/1088215

Frequency compensating capacitor C1

Capacitor C1 requires more explanation.  Real-world components aren't like their "ideal" theoretical counterparts and resistor R1 is no exception:  Even though it is a "resistor", it has some capacitance - albeit small - plus there is some stray capacitive coupling between the center pin of the RF Out connector and the nearby components.  Because of this, at higher frequencies, some RF energy "leaks" around R1, causing more voltage to appear at the junction between it and R2:  This higher voltage would cause more AGC voltage for a given power level and in testing, while 500 watts produced about -37 volts at the top of R3 on 80 meters, it took only about 150 watts to produce that much voltage on 10 meters.

Figure 3:
The as-built ALC circuit built atop the RF OUT connector,
using it for component support.  To the right of the large
resistor is the ALC adjustment potentiomteter and jack below.
Click on the image for a larger version.

We actually want this roll-off at higher frequencies to occur for the simple reason that the ALS-500M cannot output the same "maximum" power on each band - this level decreasing as frequency goes up - but as we can see from the table, whereas we could "safely" output about 400 watts at 80 meters at 12.5 volts, we'd probably want no more than 325 watts or so at 10 meters, so our ALC output voltage should be the same at those two power levels.

Capacitor C1 - placed across R2 - "compensates" for this:  Being a capacitor, it has lower impedance at increasing frequency and we can select its value to give us about the same ALC voltage at 400 watts on 80 meters as 325 watts would on 10 meters.  For the ALS-500M and our circuit, a value of 6.8pF turned out to be about right - but this would vary with components:  A variable capacitor (something adjustable over approximately the 2-15pF range) would allow easy adjustment of this compensation.  A suitable device is this

• 3-20pF variable capacitor:   https://www.digikey.com/en/products/detail/sprague-goodman/GYC20004/5278687

 Construction of the ALC circuit

Figure 4:
Another view of the ALC circuit.   R1 is the
large resistor, C2 in the foreground, C1 is the
large capacitor in the bacground.
Click on the image for a larger version.

Figures 3 and 4 show how the ALC circuit was laid out atop the "RF Out".  In the top-center of Figure 3 we see the "RF Out" connector and R1, the 12k resistor and clustered around R1 - and using the ground lug (plus an added lug) on the "RF In" connector we see the other components.  Just to the right of center in Figure 3 - between the RF Out connector and the DC connector we see R3, a 10k potentiometer and below it - partly obscured by R3 - is the "ALC Out" jack.

Figure 5 shows the rear panel of the amplifier - the "ALC ADJ" potentiometer (R3) near the top and the Phono (RCA) plug below it - both labeled.  Looking at the label of the ALC ADJ control, you will notice that the label shows that rotating it counter-clockwise will result in "minimum" power - but this corresponds with maximum ALC voltage.  While this may seem counter-intuitive, remember that the the more negative the ALC voltage, the more it will try to reduce the output power of the transmitter - but if the potentiometer were turned fully clockwise (no ALC voltage at all) it would be the same as disabling the ALC altogether.

In testing with an Icom IC-7300, setting the ALC control to "Min" (e.g. maximum ALC voltage causing the greatest amount of power reduction) resulted in no more than about 80 watts out of the amplifier, no matter the "RF Output" setting on the radio and this indicated that the ALC was doing its job.  Setting the ALC control for about 425 watts at 80 meters resulted in about 325 watts on 10 meters, maximum - both within the "linear" and safe range of the amplifier.

Figure 5:
The rear panel of the modified ALS-500M.  The ALC adjust
potentiometer is between the RF OUT and DC IN connectors
with the added "ALC OUT" jack below.
Click on the image for a larger version.

ALC Overshoot and other anomalies

In many radios, ALC isn't perfect:  There will be a slight lag in many radios between the appearance of the ALC voltage and the radio's cutting back in transmit power - some of this being due to the radio itself having "ALC Overshoot" and some being due to the ALC voltage from the amplifier being a bit slow to respond.  What this means is that it is possible for the radio to briefly output WAY more power than expected for a brief instant before throttling back.

On the air, this can cause a burst of amplifier overdrive at the beginning of words/syllables - often showing up as a "popping" (or "clicking" on CW during key-down) and over time, this burst of high power could damage the transistors and other components in the amplifier.  What this means is that you SHOULD NOT rely entirely on the ALC to limit the output power - you should, at the very least, turn down your transmit power to about 50 watts or so even if you have the ALC.

Figure 6:
Rather than remove the Filter board to get access to the
T/R relay, the cable that had connected to the input of the
amplifier deck was soldered to the ground plane to allow
splice a piece of RG-316 to reach the new attenuator on the
back panel.
Click on the image for a larger version.

Some radios have another problem:  They can do ALC overshoot even without an external amplifier - briefly driving their own amplifier  to much higher than expected power.  Some radios - even if you turn the power down - rely on feedback from their built-in wattmeter and will briefly output higher than the desired output power.  Both of these mean that you could still end up with a somewhat "dirty" signal on the air even if you believe you have taken steps to prevent it.

Overdrive protection:  Adding a 3dB pad.

While adding ALC to the ALS-500M is a "no-brainer", it would be easy to forget to connect the ALC - or your radio and amplifier combination could still cause the "popping" or "clicking" from brief overdrive conditions even if you turn down your power and/or connect the ALC.  To prevent this, it would be a very good idea to prevent too much power from ever reaching the amplifier circuitry itself.

Figure 7: 
The 3dB (actually 2.995dB)
"Pi" resistive attenuator.  At 100 watts
input approx. 16.7 watts is dissipated in R1,
around 24.5 watts in R2, and about 8.8 watts
dissipated in R3 - about 50 watts total.
Click on the image for a larger version.

As you can see from the chart above, there is never a frequency or band combination where more than 50 watts drive would yield clean output power.  What this means is that we could lose half of the drive power of a 100 watt radio and still push the amplifier to its useful limit - and protect its expensive transistors and other circuitry against an accidental "oops" should we accidentally overdrive it.

The addition of a 3dB attenuator would accomplish this, soaking up half the transmit power before it gets to the amplifier allowing the user to set their radio to 100 watts output.  The easiest place to install this attenuator would be on the input of the amplifier - but this would also affect the receive signal by about 1/2 "S" Unit:  If your S-meter reads well above S1 on even the quietest band, you won't "miss" any signals by doing so:  A 100 watt 3dB "pad" can be found commercially and on the surplus market if you look carefully.  The other down-side of having a 3dB pad inline would be that if your turn the amplifier off, you are still losing half of the transmit power.

Figure 8:
Holes drilled in the back panel in preparation
for mounting the power resistors used for the
the 3dB attenuator.
Click on the image for a larger version.

A technically "better" solution would be to place the 3dB attenuator right on the input of the RF power amplifier circuit, inside the amplifier.  Doing so avoids placing this loss in the receive path and it will also not affect the transmit signal when the amplifier is turned off.  Figure 7 shows this attenuator schematically.

These resistors must, collectively, be able to dissipate 50 watts of power and rather than trying to assemble a large mass of lower-wattage resistors, we can use thin or thick film power resistors in transistor-like package which may be bolted to a heat sink. For the ALS-500M, there is a flat area on the rear panel that is next to amplifier deck and large enough to accommodate these resistors and dissipate the power dropped.  Examples of suitable resistors include:

• 18 Ohms, 100 watts:  Bourns (Riedon) PF2472-18RF1  (DigiKey P/N:  696-PF2472-18RF1-ND - link)
• 300 ohm, 100 watts:  Bourns (Riedon) PF2472-300RF1  (DigiKey P/N:  696-PF2472-300RF1-ND - link)

Figure 9:
The three resistors comprising the attenuator, mounted on
rear panel of the amplifier for heat sinking.  The white
RG-316 coax from the T/R switch comes in from the left
while that going to the amplifier input goes to the right.
Thermal paste is used under the resistors' tables to enhance
thermal conductivity to the case.
Click on the image for a larger version.

These particular resistors are "Thin Film" and their construction is such that while not intended specifically for RF applications, they work perfectly well at HF and into VHF for a non-critical application like this - plus they are relatively cheap!  These resistors have metal heat sinks, but these are isolated from the resistor elements within, having only a few 10s of pF of capacitance coupling between the internals of the resistor and the ground when bolted to the case.

The coaxial cable from the T/R switch to the input of the amplifier will need to be extended (carefully splicing the two together, minimizing the length of the ground/shield connections) to reach the resistors when mounted on the rear panel:  RG-316 PTFE coaxial cable was used for this (but RG-174 would have been fine at this power level) and the short jumper that connected from the output of the attenuator, back to the input of the amplifier.

Figure 9 shows the attenuator, mounted to the back panel of the radio with a small amount of thermal compound:  The RF power from the T/R switch enters from the left and one leg of 300 resistor R1 is connected directly to the shield of that piece of coaxial cable.  The center conductor then connects to the junction of it and R2.  On the other side of R3, the process is repeated, the shield of the coax tied to the shield as well:  This cable then connects to the input of the amplifier module.  Between R1 (on the left) and R3 (on the right) is a piece of 12 AWG (2mm) wire that connects together the shields of the "in" and "out" coaxial cable at opposite ends of the attenuator.

Figure 10:
An internal view of the amplifier, showing all mods.  The
white cables in the foreground are to/from the 3dB rear-
panel attenuator and in the upper-left can be seen the
circuitry that was added for the ALC.
Click on the image for a larger version.

Final results:

While it might seem wasteful to throw away half of the drive power, doing so protects the power amplifier from being damaged by overdriving when one inevitably forgets to reduce the output from the transmitter.  It also protects those that might be listening on the air to a badly distorted signal:  Adding the ALC circuit is, I believe, a necessary addition as this helps prevent even mild overdriving of the amplifier that is still possible under some conditions - even with the added attenuation.

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This page stolen from ka7oei.blogspot.com

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