Tuesday, January 19, 2016

Homebrew Drake TR-3 power supply

Years ago I was a teenager with a novice license and I got for a combination Christmas/Birthday present a "brand new" (to me) Drake TR-3 transceiver - but it had no power supply.

What to do?

Fortunately, I had the original manual and in it, there was the schematic of the Drake AC-3 power supply, reproduced below for convenience.

Figure 1:
A schematic diagram of the original Drake power supply.
Click on the image for a larger version.
It looked simple enough.  At first my teenage brain puzzled over the plate and low voltage (250 volt) supply portions of the diagram, noting the seemingly-odd wiring of the diodes and capacitors, but the circuit description noted that these were voltage doublers and a quick check with my ARRL Radio Amateur's Handbook explained how this circuit worked.

Where to get the parts, then?

The chassis was no problem:  Several years prior I'd picked up a Dynaco 40 audio amplifier (or similar) with a blown power transformer (and melted tubes) for just a few dollars at a thrift store.  I'd built into this case my "Novice" transmitter, a crystal-controlled (no VFO!) unit using a 12BY7A driving a 12DQ6 design, slightly modified from that which had appeared in the (then old) 1970 ARRL Radio Amateur's Handbook that I'd borrowed from my Junior High School library and with this I used a Heathkit SB-303 receiver that I'd bought from my Elmer with lawn-mowing/misc. money.  I'd used that combination, along with a few random crystals that I'd managed to scrounge/warp (another story!) on 40 and 15 meters for 6-7 months.

Now that I had the TR-3 my plan was to scrap the homebrew transmitter and use its case for the power supply, taking me completely off the air.  Both itching to use my "new" TR-3 and knowing that I'd suffer "withdrawl" when I disassembled my only working transmitter, I wanted to complete this power supply project quickly.

Lining up parts:

Having collected parts for several years by then I rummaged around and found several power transformers, some of which I'd bought for $1-$2 each at a local thrift store, apparently having been removed from tube-type TVs or audio amplifiers (Who would bother doing that?  The store just had them on a shelf with stickers on them!)  Some of these transformers had clearly come from large, console tube-type color TVs as they were quite large and had quite a few different windings - but these windings were unmarked, aside from generally inscrutable wire colors.

In looking at the voltage requirements for the Drake TR-3 I saw that I would need around 650 volts at up to (approx.) 600 milliamps, peak, for the plate supply on the final amplifier, 250 volts for the "other" circuitry within the radio and a nominal -60 volt supply, adjustable between -45 and -90 volts for the grid bias on the finals and, of course, 12-13 volts for the filament string which, for the TR-3 - designed to be a mobile radio - consisted of a series-parallel arrangement of both 6 and 12 volt tubes.

In rummaging around my transformer collection I expected, at first, that the plate supply would be difficult, but that turned out not to be so - the difficulty would be the 250 volt supply.

Not knowing how the thrift-store transformers were wired and not having faith that their color codes would follow any of the "standards" mentioned in the ARRL handbook I first took an ohmmeter and mapped out all of the wires, noting which seemed to be connected to which and, as well as I could, the resistances of these windings.  This, alone, was useful as I now had a pretty good idea as to which ones where the likely high voltage windings (highest resistance), where the center-taps likely were, which ones might be the filament windings (lowest resistance) and finally, which one might be the primary windings.

You probably noticed a lot of "likely" and "might be" statements in the above paragraph and this meant that I from resistance measurements alone, I could not be sure that I had properly identified everything.  There was only one thing to do:  Connect it to mains power.

Even as a teenager I knew enough to not connect it directly to a wall outlet, so I decided on a two-step process.  Having a 12 volt AC transformer kicking around, I used that as the proxy for 120 volts, knowing that worst case, I'd throw only a few amps into the wrong place, the current limited by this power-limited transformer - and that is exactly what I did with a large, black transformer that had probably seen previous service in a console color TV.  Some of my guesses weren't exactly right, but after just a few minutes I found a medium-resistance winding that I'd suspected to be the primary and upon applying 12 volts to it found a winding that was suspiciously 1.2 volts and another that was in the 15 volt range - plus a few other miscellaneous taps and windings.

Now I was confident that I could "light it up" from the 120 volt mains, so I rummaged around in my dad's electrical box and found a porcelain light socket and wired it in series with the (believed) primary of the transformer, pulled a 60 watt bulb from a reading lamp and plugged it in.

Sure enough, the 60 watt bulb did not light, indicating that there was no short or gross misidentification of a wire and there was a 12.6 volt winding suitable for the filament and another winding with 155 volts and shorting either of these out caused the bulb to illuminate to what appeared to be full brilliance.  Shunting the bulb, I briefly shorted the 12.6 volt winding and noted that transformer hummed noisily while the lights in my room dimmed slightly indicating, in a rough fashion, that it was capable of supplying quite a bit of current and I surmised that it would be able to supply the filament string for the TR-3.

The 250 volt supply:

I was now wondering what I could do with the 155 volt winding? - clearly intended for some "low voltage" stuff within the TV such as IF, audio and tuners but with bridge rectification, capacitive input, from the center tap the most I could hope to get would be 200-220 volts - and that was without a load.  Using the voltage doubler method was out of the question since not only would the voltage be too high, I didn't have any suitable capacitors on-hand and being a poor teenager, I didn't have any money left over at that moment to get any!  All I had were the original "can" capacitors from the Dynaco 40 (and possibly something else) and since the cans had to be grounded, they weren't suitable for use as a doubler since one of the capacitors (both of which had to be identical) had to be "floating".

What I really needed was a 200-ish volt winding on which I could use a bridge rectifier that, when filtered with a capacitor, would get me within the target range of 250 volts, under load.

Rummaging through my pile of junk I found just the thing... sort of...  What I did find was a fairly large isolation transformer with 120 volt primary/secondary capable of at least 100 watts.  In series with the other transformer I could obtain about 275 volts AC - but this would be too much, at least for a capacitor-input power supply.  As it happened, the same piece of equipment (I have no idea what it was supplied to be) that contained the isolation transformer also had several large chokes that I managed to "ring out" to around 6.2 Henries (at no current - it likely "swings" to a lower inductance when a DC current passes through them) and by placing this in series with the full-wave rectified output I would not get the 1.4x (or so) peak voltage from the series combination, but rather something lower. 

Very carefully, I constructed the "250 volt" power supply, wiring the two transformers' "high voltage" windings in "boost".  Upon turning it on I (somewhat expectedly) got a bit over 300 volts, so I did more rummaging and found, on the same piece of equipment that had yielded the isolation transformer and choke, a 20 watt, 750 ohm "slider" type of adjustable resistor and I placed that in series with the choke.  In doing a bit of quick math and knowing approximately how much current the TR-3 pulled from the 250 volt line, I figured that I could afford to burn a few watts:  I just hoped that its voltage would be adequately stable.

Comment:  Had I the correct capacitors, I could have voltage-doubled the output of the isolation transformer, but I would have still needed a pretty hefty transformer to supply the 12.6 volt filament rail, so my "transformer count" would have remained the same based on the parts on hand at the time.

The Plate supply:

The plate supply was an easy one.  Somewhere - I don't know where - I'd managed to scrounge a Triad P-3A plate transformer with a 5 volt rectifier tube winding (of no use in this project) and a high voltage secondary of 300-0-300 volts.  I wasn't quite sure of the current rating of this transformer as it just said "300-0-300" and "0.3 amps" on the nameplate so I made the assumption (probably wrong!) that I could wire it as 600 volts, use a bridge rectifier, and safely get 300 mA.  I did know that a 600 VAC source would yield 800-900 volts DC, unloaded - a bit much for the three 12JB6 tubes in the final amplifier which "want" closer to 650 volts under load

The solution?  Another choke.  That same piece of equipment that had already yielded the isolation transformer, 750 ohm adjustable resistor also had another, identical 6.2 Henry choke on-board so I threw that in series with the output of the bridge rectifier's output.

In reading about choke input power supplies I knew that while I'd get lower operating voltage, the unloaded voltage would likely be fairly close the that of a capacitor-input power supply.  From what I could gather from my ARRL handbook it appeared that the 12JB6 tubes would be able to handle 800-ish volts on the plates at idle without difficulty, but I was wondering how much it would drop under load - and how quickly?

A bonus of the choke-input was also that it offered a bit of relief to the plate transformer that I knew that I would be overtaxing:  At maximum power output, the three 12JB6 tubes in the final amplifier would be pulling around 600 milliamps at 650 volts or so (according to the Drake manual).  By using a choke input the "peak current" at the top of the AC cycle where the filter capacitor charging would occur was significantly alleviated, effectively improving the power factor from "awful" to "not so bad."  but the degree by which the effective current capacity of the transformer would be increased was a bit vague.

It also occurred to me at the time that the 1N540 diodes that I used for the supply, rated for only 250 mA at 400 volts, were somewhat "marginal" for the plate supply when 600mA was being pulled, despite the fact that this current was being split between two legs:  The choke input and its reduction of the very high, peak repetitive currents has likely helped preserve these diodes over time.

Another point was that it somewhat "decoupled" the output of the power supply from the transformer:  Brief, high-current voice peaks would be immediately sourced from the capacitors first while being "stretched out" (averaged) as current and magnetic field builds in the choke.  I figured that that this should, in theory, work for SSB where the peaks are typically brief, but less-so for CW.  Time would tell.

The grid bias supply:

The final supply voltage needed was a low-current negative supply that was adjustable from approximately -45 to -65 volts.  In staring at the schematic I didn't see any obvious place from which I could tap a stable, isolated source of 70-120 volts of AC:  In theory, I could have coupled from one of the other transformers - most likely on the 250 volt supply - but I figured that I wanted the bias supply to be reasonably stable.

Going back to the junk box I found a small transformer that was probably from an old, tube-type UHF TV converter that had a 6 volt filament winding and a 120 volt secondary.  Interested only in the high voltage secondary knew that I could probably deal with 120-130 volt DC that I'd get from the 120 volt winding, half-wave rectify and resistively drop it to the desired -90 to -45 volt range.

This was pretty straightforward:  Just a resistive divider with some capacitive filtering.  When wired up, it seemed to perform as expected.
Figure 2:
The (recently reverse-engineered) diagram of the as-built power supply, constructed in 1983.
The 1N540 are old (1966 date code) 0.25 amp, 400 volt diodes, of which I had many at the time - likely
marginal when 600 mA of plate current is being pulled!  Each leg of diodes
on the 600 volt supply could likely be replaced with a single 1N4007 - although I'd probably
use two in series.  D17 is a tiny, 200 volt glass diode of (presently) unknown designation.
Click on the image for a larger version.

About the mechanical construction:

As mentioned previously, the enclosure for this power supply had previously been used for my homebrew Novice transmitter - and before that, a Dynaco 40 audio amplifier (I think - the small PCB was marked as "Dynaco".)

On the deck of the chassis there really wasn't quite enough room for all of the components:  On the top deck, the transformers were cheek-to-jowel, abutting each other with no room for anything else, but I managed to squeeze everything in.  Underneath  I wired the rest of the components - diodes, filter capacitors, resistors - using parts that I scrounged from my somewhat limited junk box.

With the circuits seemingly operating under no load, I obtained the correct "Jones" connector for the TR-3, wired a cable harness to plug into the octal socket that happened to already be present on the amplifier chassis, plugged it in and turned it on.


Figure 3:
The underneath of the power supply.  The inside of the bottom cover of the
supply (not visible in any of the pictures) is covered with two layers of high-
quality cloth "gaffers" tape for insulation.  The two 100uF, 450 volt plate
filter capacitors are visible along the bottom edge.  When I was building
the supply I accidentally drilled a hole in one of them, apparently without
damaging the foil inside and patched it with RTV (silicone) seal since
I couldn't afford to get another capacitor at the time!
That was back in 1983 and both capacitors are still "good".
Click in the image for a larger version.
Amazingly, nothing blew up!

The filaments lit up and the voltage strings came up to their proper voltages - within reasonable tolerances, anyway, at least once the adjustable resistors and rheostats were tweaked.

Connecting the output of the TR-3 to a 150 watt light bulb (I didn't own a large dummy load at the time) I keyed up in CW, dipped-and-loaded and observed the beautiful brilliance of RF-driven tungsten while noting that the at full load, the plate voltage sagged down from 850 volts to 625-650 volt range - approximately what the Drake manual specified that it should be.

I was on the air!

A curious problem - and a fix:

I was still a novice at this point so I used it only on CW - which was slightly awkward since the TR-3 has no sidetone.  At first I would tune the transmit frequency with my Heathkit SB-303 and use that to monitor myself, but this was quite awkward since it required another, separate speaker - very inconvenient if you were typically using headphones to avoid bothering others in the house.

I then noticed something about the power supply:  It hummed loudly.  With all of those transformers crammed into a steel box, this was to be expected, but what was particularly strange was that it hummed loudest when I was receiving, becoming nearly silent when I was keyed down.

What I also noticed was that if I left the power supply on for more than an hour or hour and a half, I started to smell hot enamel - the tell-tale indication that something transformer-related was getting very warm.

For several months I would operate CW by resting my feet on the power supply, on the floor, and use the absence of hum in lieu of the sidetone - and I got pretty good at it!  After an hour or so, when the power supply got too hot, I would then shut it off for a while an allow it to cool down before turning it on again.

After a few months of this I finally I decided that I would find out what was going on.  Without the power supply connected to the radio I discovered that if I disconnected either one of two transformers that were next to each other the hum would disappear:  Could their magnetic fields be bucking each other?

On a whim I reversed the leads of one large of the transformers (the one with fewest leads, of course) and the hum that was present when receiving was gone - it only hummed when keyed down, as it should:  I may have also done some rearrangement of the transformers on the top deck to better-separate their magnetic fields, but memory fails me on this point.

When I was done I could now could leave the rig and power supply on all day and it would barely heat up:  It took only a session or two for me to get used to the the "non-inverted" power supply hum CW sidetone!

Figure 4:
The top deck of the power supply showing the four AC
transformers, two chokes and two capacitors.  This picture
was taken after vacuuming out an inch or so of dust.
Starting from the bottom-left and working clockwise:  The plate
transformer, two filter capacitors for the 250 volt supply, the input
choke for the 250 volt supply, the filament/175 volt transformer,
the isolation transformer, the input choke for the plate supply, and
the transformer for the bias supply (the small transformer, wedged in.)
Click in the image for a larger version.
The next year I upgraded from Novice to Advanced class and used the Drake on SSB where it and the power supply worked very well.  Later, I even managed save enough money to buy a brand new, replacement AC-4 power transformer - the same as the one in the original Drake AC-3 power supply - but I never did get around to rebuilding that old power supply with its plethora of magnetics:  I still have the new-in-a-box, genuine Drake AC-4 power transformer somewhere!

Final comments:

This power supply still exist as evidenced by the fact that the pictures on this web page were taken very recently.  Both this power supply - and the Drake TR-3, now well over a half-century old - still work fine despite my having owned it decades (and some minor repair, replacement of a dried-out capacitors in the power supply) despite the fact that I've obviously abused the plate transformer.

As can be seen, some (many?) aspects of the construction techniques used are a bit "iffy" and were I to build it today - after the benefit of decades of experience and the availability of other materials I would certainly do it differently.

Is it dangerous?

Figure 5:
The power supply, assembled, showing the side with the grid bias
adjust control (far left) and the pilot light.
Click on the image for a larger version.
Not very - at least with the covers in place.  Some of the "flakiness" of the construction methods involved (e.g. excessive use of  unsecured "flying leads", components that could be better-secured, using some bits of electrical tape to insulate wire instead of heat-shrink tubing in a few places - heat shrink tubing was a bit rarer when this was built) would contribute more to reliability than issues with safety and I would most certainly not build it (exactly) this way again!

Despite the fact that this power supply been bounced around the country several times during moves, subject to very high humidity for several years (hence some of the rust!) and being of somewhat "interesting" design, it continues to function reliably.

If it does ever blow up I do have a spare AC-4 power transformer kicking around.

But then again, there's EvilBay...


This page stolen from "ka7oei.blogspot.com".