Drake 4-Line PTO

While refurbishing my Drake Twins, an early serial number R-4B and T-4XB pair, I ran into difficulty with the R-4B PTO. Mechanically it was fine after some cleaning, but suffered from a thermal intermittent. From a cold turn-on, the PTO frequency was highly variable with line voltage. I had the R-4B on a Variac and I could change the frequency a few KHz by varying the line voltage between 110 V and 125 V, which is unacceptable stability. This meant as AC loads turned on and off in my house the frequency would bobble around in a erratic fashion.

After 10 minutes or so the frequency would suddenly stabilize and be relatively immune to line voltage as it should be, with only a few Hz shift induced by varying the AC input from 110 to 125 V.

I measured the voltage at the PTO B+ input and it would initially drop to 9 V which meant the Zener was out of regulation. The voltage at this point would suddenly increase to 12 V when it warmed up, putting the Zener back in regulation. This said to me something internally was drawing excessive current until it came to operating temperature. I installed a new Zener for D9, but that did not fix the problem.

Along the way I found some excellent Drake info from many sources, including the helpful package of files put together by Garey Barrell / K4OAH (SK). Unfortunately I had been unable to find PCB layouts of the PTO, and as helpful as the Drake PTO schematic was, it is unintuitive from a signal-flow perspective and difficult to follow, so I drafted one more to my way of thinking:

Once I had removed the PTO and began to examine the PCB I realized I needed a PCB layout as well, and several hours of searching the internet did not turn one up. In the layouts below the PTO coil T13’s four connections are labeled A, B, C and D and correlate with the schematic’s references:

Component Side:

Foil Side:

I hope the time spent drafting these layouts will assist some other soul attempting a PTO repair!

I replaced the four 0.01 uF 25 V disc bypass caps (C158, 199, 200, 201) with new 50 V Vishay ceramic caps, and also replaced the 47 pF inter-stage coupling cap C142 with a 1KV C0G Vishay ceramic cap. The higher voltage caps show less modulation of capacitance with voltage. I reassembled the PTO and after several cold to hot cycles, it seems the problem has been fixed. On to the T-4XB PTO!