[MRCA] PRC-47 no transmit

[James Green]

Well, I spent some time on the PRC-47 today. I checked out the interweb and found a web page pdf that explained how to de-activate VOX:

https://military.trcvr.ru/wp-content/uploads/2015/09/AN-PRC-47_mod_1983.pdf

The process involves removing the cover from the Audio Freq Amp and adjusting R-40 CCW until it clicks. Then if it still does not transmit using the PTT adding a jumper from Pin 11 of P1 that goes to Pin 3 of the VOX relay. Mine did not transmit so I checked the circuit and the jumper is in place through a diode. This is not a modification it is in the original circuit. I suspect mine is a later model that was designed to be used either VOX or PTT.

I checked the voltage at the switching transistors. They are still working and are OEM Germanium transistors in all three radios.
I checked my voltages at the power supply J1 through J9 and all voltages are pretty close except J1 and J2 read high and negative rather than positive as shown on the schematic. I haven't measured this on the working radio yet. I will update you all when I do.

The schematic has lugs 1,2 & 3 of the primary power switch grounded. All 3 radios are wired with only lug 3 grounded. It looks as if this is a modification. This must not be the problem, because one does work.

One of the shorts tests I did earlier showed a dramatically high reading for one of the measurements on the Power supply module. The manual recommends checking TB1 the voltage regulator circuit. All three radios have the same high reading, so it must not be critical to transmitting. I did find 2 bad transistors and 1 bad Zener diode in TB1. I replaced them in my radio with equivalent parts I had on hand and it had no affect on the high reading. I ordered the original parts off ebay and when they all get there I plan to give another go and see if that fixes it.

I have also looked at some of the relays. K1 and the T/R relay do engage, but K4 and K7 aren't getting coil voltage. I’m not sure if this is normal or not. I haven't checked the working radio yet.

I am somewhat hampered by the lack of 2 power cables, 2 dummy-loads and 2 SO-239 adaptors to operate both the working and non-working radio at the same time.