[MRCA] April 20th PRC-47 no transmit update

[James Green]

I replaced the 20 amp fuse with a 5 amp fuse & removed the PL-177 tube and turned the power.

I started going through the localizing trouble procedures.

First thing  (Item 1) I found was the meter did not work on the battery test. I traced the problem to a wire from the meter to the Transmit power switch that I forgot to connect. It was hiding from me and I missed it. Easy fix once I found the wire.

Everything went OK until Item 6 no or low audio output. I switched out AM-3506 modules and found one that worked. It was the one I had marked as having one bad transistor. This transistor can affect transmit I will deal with it later when it comes up in the trouble shooting procedure.

Item 7. I’m not sure how to test the plate voltage at the 1st & 2nd amplifiers in the data translator module. If someone can enlighten me it would be very helpful.

Item 8 A5J8 to ground should read a regulated 20VDC. Mine reads 25.4VDC with 27VDC at A5J7 to ground. I pulled the power supply module and removed the cover. I removed the fasteners to the 2 PC boards on the surface. This exposed TB1 the voltage regulator. I saw that a previous technician had been in there. One of the white with red stripe wires was broken off it’s contact point & a solder splatter was shorting the emitter of Q3 to ground. I cleaned this up and removed the bases of the 3 transistors and tested them. They all test good and are OEM parts. I then removed one end of the Zener diode and tested it. It clips at exactly 10VDC as it is supposed to. I put it all back together and there was no change. On the schematic it looks as though R-22 may be an adjustment for TB1. I twiddled with it and it made no difference. I’ll have to think about this for a while. Anyway I doubt this would prevent the radio from transmitting. As I stated earlier all 3 power supplies behave the same and at least at one point one of the radios would transmit

While looking at the schematic I noticed that R3 adjusts the PA bias & R4 adjusts the driver bias. Both of these were abnormally high (negative). I made the adjustments to match the proper voltages (PA bias = -110VDC Driver Bias = -32VDC)

Next on the list are measuring the grid and plate voltages for V1, V2 & V3. V3 is the driver and that is the only one I can actually see without completely disassembling the Data Translator module. Also I don’t know how to directly measure these with or without expender cables (I don’t have them, but plan to make a set from the parts radio)