[MRCA] PRC-47 does not transmit

[James Green]

I am working on a PRC-47 project & would like to share my experiences.

I actually have 3 each PRC-47 transceivers. One works OK on transmit and receive, but has a modified face plate. One is more of a parts radio. It does not transmit, and the receiver is weak. It has been modified and the removable cover with the tuning chart is missing. The 3rd one is un-molested. It has a good receiver in it, but will not transmit. The third one is the one I want to keep and use.

Using the working PRC-47 I have tested all 3 PL-177WA final transmitting power tubes and they all produce full power.

I have tested the switching transistors and they test good.

Using the service manual I have gone through the shorts testing & have made repairs and adjustments to het the expected results. However, there are a couple anomalies that are common to all three PRC-47 transceivers. Either the service manual is in error, I have the wrong test settings or there is a undetermined failure that is common to all three. The manual does not give much information on initial settings for the shorts test.

I am only working on the working one and the one I want to keep and use. I will attempt to get the parts radio working once I am satisfied with the complete ones.

Here are the anomalous measurements.

A5J2 to ground should be 50K. The working one measures 50K to ground with the power out switch at Low Power. It measures 36K at Off and High Power. Mine measures 50K at low power and 18K at Off and High Power. The service manual does not say which position the switch should be in. The service manual says a low reading may indicate a bad switch.

A5J8 to ground should measure 30-Ohms. All three PRC-47 transceivers measure around 260-Ohms. The service manual says a high reading could indicate a bad Voltage Regulator TB1. I removed TB1 from one of the power supplies and found 2 bad transistors and a bad 9V zener diode. I replace the transistors and the zener diode with equivalent parts. I put TB1 back in the circuit and it still measures 260-Ohms. I have ordered OEM parts off ebay and when they all arrive I will try again.

I will keep updates coming if this is approved.

73
Jim