[MRCA] April 23 PRC-47 no transmit update

[Ray Fantini]

Just looked at the manual to remind me what I don't know and have to ask, did you check J2 on the Power supply (A8A5)? That's the bias supply to the PA tube. If you do not have negative bias to push the PA tube into cut off it will blow the fuse due to excessive high plate current. But with the tube out of the circuit you will get just what you have in terms of high plate voltage. On a SSB tube transceiver the bias is critical being when you are not talking the tube needs to be shut down as to not draw too much current and when you talk the RF drive will be large enough to push the tube into operation. You have a Bias pot to set bias for proper idle plate current (-110) but if you remove all bias the tube will just suck up excessive current.
You can also see the negative bias voltage on the amplifier at J105 but that's on the -ALC side.
K1 on the power supply turns on the HV inverter for transmitting, that's working because you have HV. That same supply also develops the negative bias voltage that's adjusted thru the "PA Bias Pot" R3 along with the Driver Bias pot R4 that can be seen at J1 on the power supply. Remember that you will only see these voltages when the transmitter is keyed.

The key thing is that you say that when you put the radio in transmit it will blow the fuse, and with the PA tube removed it won't. 

Though at first you may have an issue with lack of drive but when the bias system is working and adjusted correctly the driver and PA are cut off as to not cause any issue. If you have bias voltage, high voltage and drive you are getting close to it being a bad tube. The screen voltage is more than likely ok being if it were not there once again the tube would be cut off and you would get low or no output along with low or no plate current.
PA tubes sometimes short plate to screen or screen to grid and that will result in weird voltages like real high screen voltage, beyond the applied 650 volts or high or positive grid voltages keeping in mind that should always be a negative voltage when not being driven. Too bad they don't give you any way of checking screen on that radio short of looking at the voltage on L103

Ray F/KA3EKH

-----Original Message-----
From: mrca-bounces at mailman.qth.net [mailto:mrca-bounces at mailman.qth.net] On Behalf Of James Green
Sent: Monday, April 23, 2018 3:10 PM
To: mrca at mailman.qth.net
Subject: [MRCA] April 23 PRC-47 no transmit update

I measured the PL-177 plate tube B+ at J101 with the tube installed & a 5 amp slow blow fuse in the 20 amp fuse holder.  The fuse blew. I next put in a 10 amp slow blow fuse. This did not blow. I measured B+ Low at 810 VDC and Hi at 1,810 VDC Both are higher than expected. Lo should measure 650 VDC and Hi should measure 1,500 VDC.

I was just wondering if I had a shorted PL-177 tube. How does one determine this?
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