[MRCA] April 20th Update #2 PRC-47 no transmit

[Ray Fantini]

Real important safety tip, Never attempt to check the HV on the plate of a high power transmitter on the PA tube direct!
To check HV you always work on the decoupled side of the PA tank. On the PRC-47 J101 is there for your safety to provide a sample of the HV applied to the PA, if you suspect that L106 is open (that happens from poor contacts) with the power off do a DC resistance test from L104 on the tube and pin 8 on S103A for HI power and pin 8 for low power.
The PA deck is a dangerous place, one hand rule and all that sort of stuff and if in doubt find someone with experience to help.

Asked this before but got no answer, if in the CW mode when you key the transmitter do you hear it in another receiver?

Ray F/KA3EKH


From: mrca-bounces at mailman.qth.net [mailto:mrca-bounces at mailman.qth.net] On Behalf Of Tim
Sent: Friday, April 20, 2018 4:26 PM
To: James Green <jagreen3 at sbcglobal.net>
Cc: MRCA <mrca at mailman.qth.net>
Subject: Re: [MRCA] April 20th Update #2 PRC-47 no transmit

Hi Jim - Not sure I am following all this but a couple of thoughts:
Running the 47 in low power mode, including tuning, should not blow the 5 amp fuse.  So it's good to keep it at 5 amps while you are doing all this.  I have seen the HV PS oscillator transistors short under fault conditions and the 20 amp panel fuse was still good.  Beware...
Also, there is a built-in HV divider circuit in the PA compartment for measuring the HV B+.  It is J101 on the PA compartment shield. I recall it is a 10:1 divider.  The specified voltages for the PA are under load so testing them without the tube installed could be confusing things, and probably dangerous.  Especially to your meter...
FYI,,Tim
N6CC

On Fri, Apr 20, 2018 at 12:13 PM, James Green <jagreen3 at sbcglobal.net<mailto:jagreen3 at sbcglobal.net>> wrote:
Seems I can measure grid voltage at V1, V2 & V3 at A3J1, A3J2 & A3J3 in transmit mode. I get expected results. I still haven't found a good way to get at the plate voltages.

Items 12 thru 15 are ok

Item 15 is measuring the B+ at the power output tube. I measured this tube out of circuit by clipping my Fluke 77 positive lead to the cap lead of the PL-177 tube. The Negative lead to ground. I should get 650VDC Low power and 1,500VDC hi power. I get around 840VDC low power. When I switched to hi power I got a snap! like I did before with the PL-177 in circuit that led me to replace the Xmit power switch. I examined the switch very carefully and saw no problems. Also, the 5amp fuse in the 20 amp holder did not blow. I proceeded with the high power B+ test and discovered my fluke will not measure over 1,000 VDC so it registered OL when I measured Hi power. I will be building a test circuit with 2 equal resistors in series. Measuring the voltage drop across one of them should be 1/2 the total voltage. I want to be safe about this so I am going to think it over very carefully before I proceed.

I expect the high reading on low power will come down with the tube in circuit and a proper 50 load.
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