[MRCA] PRC-47 filament voltage?

[scottjohnson1]

The PRC47 has several tubes that are instant heating in the transmitter, they are on only in transmit to save battery power.  The Amperex tube data book online will give you nominal voltages for the filaments on these tubes.  They are somewhat prone to filament failures, as are most battery tubes.


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-------- Original message --------From: James Green <jagreen3 at sbcglobal.net> Date: 4/2/18  13:33  (GMT-07:00) To: mrca at mailman.qth.net Subject: [MRCA] PRC-47 filament voltage? 
Does anyone out there know the answer to this:

My PRC-47 still does not transmit. I traced the filament voltage all the way from the Driver tube back to the power supply on the schematics & it goes directly to T1 on the power supply module. T1 gets it’s primary voltage in the transmit mode only. It seems strange to me that this radio leaves the tube filaments cold until transmit is triggered. It seems like this would lead to a rather long wait before sending. Is this the actual case or am I missing something?

Next, The peak filament voltage tap used by the driver tube has no test point on the power supply. This lead goes to Relay K2 the AC power continuous relay in it’s normal position K2’s pin 2 from the Peak filament voltage at T1 goes to K2 Pin 8. From there it goes to T3. T3 has a primary made up of peak filament to filament common from T1. The secondary of T3 provides filament voltage to the drive tube in the data translator module. I should be seeing 6.3 VAC across the transformer primary in transmit mode. I do not get it. Back at the power supply I get 5-6VAC in transmit mode. However it doesn’t make it to T3. I suspect a faulty K2 relay.
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