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Old 20th Oct 2019, 11:02 pm   #1
G4YVM David
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Default FR50B query

Repairing an FR50B rx.

I found that the 6BM8 audio amp valve had been incorrectly replaced with a 12by7 giving hum and a very hot audio transformer but nowt else. I have replaced the 12BY with the correct 6BM8 and the receiver sort of works...

By sort of I mean that if I send a CW signal from my own transmitter up my own aerial and touch a receiver aerial wire to the 50B I can hear it in the phones. The S meter works, the select pot works, the BFO works...but the tuning VFO knob has no effect - the received audio is present everywhere and it sounds like crunchy white noise rather than a cw note.

I'm not transmitting masses of power - about 5 watts and it's only present when the receiver aerial is fitted...so its getting into the old Rx correctly.


Am I right to suspect the LO might be at fault? If Im receiving my signal, the detector is working and first IF I am guessing from the workshop notes. This leaves the LO?

David
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Old 21st Oct 2019, 8:41 am   #2
g4aaw pete
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Default Re: FR50B query

Hello David

Two local oscillators to deal with on these receivers. Think you'll have to poke around with a counter.

Some of these had a fixed channel option. Mine just had a dummy knob fitted instead of a switch to select the channels.
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Old 21st Oct 2019, 6:02 pm   #3
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Default Re: FR50B query

Sounds like RF overload of the receiver. First, I would check that the receiver is working properly with a reliable, calibrated sig. gen.

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Old 21st Oct 2019, 6:10 pm   #4
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Default Re: FR50B query

I'm not familiar with the frequency-conversion schema of the FR50B, but my first step would be to check that the LO is actually oscillating.

Do you have another receiver that covers the likely LO frequencies? My suspicion is that you've got an inoperative oscillator/mixer and that your local transmitter signal is just 'brute-forcing' its way through .
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Old 21st Oct 2019, 7:46 pm   #5
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Default Re: FR50B query

Thanks guys.

I also think the oscillator isn't working. There's no signal out of the VFO socket at the rear that I can detect anyway.

The VFO valve is, if I read this right, V3 but where I should get 38v I actually see 150. Summfink is badly awry.

D
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Old 21st Oct 2019, 7:54 pm   #6
David G4EBT
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Default Re: FR50B query

The manual is here if you don't already have it:

http://www.foxtango.org/ft-library/F...-70s/FR50B.pdf

November 2017 PW Magazine had an article 'Rejuvenating the Yaesu FR-50B - breathing new life into a classic receiver':

https://pocketmags.com/practical-wir.../november-2017

Hope that might be on interest.
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Old 21st Oct 2019, 9:20 pm   #7
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Default Re: FR50B query

Thanks David
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Old 22nd Oct 2019, 12:29 am   #8
Andrew B
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Default Re: FR50B query

Now following this with interest. Note G1 and G3 were stapped together on the audio out pentode.
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Old 22nd Oct 2019, 12:34 am   #9
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Default Re: FR50B query

The VFOs are transistors, the BFO is a valve.

First port of call, check their power supply.

David
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Old 22nd Oct 2019, 7:23 am   #10
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Default Re: FR50B query

Thanks.

Today's job then
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