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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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20th Oct 2019, 11:02 pm | #1 |
Heptode
Join Date: May 2014
Location: Salisbury, Wiltshire, UK.
Posts: 998
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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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David, G4YVM. Last edited by G4YVM David; 20th Oct 2019 at 11:07 pm. |
21st Oct 2019, 8:41 am | #2 |
Heptode
Join Date: Jan 2008
Location: Birchington Kent, UK.
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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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Regards Pete |
21st Oct 2019, 6:02 pm | #3 |
Rest in Peace
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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.
Al. |
21st Oct 2019, 6:10 pm | #4 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
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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 . |
21st Oct 2019, 7:46 pm | #5 |
Heptode
Join Date: May 2014
Location: Salisbury, Wiltshire, UK.
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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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21st Oct 2019, 7:54 pm | #6 |
Dekatron
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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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David. BVWS Member. G-QRP Club member 1339. |
21st Oct 2019, 9:20 pm | #7 |
Heptode
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Re: FR50B query
Thanks David
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David, G4YVM. |
22nd Oct 2019, 12:29 am | #8 |
Heptode
Join Date: May 2015
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Re: FR50B query
Now following this with interest. Note G1 and G3 were stapped together on the audio out pentode.
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worried about the electrons entering the circuit and the smoke leaving Andrew |
22nd Oct 2019, 12:34 am | #9 |
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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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22nd Oct 2019, 7:23 am | #10 |
Heptode
Join Date: May 2014
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Re: FR50B query
Thanks.
Today's job then
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David, G4YVM. |