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Old 4th February 2012, 07:42 PM   #1
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Default Fisher 500c radio hum

I have a fisher 500 c that works fine with the aux input and my record player ,but when I listen to the fm radio specifically fm automatic... It will only play in mono .. When I put on fm stereo it plays in stereo but hums...in fact when I first switch it on and the fm auto is on the stereo beacon comes on along with a hum ..after 15 seconds the hum goes away ,but again only plays in mono... When I switch to fm stereo the hum stays on... Any ideas what might be wrong?
Thanks
John denicola
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Old 4th February 2012, 08:29 PM   #2
DF96 is offline DF96  England
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Fault in the stereo decoder?
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Old 5th February 2012, 12:13 AM   #3
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If that "hum" is not the same pitch as a normal AC hum, it could be that the 38 KHz oscillator in the stereo decoder is off frequency, not able to lock onto the 19 KHz pilot signal that's part of a stereo broadcast. If that is the case, you also won't get any stereo separation. Adjusting the ferrite slugs in the coils in the decoder most likely requires a plastic tool with a hex tip, like an Allen wrench (some have slots,, still need plastic tools however). Do NOT use an allen wrench. It can crack the cores very easily, and a metal tool would detune the coil making an adjustment invalid. If the frequency is off, starting to insert a ferrous nail, allen tool, maybe even the end of a paper clip into coil Z101 will change the pitch of the hum. If so, a proper tool should be found to tune the hum down in pitch until you get a stereo lock, then set it for maximum audio from a speaker connected BETWEEN the left and right outputs controls flat and balance centered (that's the stereo difference signal, will sound strange, lead vocals cancelled out etc). Z100 probably is okay left alone if the stereo light comes on. It's likely normally adjusted for maximum 19 kHz signal seen on a scope at grid pin 7 of V100. It is likely distorted to help generate a 38 KHz harmonic for the oscillator (next stage) to lock to. I think peaking for maximum negative voltage at test point 104, the anode of diode CR100 would do the same thing. If you adjust that, go back and repeak the other again for volume. Coil L100 should be left alone. (It not clear where that one is hiding on the decoder) Its a 67KHz filter to keep a subcarrier (the old Musak background music to have teeth pulled by service etc) from making extra noises in stereo reception. It takes a scope and signal generator to adjust that one. All of that is on the stereo decoder module. It also has a small 1 uF electrolytic that's likely bad, as are the small electrolytics in the audio section of the main chassis. At some point all should be changed out. Observe polarity markings. If the hum didn't change pitch in that early step, I'd be checking for power supply hum, but that seems unlikely. The unit also has some caps in what looks like a switched-diode muting circuit. Perhaps the control voltage there could leak hum if caps are bad. Coil Z100 is by tube V100, at the end of the decoder most distant from the big audio transformers. Coil Z101 is near the middle of the decoder, between V101 and V102. I doubt that any of the three 12AX7s are bad, but you can try switching them with any of the three along the front of the chassis to see if that makes any difference. Try only one at a time. The other three are the phono and preamp filter section tubes. The tuning adjustments above end up controlling the level and phase of a 38 KHz signal. That signal drives two 4-diode balanced mixers. You can think of it as receiving double sideband, or imagine it as taking an incoming signal that is switching between left and right 38,000 times a second, and decoding by flipping a (diode) switch at the same speed and phase to switch to the left and to the right in step with the switching in the broadcast. The part of the received audio above the hearing range has basically a tone, 19 KHz, that locks the decoding and is detected to light the light. Higher in the spectrum the difference between left and right has become 38 KHz double sideband. Sorta like AM radio, but the carrier is missing. The 38 KHz oscillator provides the needed carrier. By adding or subtracting the recover left-minus right from the main mono (left plus right), separate left and right are produced. There's a pot to trim levels for best separation, but it is better to leave that alone with no proper test signals. Good luck

Last edited by riccoryder; 5th February 2012 at 12:18 AM. Reason: small but important typo
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Old 6th February 2012, 08:18 PM   #4
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I had a 500-C with a similar issue and just poor FM stereo performance. I might recommend that you get the Service Manual for it (i got mine at HiFi Engine | Download Free User/ Service Manuals, Amplifier, Receiver, CD, Tape, Tuner, Video ) and follow the alignment procedure exactly. I was able to do it with just a signal generator and a scope. I worked great after that. No parts replacement ended up being necessary (however later i did upgrade all caps to very good effect.)

Good luck.
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Old 9th February 2012, 05:33 PM   #5
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Switched out the stereo beacon tube. 6gk5... Now the stereo tube beacon stays on when switched to fm automatic and the hum continues ( and plays stereo) ...whereas with the old tube in there the stereo beacon shuts off after a minute and switched to the non humming fm mono .. Does that mean anything?
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Old 9th February 2012, 07:11 PM   #6
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All that 6gk5 does is drive a relay that turns on the indicator light and shorts the stereo into mono. The hum is probably a combo of old electrolytic filtering caps and misalignment of the FM circuitry.

On mine, the 6gk5 was physically broken so i replaced it outright. Then i replaced all electrolytics. Then i realigned the FM per the service manual. Works great.
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