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Fisher 500B MPX Function

VinAudioRev

VinAudioRev

Member
I jut finished recapping my 500B.
I was testing out the Tuner before deciding on an alignment. I am amazed at the amount of stations I am receiving.

I switched to FM MPX and tuned to a nearby station. When the station locked on, The "Stereo Beam" Closed quickly and I definitely could hear the "Stereo" Effect. After about 5 seconds, the "Stereo Beam" Opened and I lost Stereo.

The Tuner still pulls in tons of stations and the Beam works while tuning, but as soon as I switch to MPX, the beam opens and I hear no difference in the sound.

Any Idea where to start?

I Appreciate the help
 
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Have you replaced the electrolytic cap in the mpx unit? When it goes open it usually makes the decoder quit.

No beam indicator means no 19kc Pilot tone, and i seem to recall that cap is on the Pilot amplifier section of the first tube
 
gadget73 said:
Have you replaced the electrolytic cap in the mpx unit? When it goes open it usually makes the decoder quit.
Click to expand...
I did. And actually. I just put it back on the bench and when I critically listen It seems that the decoder is working, when I switch to MPX I can definitely hear the switch to “stereo” on a strong station. But the Beam (which is closed on standard FM) opens when I switch to MPX.
 
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see if you can measure a variable voltage on the connection marked 'tuning indicator'. When you get a good strong stereo station the voltage on your meter ought to change to close the eye. If you're getting voltage change there but not actually on the eye it might be a crusty contact in the selector switch not getting the signal where it needs to go.
 
Thats what I am looking at now. It seems that when in MPX The function switch disconnects the indicator from the FM circuit and connects it to the MPX circuit.

When I check the voltage while tuning in MPX mode, I get nothing at all at the "Tuning Indicator" Junction, but I do get varying voltage Just after the Diode, Both the R206 and R 207 are good. Removing the caps (C212, C213) to test now.
1726679807270.png
 
that would do it. Its a very low current circuit so it wouldn't take a whole lot of leakage to kill it. What type of cap was it?

one of the annoying things about most cap testers is they won't tell you the leakage at rated voltage. Sometimes thats really the critical one.
 
gadget73 said:
that would do it. Its a very low current circuit so it wouldn't take a whole lot of leakage to kill it. What type of cap was it?
Click to expand...
It was a little Ceramic Disk. When I removed it and tested it out of circuit. My tester thought it was a 1K resistor.

I replaced it with a .047 Film cap.
 
gadget73 said:
Interesting. Ceramics tend to be pretty reliable, not super common to have one fail with that much leakage.
Click to expand...
Of course. When I got it. The 12AX7 and one of the 12AT7’s in the MPX adapter were switched. The beam is run off the 12AX7, so being on an AT7 shouldn’t have hurt it. But who knows.
 
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