My Fisher 69T has FM Mono Only, No Stereo

[nogden]

Hi all, I have a Fisher F-591 console system (69T chassis) that I've been using daily for about a year. Appears to be stock (even the electrolytics). A few days ago, I noticed the audio (at least on FM mode) was noticeably distorted. I also noticed that the stereo indicator wasn't lit (and it definitely wasn't receiving stereo sound).

Since the 'lytics were original, they were my first suspect. Sure enough, there was excessive AC ripple on a few of the DC rails. New caps took care of that problem. Audio sounds fine now.

However, I still don't have FM stereo (mono is fine). Checked the basics; incoming signal strength is adequate, station is broadcasting in stereo, mono switch is not engaged, stereo indicator lamp is good. If I generate an RF test signal with a strong enough 19KHz stereo pilot, I can get the stereo multiplex decoder to become active. Additionally, if I add in my own 19KHz audio signal to a strong broadcast signal, I can get the stereo decoder to activate. Its just that broadcast signals on their own aren't strong enough.

I checked the alignment and it is good. (Why would alignment suddenly change, anyway? Stereo worked fine a few days ago). I also adjusted the AGC trigger control with no improvement.

I traced the 19KHz signal through the decoder circuitry right up to (what I assume to be) the frequency doubler (Z403 on the schematic). I'm not getting anything out of Z403. However, the 19KHz signal at the collector of Q402 (input of Z403) is only about 175mV. Considering the forward voltage drop of the diodes in Z403, I don't think the 175mV signal is sufficiently strong. This is all when tuned to a broadcast station. Again, I can generate a signal that's strong enough, but for some reason, typical broadcast signal stereo pilots just aren't quite strong enough by the time they get to Z403.

For testing, I also tried amplifying the baseband audio signal (from the ratio detector) going into the decoder. Using an external amplifier, I can amplify it enough to get the decoder to activate, but then the audio is noticeably distorted.

Where in the circuit might the higher audio frequencies be rolled off? It seems like the 19KHz pilot (and presumably higher frequencies) are being rolled off or attenuated somewhere in the circuit, presumably before the stereo decoder. The schematic says the 19KHz signal at the input of the decoder should be 30mV. If I generate a signal of that amplitude, the decoder works, so it seems like the issue is in the IF amplifier (which includes the radio detector).

I'd appreciate any suggestions or insight! Sorry for the long post but I wanted to be thorough in my description.

 

[chazix]

I'd appreciate any suggestions or insight!

I got nothin' myself, or pretty near. But did you know there is a Fisher-specific sub-forum? Might be worth moving this thread there:

http://audiokarma.org/forums/index.php?forums/fisher.172/

The only thing I can think of that might be worth checking is detector health. I take it you have an RF generator, so perhaps inject a signal and see whether the audio output from the detector exhibits DC voltage that swings from + to - as the injected frequency is adjusted to either side of center?

Good luck,

chazix

 

[Watthour]

Is that a hybrid tube / SS unit? It may have germanium devices in the front end, making any replacement more challenging.

Is there any apparent amplification at Q402?

 

[nogden]

I did not know there was a Fisher sub-forum! How'd I miss that? If I could move this thread, I would!

chazix, thank you for the suggestion. I performed that test and had good + and - swing.

Watthour, yes, hybrid. Nuvistors in the FM front end; transistors everywhere else. And yes, Q402 has gain. With my current input signal, the 19KHz signal is about 60mV at the base and 200mV at the collector of Q402.

Additionally, I confirmed that a 19KHz test signal as small as 20mV at the input to the stereo decoder will activate the decoder and the manual states that the 19KHz signal should be 30mV at that point. Since I'm getting less than 20mV at 19KHz at that point when tuned to a broadcast station, it seems like the issue is not with the stereo decoder but with the IF or detector stage.

 

[Watthour]

I would agree with that theory. The IF stages might need a little investigation. Low output at any of the IF stages could be an issue.

I've had a few trips around the block with Ge transistors in older amps. If you can find replacement germaniums, you would be very fortunate. Otherwise, refitting something like a BC212A with a replacement resistor array to bias up the junction properly may be an alternative. I had to do that in an older Sansui in the AM section. Fun - NOT.