SX-828 FM only works in mono

[tutor_turtle]

During a re-cap I mis-wired the ground of the capacitor that supplies 7 volts to the signal meter.
After fixing the mis-wire, FM only works clear in mono, in stereo (auto) mode, it's distorted, both channels.
I've been spinning my wheels for weeks on this one, but I am not well-versed in trouble shooting tuners.

My understanding of how de-multiplexing is supposed to work has me mystified.

It didn't help any that my old Tektronix scope gave up the ghost in the process.
I bought a used Siglent, that failed within a week.
I broke down and bought a brandy new Rigol DS2202A. Nice 'scope, so far.

Any help would be much appreciated.

 

[dr*audio]

Lets have the reference designator of the cap you mis-wired. How did you even do that? Did you put it in backwards?

 

[tutor_turtle]

Capacitor C9 sits on a terminal strip. Looking top to bottom, tab 3 is ground, tab 2 is where L-3, R8 & R7 connect to AGC 2 (FM front end).
I connected the ground of C9 to tab 2, instead of tab 3, off by 1.

 

[dr*audio]

AGC1 goes right to the gate of Q1 in the front end of the tuner. I think you blew the FET. If you tune a known strong station, does the stereo lamp light? You don't have a signal strength meter, only a tuning meter so that's the quickest way to tell if you have a strong signal.

 

[tutor_turtle]

AGC1 goes right to the gate of Q1 in the front end of the tuner. I think you blew the FET. If you tune a known strong station, does the stereo lamp light? You don't have a signal strength meter, only a tuning meter so that's the quickest way to tell if you have a strong signal.

Yes when I receive a strong station I get full signal strength meter deflection and a stereo light. Tuning meter also performs as normal.
I have changed the entire front end assembly with a replacement from eBay after repairing the mis-wire.

To compound my troubles, I bought a working tuner board and swapped it out before finding my mis-wire (Duh)
So whatever damage I did to the original board, also damaged the replacement.

The fact that it sounds perfect in mono tells me the issue must be in the multiplex portion of the circuit of the tuner board, not the front end.

I have also replaced the MPX Filter Unit AWX-009.
The audio signal I'm seeing at the MPX filter is positive DC biased, the signal is super saturated at the upper limit, ~ 5 volts.
It is a clipped half wave signal, both channels, where the negative half wave emerges from the 5 volt ceiling.
Again, I don't know what I am supposed to be seeing at this juncture.

Thank you for your patience. This is part of a complete restoration. This issue is the final problem to make this a success.

 

[dr*audio]

I reviewed the schematic and installing the negative side of that cap in that position could not possibly cause the problem you are seeing. It's something else you did, probably in the MPX section. Do you have a Stereo FM alignment generator? If not, go with a known strong station. Start at point C. do you have clipping there? Should be clean if everything before the MPX is ok.

 

[dr*audio]

 

[tutor_turtle]

View attachment 3775683

Thank you for all your help. This was a very involved restoration.
One mistake I made was not to test the radio before starting. I tested everything else...
This has set in an attic for well over 20 years.
It could very well be it was already broken and that why it was shelved.
The customer said it was his uncle's who got it in Vietnam at the PX.
Uncle was a heavy smoker.... very heavy, like the front panel was a uniform amber hue, heavy.
The rest of the repairs came out outstanding. It cleaned right up.

Going at it again today,
Yesterday was a skip day from work.
After an especially brutal winter and cold wet spring here in the north east, I savor every sunny day I can.
I'm not getting any younger.

I'll send you pictures when it is all running and reassembled.