Pioneer SX-680 Receiver FM Issues

[lazio200]

To all vintage enthusiast & FM experts, I had this unit on my bench for a while now.
When I feed the unit (via Variac) 60 VAC the FM band is perfect, low noise, sensitive station tuning even without an antenna.
As soon as I plug it directly to 120 V there is distortion & noise all across the FM.
AM & all other inputs are working fine.
The 76 Khz signal on PA1001A demodulator is present until the a certain threshold then it disappears.
The majority of the elect. capacitors have been replaced, perhaps it is a power supply issue or one or more components that are failing beyond a certain voltage.

[FIXITNOW2003]

there has been some chatter /mentions of integral toko caps going bad. does this unit use Toko coils ? not seen this model but the SX590 has them and know for the caps breaking down may need to remove coils and test caps at working voltage (12v?)

[lazio200]

Yes this unit uses Toko IF transformer, by caps do you mean the internal dog-bone small capacitors inside the IF transformer ?

[andy3055]

Those Toko coils will rarely if ever go bad. Don't mess with them unless you can realign them!
Look for interference sources outside the unit. Also any possible screening in the tuner that may have failed.

[lazio200]

The unit is perfectly aligned and tuning meter dead center.
What do you mean by screening ?

[andy3055]

That is the reason not to touch those coils. Is there some sort of shielding in the tuner section that may be missing? Do you have any outside environmental factors that may be contributing... like lighting that emits RF? Is there a loose ground lead somewhere? Look at those first. It is very unlikely that there is a faulty component. Can you move it to another location in the house and test it?

[lazio200]

I can do that testing but my gut instinct tells me that one or more components are failing beyond a certain voltage, or there is something wonky in the power supply.

[wasedadoc]

My story may not seem relevant at first but think about it.

I had a Sony Hi-8 VCR. One day I wanted to replay an old Video-8 tape to transfer to DVD.  The video was fine but the audio was distorted and very noisy.  However another tape played back OK. When I tried that first tape in the Video-8 camcorder it had been recorded on, no problem.  I soon discovered the same with other old tapes.  Fine in the camcorder but not in the deck.  But more recent recordings were all OK when played using the deck.  Clearly the deck had some problem.

I realised that the old tapes having unusable audio when played in the deck had mono sound whereas the later tapes were stereo.  Sony's system used frequency modulated audio subcarriers.  The initial Video-8 camcorder had just one for its mono sound.  My later camcorder had stereo recording.  Two subcarriers. One for L+R and one for L-R.  I was doing a lot of faultfinding in the subcarrier recovery and demodulation circuits of the deck but getting nowhere.

At this point the Vacuum Fluorescent Display failed so I switched my attention to that and soon found that the 30 volts (maybe it was -30) for the display wasn't there.  Supposed to come from a dedicated dc-dc converter consisting of oscillator, step-up transformer, rectifier and electrolytic.  All inside a metal screening can on a different board (part of the motorised tray assembly with the full array of control knobs and switches that glides out) from the main board with the audio circuits.

Sure enough faulty electrolytic and transistor in there.  Sorted those out and the display was working again.  And to my surprise the audio was working properly too.

Seems that the failing electrolytic had caused the dc-dc converter to be working very hard (eventually killing the transistor) and despite the screening can there was sufficient pickup of its switching frequency by the audio replay circuits.  The deck used the presence of the L-R subcarrier to decide that the tape is a stereo one.    The dc-dc converter frequency leakage had been sufficiently strong and close enough in frequency that the deck considered that the mono tapes were actually stereo and was adding the nonsense noise from the L-R demodulator to the L+R audio. When playing stereo tapes the L-R subcarrier was strong enough to swamp the interference.

[lazio200]

Something puzzles me on the FM front end: the voltage on the Drain of the dual gate mosfet is 11.6 V (it supposed to be 10.2 V) & only when I reduce the AC, to obtain such voltage the FM works fine.

[FIXITNOW2003]

Yes this unit uses Toko IF transformer, by caps do you mean the internal dog-bone small capacitors inside the IF transformer ?

yes the internal ones

[lazio200]

Thanks, I tried swapping for another IFT transformer from a SX-580 & the results are more less the same.
I need to understand if the local oscillator is working & if the FM front end is badly misaligned.
I like the challenge & not giving-up but it is keeping me up at night.
Regards