[Premium-Rx] WJ-8716 intermittent problem

[Tim Shoppa]

After sitting on the shelf for a few months, I fired my WJ-8716
back up again.

It worked fine for about 20 minutes, and then went into this weird
situation where the symptom was: on all filters except for 16kHz,
it sounded like a regenerative receiver with the gain set slightly
too high. As signals went up and down it sounded like it was
breaking out in self-oscillation. This happened in all modes
and all bandwidths EXCEPT 16kHz.

But there was also this weird interaction with main tuning: it
was most likely to obviously exhibit this symptom when tuned just below
X.XX5 MHz. e.g. it would do it if I was listening to 5.924999 MHz
but then largely (but not completely) go away if I went to 5.925 MHz.

After leaving it on a few days, this all went away.

Is this a known problem? If I look at the schematics, then the
self-oscillation except on 16kHz bandwidth
looks like it might have something to do with the
IF strip. I see all these switching transistors to allow digital
switching between the different IF bandwidths and I suppose it's
possible that one of those might be leaky in a way allowing
regeneration possibly through coupling in the digital control lines.

But then there's that weird interaction with main tuning,
which takes me into the world of the synthesizer, a place I'd
rather not go.

I suppose it's possible that it's the IF strip interacting with
the synthesizer in some weird way, either feeding back into the
synth's local oscillator outputs, or maybe even coupling in the
power supply despite all those bypass caps.

Any advice or collective wisdom? Maybe I'll just be sure to use
it a couple hours a day from now on, and not let it sit on the
shelf!

Tim.