Pioneer SX-3900 FM issues

[goldswimmerb]

Hey, so I restored an SX-3900 a while back, new parts throughout, and it worked well for around 5 months, had the FM tuner pulling in stations strong, everything working great.

Now, all of a sudden there's no FM sensitivity, I notice when off station it quite consistently shows two bars on the meter, but when on station I have to turn muting off and there's little to no sensitivity.

Checking voltages it looks like everything is good except a few of the voltages on the PA3007IC, is this a likely culprit or am I missing something?

Most notably pin 9 on the IC only has 1.9v or so...

 

[markthefixer]

You have not determined whether or not the 10.7 MHz signal to pin 8 is the same as before or reduced.

If your IF strip is misbehaving, there is a quick test:
tune in a strong local station
then using your DRY FINGERS
bridge these components IN THIS ORDER:
F1
Q6
F2
Q7
F3

If a strong IF signal is stopped by a bad filter (F1, F2 or F3) or a duff amplifier Q6 & Q7 HA1202 the "skin" leakage induced by a finger bridge
will be greater than the dead component. Should see a jump in signal strength.
It's a quick test, just have to find exposed metal conductors at that point in the stage.

 

[goldswimmerb]

You have not determined whether or not the 10.7 MHz signal to pin 8 is the same as before or reduced.

If your IF strip is misbehaving, there is a quick test:
tune in a strong local station
then using your DRY FINGERS
bridge these components IN THIS ORDER:
F1
Q6
F2
Q7
F3

If a strong IF signal is stopped by a bad filter (F1, F2 or F3) or a duff amplifier Q6 & Q7 HA1202 the "skin" leakage induced by a finger bridge
will be greater than the dead component. Should see a jump in signal strength.
It's a quick test, just have to find exposed metal conductors at that point in the stage.

View attachment 2291081

Thanks for that, the voltages up to the FM IC were pretty close to the manual, when I dig into it more I'll probably update this thread.

 

[markthefixer]

title: SX-3900 IF Amplifier test points for finger bypass

Those IF amplifier test points are best accessed through the bottom of the unit. It does make the finger test lots easier!!

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