Pioneer SX-780 Stereo LED doesn't work (LED is good?)

Raccoon1400

Super Member
I'm working on a pioneer 780, and I can't get the FM stereo light to come on. I can get stations in stereo.
The "source" light comes on but it is dimmer than it should be. The source and tape monitor lights get about 2V across them, and the FM stereo light gets about 1.2V. But the voltage going to it look correct according to the schematic. 7.4V on one side of it and 6.5V on pin 6 of the tuner IC that drives it. When I test the LEDs with my meter, I get a double diode drop worth of voltage across each one.

What could be going on here?

EDIT: I'm getting the 6.3V at the chip whether it is in stereo or not. So the chip isn't turning the light on?
 
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Review the schematic. The STEREO LED (D21) receives constant power. The ground side of that LED is switched by the PA1001 IC.

The chances are statistically very low that the PA1001 has a problem, and it is more likely that the tuner is not decoding the MPX signal properly. It may help to turn the FM MUTING to off in case there is a something pulling that signal up at Pin 16

Are you certain that the audio is in stereo? If so, the VCO for the 19KHz quad detector might be adjusted. If VR1 in the tuner section tests O.K. (4.7KΩ), carefully note/photo document the position of VR1 and attempt to adjust it using a strong local stereo broadcast as a source. If no progress is made, return VR1 to its original position.

The method is described here:

...the VCO center frequency adjust ... can be set with a strong over the air FM signal.
That is set by finding the OFF to ON stereo lamp transition POINT on CW rotation of the adjustment pot and then finding the OFF to ON stereo lamp transition POINT on CCW rotation of the adjustment pot.
Then the set point on the POT is the midpoint between the two points you just located.
MUST ONLY BE THE OFF TO ON stereo lamp transitions!!!!!!
NEVER NEVER NEVER the ON to OFF transition!!!!!!
 
Review the schematic. The STEREO LED (D21) receives constant power. The ground side of that LED is switched by the PA1001 IC.

The chances are statistically very low that the PA1001 has a problem, and it is more likely that the tuner is not decoding the MPX signal properly. It may help to turn the FM MUTING to off in case there is a something pulling that signal up at Pin 16

Are you certain that the audio is in stereo? If so, the VCO for the 19KHz quad detector might be adjusted. If VR1 in the tuner section tests O.K. (4.7KΩ), carefully note/photo document the position of VR1 and attempt to adjust it using a strong local stereo broadcast as a source. If no progress is made, return VR1 to its original position.

The method is described here:
It is definitely in stereo. I tried that adjustment and I could hear it go in and out of stereo but no light still.
 
If you have an oscilloscope you can check the oscillator frequency (and amplitude). The X-Y plot method is in the service manual, or you can monitor Pin 15 on the PA1001 for a 76KHz rate.

If you don't have a 'scope handy, you can attempt to adjust as shown above.
 
If you have an oscilloscope you can check the oscillator frequency (and amplitude). The X-Y plot method is in the service manual, or you can monitor Pin 15 on the PA1001 for a 76KHz rate.

If you don't have a 'scope handy, you can attempt to adjust as shown above.
The act of touching the probe to 15 throws off the adjustment.

Vr2 (4.7k) looks like it is the pilot adjustment but it has no effect that I can notice.

In your first post you said Vr1 is 4.7k but it is 22k. Vr2 is 4.7k
 
The capacitance of the scope probe may be affecting the circuit. It may be better to try to pick the signal from one end of C40.

VR2 is the pilot / subcarrier cancelling adjustment, and probably should be left alone unless using a siggen to set the input and and modulation levels.

VR1 is the adjustment for the VCO (Voltage Controlled Oscillator) and it determines the frequency. The PA1001 divides that 76kHz signal (twice) to derive a 19 kHz pilot source. Checking both the frequency and the P-P level could be helpful.
 
The capacitance of the scope probe may be affecting the circuit. It may be better to try to pick the signal from one end of C40.

VR2 is the pilot / subcarrier cancelling adjustment, and probably should be left alone unless using a siggen to set the input and and modulation levels.

VR1 is the adjustment for the VCO (Voltage Controlled Oscillator) and it determines the frequency. The PA1001 divides that 76kHz signal (twice) to derive a 19 kHz pilot source. Checking both the frequency and the P-P level could be helpful.
Here's what the scope shows me at C40

Can I try shorting IC pin 6 to ground to try turning on the lamp?

I'm not convinced it isn't an issue with that part of the chip, since the FM stereo seems to be working fine.
How can I verify it is getting good signal strength, since there is no signal strength meter?

I do have an fm signal generator. (sencore)
 

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Sorry for the confusion earlier, but I didn't consider the exchange factor. I think 76kHz here might be about 90kHz Canadian. Then again, that market has been all over the map lately.

Here's what the scope shows me at C40

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76kHz should be well within the range of your 2215. Is that the cleanest, most stable wave you could get? Did you try with different trigger modes? If that doesn't clean it up, maybe connecting to Pins 10/11 will give you a clearer 19kHz image.

Can I try shorting IC pin 6 to ground to try turning on the lamp?

Yes, Pin 6 of PA1001can be grounded to verify the rest of the circuit is intact (just in case a solder joint or resistor is open). It may be safer to ground this at R38, or connect a chip clip and mini-grabber BEFORE powering on the unit to avoid shorting anything else.

I'm not convinced it isn't an issue with that part of the chip, since the FM stereo seems to be working fine.
How can I verify it is getting good signal strength, since there is no signal strength meter?

I do have an fm signal generator. (sencore)

If yours has a multiplex capability, follow the service manual alignment instructions, you should be able to see the sync pattern in X-Y mode on the scope.
 
O.K. For the sake of "just because" I pulled a spare 780 out of the storage rack and made some room on my bench.

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Then connected the check-a-ma-doodles:

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The faster scope showed this at Pin 15:

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The slower scope showed this at Pin 15:

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And the counter showed this at TP23:

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Sorry for the confusion earlier, but I didn't consider the exchange factor. I think 76kHz here might be about 90kHz Canadian. Then again, that market has been all over the map lately.


76kHz should be well within the range of your 2215. Is that the cleanest, most stable wave you could get? Did you try with different trigger modes? If that doesn't clean it up, maybe connecting to Pins 10/11 will give you a clearer 19kHz image.


Yes, Pin 6 of PA1001can be grounded to verify the rest of the circuit is intact (just in case a solder joint or resistor is open). It may be safer to ground this at R38, or connect a chip clip and mini-grabber BEFORE powering on the unit to avoid shorting anything else.

If yours has a multiplex capability, follow the service manual alignment instructions, you should be able to see the sync pattern in X-Y mode on the scope.

Here's another picture but with different triggering settings. I get the same thing at C40 as I do at pins 10-11.

I can't quite figure out how to set up the MPX alignment with the scope and XY mode.

If the receiver gets stereo, then I don't understand what could prevent the light from coming on except a problem with the part of the chip that turns the lamp on.

EDIT: just saw your last reply. Your waveform at pin 15 looks about the same as what I get there. I can't get a good frequency reading at tp23 though, it shows about 60Hz.
 

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It could be a case of a failed PA1001. There are supposedly NOS pieces in China, or at least they are being advertised that way.
 
is r38 good ?
am sure i remember reading something about shorting pin 6 to ground as not being a good plan . ?
 
is r38 good ?
am sure i remember reading something about shorting pin 6 to ground as not being a good plan . ?

It's probably not the best method, but PRESUMING (since the PA1001 internal schematic isn't published) that it's a typical sinking open collector output, grounding it shouldn't necessarily cause failure. I know that's a BIG presumption, but it's a common TTL and CMOS scheme.
 
DIP clip..I like chack-a-na-doodles..:D

I just about had to. Typing in cap-lock usually gives me a my-brain headache. Besides, it fits pretty well alongside some of the slobber joints in the repair work.

Coincidentally, and if I'm correct, the very first dim-bulb test in history was my ASVAB.
 
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