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Pioneer sx 3600 fm stereo indicator not working

klamsoss

klamsoss

Be willing to fail and try again.
I picked up this little pioneer at a garage sale. It needed some new bulbs and a cleanup, and I went ahead and recapped it. The fm stereo indicator didn't work when I got it, and still doesn't work even after the recap. The bulb isn't dead, though.
The posts feeding the bulb are obviously not feeding the right voltage when a stereo station is coming in. Any suggestions for how to resolve the problem?
 
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Measure the voltage with a strong FM stereo station tuned in. Then detune the station. Note the differences ,if any.
 
The voltage doesn't stabilize with a strong stereo signal. It does change, but not to a constant value.

As for stereo signal, I strongly believe that I am, but I should probably do some additional testing to double check.

Many thanks! Keep the input coming! I'll do some testing tomorrow!
 
@Tom B yeah I am getting good stereo separation. Not the ABSOLUTE best I have ever heard, but it's definitely not borderline. It's very clear.
EDIT: it's NOT actually very clear. Or at least it isn't as definitive when I revisited as it was when I made this original post.

@avionic R36 is bang on spec and so is the first resistor I can trace the opposite direction.
 
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Don't suppose you have a frequency counter. Really need to check VCO freq. Make sure its at 19khz.
 
I have a frequency counter. I wouldn't know how to set up to check the VCO. I suppose it's in the service manual?

I'll double check the voltage and report back but it never went above 4 VAC and even while I am getting a steady, stable stereo reception, it's not stable at all. I'll see if I can check the voltage tomorrow morning sometime, if I can make some time.
 
@avionic I'm now not sure that I'm getting stereo separation. It seemed fairly clear that it was coming in just fine last time I was checking things out, but it's not quite as definitive to my ears now. It's a bit hard without an FM stereo signal generator to be sure. Muting works well. No DC voltage to speak of... less than 0.1V.

I am getting 0.6 to 0.75 VAC on the lamp with a stereo station tuned in, if I'm not touching the chassis. 0.4 to 0.5 VAC if I touch the chassis.
THEN, if I hold the probe wires (i use mini grabbers) so that they don't touch any part of the receiver, I get between 2 and 3 VAC.IF I USE BOTH HANDS... I get about 1.6 VAC if I use just one hand to hold the probe wires up.

This is partly why I described the voltage as being unstable... it shouldn't depend on if I have my hand on the probe wires or not, and definitely shouldn't shift based on having one vs two hands on the probe wires....

If you can explain how to hook up my frequency counter to check VCO, I can do that.
 
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I'll let avionic followup on the VCO, but notice you are having your meter set on "ac" volts. The voltage for the stereo light is DC, so you need to set your meter for that.
 
@Tom B thanks for that. I had typed in that I'm getting no meaningful DC volts but must have deleted it when I rearranged my post. I've now edited it.
 
Go to Hi Fi engine, sign up free, and get the service manual for the sx-3600.
page 14 shows the test point, and VR5 adjuster for the 19khz VCO. They are near the AM rod antenna.

Easy way to determine if you are receiving stereo is to use headphones, tune in a strong station, and identify if it seems to be stereo. Then to confirm, toggle the ode switch to mono and make sure you have a noticable difference.
 
I have the service manual. So I'm just hooking up the frequency counter between pin 13 on the PA3001-A and ground? Then adjust VR5 for 19kHz? All while tuned to what I know is a strong stereo signal (I have more than a few that I can pull in, no problem).

Full disclosure: Tuner alignments aren't something I have ever tried to mess with. Partly because I haven't needed to, and partly because I don't have a reliable signal generator or ANY mpx signal generator. I assumed both were necessary to get a reliable alignment (and also that without them, and some instruction, I'm more likely to do harm than I am to do good).

The headphone method is what I have bene using. Comparing the 'mono/muting off' and 'stereo/muting on' sound, and also using the balance to shut off one side or the other. My impression is that there is maybe some stereo effect happening, but not what it should be.
 
I would measure from the Test Point, as shown in the manual, rather than clipping to the pins of the IC.
(covering for Avionic, as he may be busy)
 
Ugh... duh. I fell dumb now. Didn't pay attention to the whole drawing.
So, just hook my frequency counter up to the TP and chassis ground, then adjust VR5 for 19kHz when I have a good FM stereo broadcast signal coming in?

It doesn't matter if the receiver is getting signal from the airwaves rather than an FM SG with MPX SG hooked up?

Many MANY thanks for the hand holding! Very much appreciated! I'm learning.
 
Okay, I tweaked VR5 and have 19kHz on my frequency counter. It had been in the mid 17s but as soon as I hit 19kHz, it locked in, and I got stereo sound. It seems like once you hit 19kHz, it stays there no matter how far you push that pot. Is that normal?
I turned it back to the original 17.5ish and worked my way slowly back to 19kHz and left it as soon as it hit 19.

I'm most grateful for the guidance!!!!

I'm now getting 16.5V DC across the lamp posts, if i put the bulb back in, I get 6.3 V DC.... Am I just tired or does that not make sense??
 
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it probably is designed to sync with broadcast stations but it can only do that if its at least in the neighborhood of right. Manual may specify some process for setting it but if I had to guess it involves tuning to a dead spot in the band and setting it for as close to 19khz as you can get with no signal in.


voltage drop with load from the bulb is sensible.
 
Glad you got it right, those are pretty nice receivers. You can get away with not having a FM sig generator and such for VCO settings. Setting for 19K on a strong station usually will turn on stereo and the lite.
 
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