Stereo LED lighting up dim

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Superduper

Superduper

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I don’t know what is going on with that resistor. Details matter, they really do, so you really need to be really specific. Really. When you say resistor failed, burned up, did it fail opened or shorted or just drifted?

As for pin 9, I’m just going with the datasheet notes which specifies that stereo mode, voltage at pin 9 needs to be LESS than 0.7V. Forced mono occurs between 0.7 and 2.1V. Over 2.1V and the VCO will turn off. Definitely think there will be no stereo if that oscillator is off. This is why I said to disconnect pin 9 and retest FM. Disconnecting that pin will remove it from any power provided by the stereo/mono switch if any. Btw, resistors generally don’t toast all on their own. Over current for some reason which could be due to over-voltage condition, short circuit possibly while live testing and probing, or a problem with the Sanyo mpx decoder consuming too much current through that pin.

Oh and you replaced a bad resistor with a “used” one without testing? It goes without saying, that’s really bad practice. You should test & verify it, like right now.
 
baddboybill

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Ok so pin 9 disconnected and readings as follows. Pin 9 no reading in stereo or mono which I figured. In stereo 0 reading both sides of resistor. And mono 5.74v on resistor side of switch and 5.61 on pin9 side of resistor.
 
baddboybill

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It’s still not working 100%. I didn’t put cap in but have been working on it still. Seems like only 1 station comes in good on high end and some try. Low side nothing.
 
Superduper

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Ok, so here’s my 2-cents. The MPX system was probably working fine although I don’t understand your readings. First time you said 9.5, then it was 9.5k but I’m guessing you simply not understanding how to properly read your meter. The same with the cap... was probably fine. You probably mistaken an infinity reading as zero and while electrolytic frequently do count, small value caps like this charge up so quickly that the meter almost immediately reads O/C or - - - - which is infinity, which is probably ok. The 22k resistor you presumed it bad by appearance where you really should have checked first. I asked for voltage checks at pin 9 and you said it was 5.7v regardless of stereo or mono mode. This is wrong and would result in no stereo since 5.7 will turn off the internal VCO in the decoder. This is why i suggested to disconnect that pin, to “force” stereo mode. It also sort of made sense that the jumper was only soldered on one end because someone might have discovered such discrepancy and wanted to “force” stereo mode. But then on your post above #46, your readings now reflect the proper expected amounts which is inconsistent with your previous readings where you suggested that those voltages were 5.7 in both stereo & mono so now I’m confused.

I suspect that you are not having problems in the MPX circuit. Rather I think you have a weak tuner. Whether this is due to a weak RF/IF amp or a problem in the front end is unclear. But if the tuned channel is weak, the decoder will not lock stereo. The fact that you can capture one strong channel sort of supports this. If you take the boombox to an area with strong signal strengths, I’ll bet you will capture more stereo stations. Anyhow, that’s just my guess based on what i see.
 
baddboybill

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Norm I do appreciate your help. When I said 9.5 I just left out K. Lol but mentioned it was k ohm setting.

I agree it looked like resistor was burned and puffed out on end so I replaced it.

And the 5.7v readings were exactly what I got as I measured 3 separate times on both mono and stereo. I have no idea why it’s different.

So is there any way to strengthen tuner then?
 
baddboybill

baddboybill

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Weird I’m able to lock another station but only by touching pin 9 with my probe as I was trying to see voltage in stereo mode. I resoldered it but that don’t seem to be issue and it tunes and stays tuned unless I change station and go back again to that one. Or if I power off then on I have to touch pin 9 again with probe.
 
Superduper

Superduper

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1, like I said, details matter. On the one hand, 9.5 probably indicates a short in the circuit but 9.5k is normal.

2, Yeah, fix whatever is wrong and the you’ll have a stronger tuner, lol. I can’t tell you from what little we’ve learned here the exact cause, no one can. In a tuner, you have the RF stage, local oscillator, mixer, IF stage, detector, sometimes more than 1, all these precedes the MPX stage. Each stage may have an amp (transistor), and because these transistors operate at much higher frequencies than audio circuit transistors, it seems they are more prone (to me anyhow) to weakening or failure over time. Of course coupling capacitors can fail too, and if the tuner is out of alignment, you’ll never get the best reception.
 
Superduper

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baddboybill said:
Weird I’m able to lock another station but only by touching pin 9 with my probe as I was trying to see voltage in stereo mode. I resoldered it but that don’t seem to be issue and it tunes and stays tuned unless I change station and go back again to that one. Or if I power off then on I have to touch pin 9 again with probe.
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This really just confirms to me that your tuner is weak and you are able to lock stereo if you momentarily alter the circuits behavior by introducing changes to the circuit, whether through capacitance changes or loading down a pin, etc. remember that there may be some comparators at work. You’ve never had a BOOMBOX on a marginal station suddenly lock when you approach it, touch antenna, switch from mono-stereo, etc? I see that all the time. Don’t forget that pin 9 is measuring voltage, and depending on your dmm impedance in voltmeter mode, you’ve introduced a path to ground. Bottom line is don’t read too much into probing that pin to lock stereo, it’s simply indicative of reception being weak & stereo lock being at the margin. Once locked, the “lock” usually can hold if the strength of the signal doesn’t deteriorate too much but it needed a little push to initiate the lock.
 
Superduper

Superduper

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For expert or very experienced tuner techs, possibly as they have a deep understanding of the various stages and can recognize at a glance the design (there are different types) and knows which adjustment point will net a particular effect.

For the 99% rest of the crowd, a SM is necessary for guidance. You’ll also need an RF signal generator to inject the 455khz and 10.7mhz IF signals and AM/FM broadcast frequencies to adjust band coverage. And of course you will need an oscilloscope for peaking of the coils. While some can do the am without a scope by ear, you can’t do that with fm. And sometimes, a manufacturer will use non-standard IF frequencies for adjustment & you would have no way to know without the SM.
 
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