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#31 baddboybill

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Posted 19 October 2017 - 08:25 PM

Thank you Norm and funny thing is I replaced 1 resistor that was burned and resoldered a leg from a jump lead that looked like it may have never even been soldered in first place. Then I replaced the 19k variable resistor with 5k and hooked up to test it. Tried adjusting resistor and nothing happened at first but then somehow while adjusting it I went to hard past the stop point on it and all of a sudden stereo came in and led on. But then I tried centering it and went to change stations and the stereo was in but led stayed on whole time. Then I turned of radio and put back on and no stereo or led. Then I moved vr again and it came back on. Went through it again with same results. Then soldered another new vr in and nothing. So not sure what to think but that’s when I checked another resistor and the polypropylene cap. Resistor was fine but cap had nothing. This was after I took them out.

Also Norm since you posted I decided to put cap back in and test radio with same exact reaction so I’m still stuck. Here are pics of the area both front and backAttached File  A10B6269-A84C-4305-A989-6F4FC9F4E46B.jpeg   148.45K   2 downloadsAttached File  1AF37417-90F3-4779-9975-63AA7A1D96E0.jpeg   182.41K   3 downloadsAttached File  74F73CCA-3E93-4E0C-9C1A-92817B99E305.jpeg   122.79K   1 downloads

Arrow is where lead is for stereo led, 2nd lead from leftAttached File  9D026C96-32F7-4017-9E5D-41E2ABAC79B2.jpeg   178.11K   2 downloads

Yellow arrow was bad resistor I replaced and green arrow was the jump lead that was only 1 leg soldered in.Attached File  6BC7B2AC-9D3F-42AB-974C-71457E9C5EC0.jpeg   157.56K   1 downloads

#32 Superduper

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Posted 19 October 2017 - 09:19 PM

If the cap is bad, then it's bad so reinstalling won't fix anything.  However, to be clear, when you tested the cap, you tested it in OHM mode and you got "0" or did you get infinity, no reading - - - - or something like that?  You wouldn't be the first to make that mistake but zero is "0" and nothing (like as if nothing was connected) is not nothing, it is infinity.  Zero is definitely shorted but most meters will almost never read zero.  It would be more like 0.2 or some other low number.  Even the leads have resistance so unless you rel out the built in resistance, you almost never get a true zero reading.  Now, if you were getting no reading, then that's not necessarily indicating a bad cap.  It might be perfectly fine.  That's not how you test a low value cap.

 

As for your circuit, we can now see that your circuit indeed uses an MPX decoder chip, a Sanyo LA3361 (common chip).  This is why you should always post extra details whenever possible so as not to waste time for others guessing.  Here is block diagram of that decoder.

 

 

Attached File  la3361-block.png   323.16K   0 downloads

 

If you look carefully at the 1000p cap, which I presume is the one you removed, it has a caution sign next to it indicating that exact replacement is required.  It states polyester but whatever was in there should be replaced with exact part.  Mica, tantalum, ceramic, electrolytic, etc definitely shouldn't be used.  Also look carefully and see that one leg is connected internally to the MPX decoders VCO (voltage controlled oscillator).  This for sure requires a critical stable circuit.  Also note that the is cap is connected to ground on the other leg creating a parallel circuit to the resistors.  IF it were indeed shorted, that means that leg#16 would be grounded and no amount of adjusting of the resistors would have any effect.  If that cap is truly shorted, you can certainly check now whether pin #16 is grounded with the cap in place.  If yes, then the cap is definitely bad.  If however you are getting some reading, say something between 6.8k and 16.8k, then the cap is likely fine or certainly not shorted.

 

Here is the sample circuit diagram.  I am guessing that it should mirror your circuit quite closely.

 

Attached File  la3361_sample_circuit.png   54.09K   1 downloads

 

I suggest you compare the circuit to yours and see if your resistance network to pin 16 is working properly.  In other words, check and verify that pin 16 to ground with an ohmmeter (radio powered off) reads a range consistent with the series resistors you have installed.  The circuit is calling for 5k VR with a 9.1k fixed resistor, OR 10k VR with a 6.8k fixed resistor.

 

Also check out the notes regarding pin 9.  According to notes, in stereo mode, should read less than 0.7v (or off).  In mono, it should be between 0.7v and 2.1v.  It sounds like the VCO will turn off if voltage exceeds 2.1v.  It's worth confirming those numbers.



#33 baddboybill

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Posted 20 October 2017 - 06:38 AM

Ohms set to 200k on meter Pin 16 to ground I get 9.5 reading. Board is out of unit so to test pin 9 I will do later after work.

Any other tests I need to do Norm?

#34 Superduper

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Posted 20 October 2017 - 11:05 AM

9.5 ohm or 9.5k ohm?

#35 baddboybill

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Posted 20 October 2017 - 11:23 AM

9.5 ohm or 9.5k ohm?


09.5 on meter

#36 baddboybill

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Posted 20 October 2017 - 07:19 PM

Using 20k on my meter my reading is 9.58k

#37 baddboybill

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Posted 20 October 2017 - 07:32 PM

While powered up I get on both stereo and mono 5.71v as well on pin 9

#38 Superduper

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Posted 20 October 2017 - 09:54 PM

Using 20k on my meter my reading is 9.58k


You need to be sure and maybe understand your meter better. 9.58k is within the normal range with the trimpot near the low extreme. If 9.58k, there is no way that the cap is shorted since it’s a parallel circuit to ground with that resistor network which would read virtual zero if that were the case.

While powered up I get on both stereo and mono 5.71v as well on pin 9


The data sheet only shows the snippet relevant to this circuit. Without the full schematic, it’s unclear what’s happening on the circuit connected to pin 9. Maybe the issue is downstream. Try this, disconnect pin 9 by removing solder from the pad, either with a sucker or wicking with desoldering braid & liquid rosin (flux) & see if it makes a difference with pin 9 out of circuit?

#39 baddboybill

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Posted 20 October 2017 - 10:06 PM

Using 20k on my meter my reading is 9.58k

You need to be sure and maybe understand your meter better. 9.58k is within the normal range with the trimpot near the low extreme. If 9.58k, there is no way that the cap is shorted since it’s a parallel circuit to ground with that resistor network which would read virtual zero if that were the case.

While powered up I get on both stereo and mono 5.71v as well on pin 9

The data sheet only shows the snippet relevant to this circuit. Without the full schematic, it’s unclear what’s happening on the circuit connected to pin 9. Maybe the issue is downstream. Try this, disconnect pin 9 by removing solder from the pad, either with a sucker or wicking with desoldering braid & liquid rosin (flux) & see if it makes a difference with pin 9 out of circuit?

So the test I performed on pin 9 I did while hooked up and powered on. So you want me to remove solder from pin 9 and test fm stereo?

#40 baddboybill

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Posted 20 October 2017 - 10:25 PM

Pin 9 leads to the resistor I replaced that was bad and then to the band switch

It’s a 22k and it had been blown so I replaced it with used so maybe it’s no good cuz I never tested it.Attached File  7E651327-A823-4B02-A673-45681B4ABFE4.jpeg   94.42K   2 downloadsAttached File  A24A0D73-CAF1-4240-9D49-8AEA6D281C5A.jpeg   87.9K   2 downloadsAttached File  75255E61-3FD8-4E13-8BD4-3BCAD1224DE8.jpeg   73.31K   2 downloads

#41 Superduper

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Posted 20 October 2017 - 11:45 PM

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.

#42 baddboybill

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Posted 21 October 2017 - 05:10 AM

the resistor seemed burned and protruding. I just tested them both and readings were around 22k so I guess it wasn’t bad

#43 baddboybill

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Posted 21 October 2017 - 05:35 AM

I will remove solder after work and give findings

#44 Superduper

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Posted 21 October 2017 - 09:04 AM

I will remove solder after work and give findings


When you remove the solder, also check to see what voltages you observe at the r22k resistor in both stereo & mono modes too with p9 disconnected.

#45 baddboybill

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Posted 21 October 2017 - 09:09 AM

Ok thanks

#46 baddboybill

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Posted 21 October 2017 - 08:05 PM

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.

#47 baddboybill

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Posted 21 October 2017 - 08:23 PM

I also tested FM and still no stereo or LED. I also slowly adjusted VR with zero response

#48 baddboybill

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Posted 21 October 2017 - 09:03 PM

Ok so trying to play with it I was able to tune into a couple stations and get stereo and ledAttached File  E844251F-6440-40DE-A78A-8F69229BC718.jpeg   123.6K   0 downloads

#49 caution

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Posted 21 October 2017 - 09:14 PM

It's working again! Did you put the cap in I sent you or was the original actually OK all this time?



#50 baddboybill

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Posted 21 October 2017 - 09:18 PM

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.

#51 baddboybill

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Posted 21 October 2017 - 09:41 PM

The radio station that comes in best is about 10 miles away so others that are farther don’t seem to work.

#52 Superduper

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Posted 21 October 2017 - 09:43 PM

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.

#53 baddboybill

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Posted 21 October 2017 - 11:09 PM

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?

#54 baddboybill

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Posted 21 October 2017 - 11:57 PM

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.

#55 Superduper

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Posted 22 October 2017 - 12:05 AM

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.

#56 Superduper

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Posted 22 October 2017 - 12:25 AM

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.


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.

#57 baddboybill

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Posted 22 October 2017 - 06:59 AM

So switching out the chip won’t fix it either?

#58 Superduper

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Posted 22 October 2017 - 09:23 AM

If the chip is bad = yes.

If tuner is out of alignment, or your RF amp is weak = no.

In other words, extracting your molar won’t cure your broken leg.

#59 baddboybill

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Posted 22 October 2017 - 09:39 AM

Can tuner be realigned without a scheme? I know stations are off by a little bit.

#60 Superduper

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Posted 22 October 2017 - 11:08 AM

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.