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LC Squealing Continues After Recap and Several Washes


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I would never call another member's work as shoddy or terrible. In fact this LC board's recap was done better than what I could have in comparing it to those I did. And I praised him for it, he does great work. Sorry Uniserver, it's not you. I'm talking about CompuNerd.

 

But this LC board still eludes me. At first I thought it was noise from the PSU. Nope. The PSU works fine and without noise on other LC Boards, including LC II, LC III and LC 475. So I decided to try a known perfectly working LC PSU from another LC III to this LC. IT SQUEALS!!!

 

So it's not the PSU creating noise on the lines.

 

But CompuNerd admitted long ago that he put one of the ROMs backwards on this board and killed the ROM. An honest mistake, we all done that at one time or another. So it took me a while to get a new set of ROMs, having to buy a complete set of 4 ROMs from ebay for $9. I replaced that 1 dead ROM and got the LC working again but it squeals heavily! Today I decided to replace the other three ROMs, and it still squeals but not as badly. Put back in the old ROMS and it squeals loudly. Replace the ROMs, and its quieted down but still squealing. Interesting...

 

So part of the problem is solved but it is still there. Like Schödinger's Cat being there and not there at the same time. So I'm thinking, could a replacement cap have blown at the audio section from the ROM being placed backwards? To me they look fine under a magnifying glass. Voltage is OK. The problem is that it continues to squeal.

 

Furthermore - with the ROMs replaced, it will quiet down after a long while (a 1/2 hour or so) to a white noise static background noise. Audio Amp Chip problem?

 

To make sure that it is not remaining cap goo on the board causing this, I have washed this board again and again.

 

Other than this "little problem" the machine works fine!  CompuNerd did an excellent job of recappiing and repairing this board when he had it. Things happen and I'm not blaming him for anything bad with the board. Who knows, it may have had this problem all this time and long before CompuNerd got it!

 

What you guys think - bad cap(s) or bad audio chip? This one really has my head scratching.

Edited by Elfen
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1) Opps. I thought I did posted it in the LC Section. Obviously it is not the case. Hopefully one of the admins will move it into the LC section where it belongs. But in the very least I am not alone making this mistake as there are a couple of misplaced postings on the forum; I can count 2 here in the classic section.

 

2) There is a history this board has and we have been trying to figure out this for a long time. It's that now I was well enough to look at it again. Last possible ideas was that the PSU was creating the noise. Today I just concluded that it is not the PSU.

 

And thanks.

 

Now back to the issue at hand... Since he did the recapping, he used Solid State Tantalum caps for this board and I'm crediting him for the neat and great job he did. Now as demonstrated on You Tube, when you reverse feed or over feed a Tantalum Cap with enough power, they go up in a dramatically with an explosive bang. But somewhere on this forum, I think it was Uniserver that stated that a Tantalum Cap can quietly fail and show no symptom of failing except for random issues that looks like something else is at fault.

 

So I'm rather perplexed by this.

 

The only hint I have to this problem was that one of the ROMs was accidently put in backwards and the ROM blew out though the rest of the board and ROM Chips survived. And that is not even a hint as the board could have had the problem before this happened. So this narrows it down to:

 

1) One or more of the caps going bad, or 

 

2) The Amplifier Chip being bad and generating random signals.

 

The only hint here is that when all of the ROMs were replaced, the volume of the squealing went down but is still there.

 

 

I'm just posting this because noise after a recap is usually blamed on cap goo still on the board. But I washed this board again and again, and I know CompuNerd washed it because this board was awesomely clean along with the case and like I said, he did a great job with it.
 
Ever heard of a "signal monster?" It's a noise on the signal line that when you look at it repeats within it self on a lower voltage and if you look into that it repeats itself again on a lower voltage and it continues to repeat again and again. The signal monster is not strong enough to offset a 5v digital signal as it fluctuates from 4.95 to 5.05 but put it into an audio line, it becomes noise from squealing to static noise depending on the repeating "monster" pattern. This was where that guy who created the mathematics of fractals got his idea from. See:
 
To me this sounds like such a signal monster has crept into the circuit. The problem is tracking it, finding it and then eliminating it. Question is where to begin.

 

No disrespect to you or anyone else, as we say in the Life Guard Corps, "if you can't help, leave it to somebody who can." So lets ping-pong this question back to you and everyone else: What do you think it is? Bad cap or bad audio chip?

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can we see a picture of the upper left hand side of the board.   Where all those 10uf caps are.

maybe there will be something tell tale?

 

Thanks.

Charles

 

Let me see if I can barrow a Digicam sometime this week and let my friend to take the pic. My shakey hands will blur the picture.

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