Amiga 4000 emits high pitch sound

[oblivion]

So I bought my Amiga 4000 some time ago in an Amiga lot. To make a long story short there was a sound issue in that the volume was extremely low and at times the left channel would go out completely. It was suggested I get a recap so that's what I did. unfortunately after I got the board back the system emitted a very high pitched sound making the sound on it pretty much unusable.....it gets worse. me, not wanting to pay shipping again decided to try and resolder the caps by the RCA jacks myself but was unaware of solder pads and pulled up the pads of the caps dealing with the sound. so anyways I finally sent the board to a friend whos really good at solder work and he managed to bridge everything.

so here we are today. problem is now I have stereo sound back but I also have that damn high pitched screech. we have no idea what the problem could be. any ideas? Is the board a lost cause? were there ever any Amiga sound cards so maybe I can bypass the onboard sound? I made a video of the issue to help show what's going on.


https://youtu.be/cjkER2tOOak

 

[tbtorro]

Here we go again, recaps done bad, in this case 3 times over..

You probably have that pitch because those caps are the incorrect type and some of them acts as an antenna. Btw. by pulling one jack off is not a proof that the pitch is coming from both sides as the RCA jacks are short barred, meaning if only one side plugged in then both left and right channels will be down-mixed to that output.

 

[oblivion]

Here we go again, recaps done bad, in this case 3 times over..

You probably have that pitch because those caps are the incorrect type and some of them acts as an antenna. Btw. by pulling one jack off is not a proof that the pitch is coming from both sides as the RCA jacks are short barred, meaning if only one side plugged in then both left and right channels will be down-mixed to that output.

okay, do you know what the correct type is?

 

[ajk]

When the board was first recapped, do you know if it was also cleaned properly? It's hard to say from the video, but sometimes the leaked electrolyte gets underneath the other surface mounted components and all that has to be cleaned.

Also again it's hard to see from the video, but are the capacitors marked C433 and C443 of different types? A small ceramic brown one and a big blue electrolytic one? If you can get a nice closeup shot of the area in good light, it'll be easier to see what might be wrong.

Here's someone else with the same problem, although the thread is inconclusive: http://eab.abime.net/showthread.php?t=69873

 

[oblivion]

When the board was first recapped, do you know if it was also cleaned properly? It's hard to say from the video, but sometimes the leaked electrolyte gets underneath the other surface mounted components and all that has to be cleaned.

Also again it's hard to see from the video, but are the capacitors marked C433 and C443 of different types? A small ceramic brown one and a big blue electrolytic one? If you can get a nice closeup shot of the area in good light, it'll be easier to see what might be wrong.

Here's someone else with the same problem, although the thread is inconclusive: http://eab.abime.net/showthread.php?t=69873

the guy that originally capped the board did clean it. he supposedly had some experience with Amiga boards. Yes there are two different caps being used for C433 and C443. one is a 22uf bi-polar electrolytic cap and the other is a ceramic. The other person that fixed the issue once the solder pads got lifted swears up and down this couldn't cause the problem. I can take some close up still pictures if that would help.

 

[ajk]

Okay. I'm quite rusty with op-amp circuits but looking at the schematics, the audio section of the A4000 is moderately complex and I wouldn't be surprised if an imbalance in the components could cause some kind of oscillation.

There could also be a broken trace or a short circuit hiding under one of the components, unless everything has been thoroughly verified after the repair. The schematics are available here: http://www.amigawiki.de/doku.php?id=en:service:schematics#a4000

 

[oblivion]

Thanks, I'll give it a look. If it makes any diffrence after the first recap both caps were ceramic and I still had that high pitched screech.

 

[bebek]

Most likely there is a broken connection in audio circuit. Many times the track is gone just under SMT resistor so you can not see. Also damage to any of power amp is very possible.

 

[oblivion]

can anyone tell me why the guitar solo at the beginning of Elite Advanced is the only time the screeching goes away. is a completely different audio circuit being used here to output that sound?

 

[bebek]

can anyone tell me why the guitar solo at the beginning of Elite Advanced is the only time the screeching goes away. is a completely different audio circuit being used here to output that sound?

eee, no
it maybe just a range of frequences used ...

 

[ajk]

Maybe the screech is related to the audio filter circuitry, which can be turned on or off? Does the power LED by any chance dim or brighten when that guitar solo is playing?