A4000 Audio Problem

[spaceman88]

Hi,
A couple of months ago I posted a question about my newly aquired A4000's right mouse button not bringing down the top menu (the keyboard equivalent would, right amiga + Alt). I got some suggestions, but didn't get a chance to seriously work on  it until today. Someone asked me about the sound, when I checked it was terrible (low, static). The funny thing is it gets about five times louder (according to my ocsilloscope) when I press the otherwise useless right mouse button!! I downloaded the audio test program from the excellent Amiga Technical Resource guide. I measured some voltages on U402, my +12v & -12v are OK but the VREF on pins 10 & 12 are too high at 4.1 - 4.2v. I checked the resistance between pins 13-14 and pins 8-9 and got about 770 ohms. The output on pin 8 is only about 120mV (when the mouse button is pushed) and is clipped on the bottom. I used to do surface mount repairs on Super Nintendos 10 years ago, but no longer have the hot air soldering system, so I hope this doesn't get too involved :-). Any ideas what to check next?
Thanks, Len

[spaceman88]

Well I'm not quite 70 yet (46), but the eyes are not quite as good as they were. This was an Ebay special (works fine, but sold as is!!) When I first received it I didn't check it well enough and even left positive feedback!!! I bought this with the idea of building a Video Toaster setup, but now I just want my sound back :-(. I had found the audio repair site before and was using it to trace through the circuit. This unit has a 68030 so I won't have to worry about the A3640 problem. There doesn't seem to be any odour and I would hate to change all the audio parts and find out that Paula is bad. I may try the audio fix without hot air but NOT Paula!
Len

[spaceman88]

Hi again,
Unfortunatly Paula doesn't have a socket on my board (Rev. C), it's soldered (with VERY small legs) directly to the board.
Thanks, Len

[spaceman88]

Hi guys,
I should have mentioned that I tried another mouse, what really bugs me is that the sound gets louder when the RMB is pushed. The battery was removed before I got it so I don't know how bad any leakage was.
Len

[spaceman88]

Hi Guys,
Thanks for all the info. I'll reread the posts a few times and order some caps & do a little cleaning. If I get desperate enough maybe I'll part it out on Ebay!! :-).
Len

[spaceman88]

Yes, I tried bypassing some of the audio caps with no improvement (this method works quite well troubleshooting some circuits, like vertical output sections of T.V.s or monitors). If you have the part #'s handy ...:-). BTW I turned the computer on a couple of times yesterday and both times the sound was good and the right mouse button worked for 2-3 minutes. Then the sound slowly faded, became distorted (bottom of the sine wave clipped) and the RMB stopped working. I tried cooling Paula and some audio parts with freeze spray, but no effect. I'm going remove the main board and clean it as suggested by Kin-Hell and in the link you provided. then we'll see what happens.
Thanks, Len

[spaceman88]

@da9000

Thanks for all the info. I'm going to order the caps and scrub the board. Hopefully it will make a change in performance (for the better I mean :-)).

Len