Do I have a dead Paula chip in my A1200?

[tom16mm]

Hello all.

My A1200 recently developed an audio issue, where the bottom half of both stereo audio signals are clipped. The following video of my oscilloscope display, taken from the right channel output while playing a mod file in ProTracker, clearly shows what's wrong (sorry, no audio, but it sounds as bad as it looks):

http://www.youtube.com/watch?v=pSqCU4FU_n8

The audio waveform is distorted in the same way all the way back to pins 33 and 34 on Paula/U3. I replaced coupling capacitors C324 and C334 with non-polar ones, and the 470µF electrolytic filter cap (C307) in Paula's power circuit, and the problem persists.

Power in to Paula is close to 5v, and very stable. I also did not detect any leaking/bulged capacitors.

Is Paula really dead, or can something else be at fault?

[tom16mm]

Forgot to mention, it's a revision 2B A1200, if that makes any difference.

[Buzzfuzz]

Is Paula really dead, or can something else be at fault?

No, if Paula was dead equals no sound at all, and you should have color warning @ startup.
 
The problem is probably in lf347/tl084, either one of those 2.

[shortcircuit]

Hello all.

My A1200 recently developed an audio issue, where the bottom half of both stereo audio signals are clipped. The following video of my oscilloscope display, taken from the right channel output while playing a mod file in ProTracker, clearly shows what's wrong (sorry, no audio, but it sounds as bad as it looks):

http://www.youtube.com/watch?v=pSqCU4FU_n8

The audio waveform is distorted in the same way all the way back to pins 33 and 34 on Paula/U3. I replaced coupling capacitors C324 and C334 with non-polar ones, and the 470µF electrolytic filter cap (C307) in Paula's power circuit, and the problem persists.

Power in to Paula is close to 5v, and very stable. I also did not detect any leaking/bulged capacitors.

Is Paula really dead, or can something else be at fault?

Check power to U15. Should be +12v at pin 4 & -12v at pin 11.

[Karlos]

Power in to Paula is close to 5v, and very stable. I also did not detect any leaking/bulged capacitors.

Is Paula really dead, or can something else be at fault?

Sounds like you lost the -12V rail. The amplifiers (and presumably DAC) require a split power supply. Very little else depends on this (possibly the RS232 output, but that might be implemented differently), so if you lose it, your audio will clip whenever the waveform goes negative and you may have serial port problems too.

[tom16mm]

-12v rail was the issue. The A1200 board came out of an old MagicBox AlphaGen character generator, and they had shorted the -12v input to ground. Since they never had sound, it wasn't an issue, and since I'm running it off a modified ATX power supply, I never noticed that it even needed -12v until now.

Can't thank you all enough for the answer. It really saved my project. I was not looking forward to trying to find another cheap A1200.

I do recommend the AlphaGen as a source of A1200 boards. They were all A1200 based, and they are dirt cheap when they show up. They have '030 accelerators too.

[Daedalus]

Glad you got it sorted. The RS232 and sound do indeed require -12V and there's no checking done, so they just try their best when it's not available, usually without success.
Must have a look at these AlphaGen boxes :-)