Repair Notes: A1200 distorted audio

Repair notes are an occasional series of (fairly!) quick fixes to common issues. Hopefully, if Google bought you here then you might find this useful, it might even solve your problem!

The most common revisions of A1200 are 1D.4 and 2B, and whilst there’s minor differences between them for the majority of the production span Commodore changed from one to the other depending on which RAM ICs were cheapest – the 1D.4 uses 4 256×16 chips and the 2B uses 4 512×8 chips. Both 2MB, different arrangements, and it probably saved them a few pennies per board to buy whatever was available at the time.

Anyway. There are earlier versions, and here we’re going all the way back to 1A which sounds like we can safely call it “the first revision”. It’s fairly stable, there’s no reason to upgrade to a later one but it does have a bit of an issue with audio output which is quite frankly, terrible. Namely, it’s too loud, it clips, it distorts. If you’re listening to a demo or module with a decent bassline you’ll hear awful crackling.

Don’t worry, it’s an easy fix!

The audio is produced from the op-amp at U15, located just below the keyboard connector, and this has a feedback loop to control the amplitude (volume) of the output. For the right channel this is R331 and C331 in parallel, and for the left it’s R321 and C321. If you’re already lost, don’t worry – all you really need to know is these values control how loud the eventual output is.

The values from the factory for the Rev 1A were

  • R321, R331: 1500 ohms
  • C321, C322: 3900pF

Compare these to the Rev 2 machines:

  • R321, R331: 680 ohms
  • C321, C322: 6800pf

Clearly Commodore realised they’d screwed up a bit and modified it for later machines – by raising the value of the capacitor and lowering the resistance there’s more signal fed back so it gets attenuated more. In fact the A600 takes this even further and the resistors are 340 ohms and the audio signal is probably the best of all the examples. The odd thing here is the A600 is an earlier design, Commodore got it right and then used the wrong values later.

So, by changing these four components to a mix of what’s used across the A600 and later A1200s, we should be able to make the audio quieter and less distorted. I used the following values:

  • R321, R331: 330 ohms (slightly less than the A600 but it’s what I had available)
  • C321, C331: 6800pF
  • All components are 1206 size

And it works! The playback volume is still slightly higher than an A600 but much more acceptable, no distortion of bass and no audible clipping.

You’ll need hot air to remove the old components so you might need to ask a friend with the right kit for help (or us!), but it’s a really cheap and easy upgrade.

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