exxos's Atari, Amiga & retro forum

Hi,

I will add more detail, but during a repair of a 1200 I was made aware that Commodore fitted incorrect resistors in the audio circuit on early motherboards. At least on 1A and 1B.

They fitted 1.5K ohm instead of 680 ohm (later boards have lower values installed).
This causes a number of capacitors to become 'reversed biased'. And I wonder if this leads to the capacitors leaking.

The change is pretty simple. Just need two 1206 sized SMD resistors. It also improves the sound as the OpAmp is not being driven so hard.

I checked and this was also the case on my other 1B PCB!

I have to check what rev I have on my 1200 then.

I had a quick look at mine and it appears to have 820 ohm resistors fitted, assuming you're talking about R321 and R331 ?

Mine is a revision 1B.

Hi.
Yep that's the ones. Yours looks nice and clean.
From the bit of yellow wire I guess your 1B has the lone chip soldered and patch wire.
Also your edge connector bevelled?

Looks like the yellow wire goes to an 74HC32T near the front of the board.

And it appears the edge connector is not bevelled.

Higgy wrote: 15 May 2021 08:43 This causes a number of capacitors to become 'reversed biased'. And I wonder if this leads to the capacitors leaking.

I havent simulated this but since those resistors are the in feedback path for the op amp nothing will be revise biased. All that will happen is voltages will be too high and the op amp will saturate and clip. Its just a signal quality issue nothing more.