Atari > Hardware

Fixing STE microwire "mixer" error

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Some people observed problems with unbalanced audio volumes of PSG and DMA by STE, Mega STE years ago . There should be possible to set PSG volume -12db with microwire in SW, but it never worked - I talk about serious sources like Atari ProfiBuch.
I looked schematic of microwire in STE, and there is something very wrong.

Correction took some 15 minutes:

How to perform fix: no need to desolder anything - just cut pin of resistor R531 which leads to 2 capacitors connected to GND. Solder there, to resistor new 10K resistor, and other pin of it to point where R531 was soldered. Add shortcut from common point of 2 resistors to pin 5 of IC. That's all.
This divides PSG auidio level to 18%, what is aprox. -12db . With different resistor value may change that ratio.

And proper setting of microwire mixer is not %01 for this, but %10 :D I wonder how this went this way in Atari . Seems that communication between SW and HW crews was bad.
Anyway, this correction is very useful. For instance, now sound quality in modded Xenon 2 is better, because PSG volume settings remain same, 4-bit, while I needed to lower them down, so used less bits, what made not so good volume changes by sound effects. And will help in other cases where DMA and PSG audio works simultaneously in STE.

Applies and for the STE any models?

I'm sure that yes. Hardly that anything is changed in that circuit part in later STE revisions. It is same in Mega STE too.
And should be same in TTs too, according to some people - same problem is present, that PSG is much louder.
Checked TT schematic - 100% same circuit, with useless muted inputs.
Only by Falcon it is balanced.

If i found free time i will do this mod, (i think my STE need inside cleaning also,so is time to opening..)
thanks petari.

Before mod 1040 STE motherboard By Anemos



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