A1200 display went almost totally dark + flickers on floppy access

[drk421]

Ok, so my A1200 (rev 1D.1) recently went almost totally dark, I can barely see any output, until I tried messing with the capacitor at D215A. If I short across it with a 470K resistor, it works for a brief time, then goes dark again. I tried replacing the electrolytic cap there with a similar value, and it didn't change much. One side is ground, the other is about 4V DC.

Another thing I noticed was that the display flickers a lot during floppy access, when it didn't used to do this.

Is there another part that might have failed, that I should check? Does anyone have a schematic of what that cap (D215A) is hooked up to?

I'd like to resolve this problem, and get my A1200 back up and running correctly!

Thanks!

- drk421

[CLS2086]

Hi,
4V is not a good value, you should have very close to 5V !
You should try to power your 1200 with only an AT PSU throught the Floppy power Connector first.

[drk421]

Ok, I'll give that a shot.

[drk421]

Tried using a 300W PC PSU, same results.

Does anyone know what the other side of that cap is connected to? (D215A)

(one side is GND)

I'm going to trace it out tonight.

[drk421]

If anyone wants to know...

I fixed the problem.

It seems commodore new about this problem, as their original circuit works fine.

Here is what I did, soldered 2 1N4148 diodes in series across the cap at D215A, and added a 1.5K pull up resistor to +5V (vid supply).

It seems that commodore decided to rely upon the internal pull up resistor in the BT101 video DAC (bad idea), for the VREF (pin 18). I have no idea why they left it floating, and simply used a .1uF ceramic surface mount cap to +VID. It seems the chip wants to see 1.2V on that pin.

If you just follow commodore's original schematic and put the parts they "forgot" to put in, it will solve the "flicker" problem during floppy drive access, or the display going out completely.

[Doppie1200]

This is great info. I beleive I have a board somewhere with the same symptoms. Never got around to fixing it. Good job!