Atari XEGS/XE GS totally dead

[RTBaker]

Hi,

 

Looking for some hardware repair advice regarding my XEGS.

 

I brought it on Fleabay as 'working when I put it away but unable to test before selling' - not surprisingly its dead.

 

On power up there is no sound and no video output.

 

There are some 0.5v pulses on the video output which occur at PAL line rate but they are too small for a tv to lock on to - no video data is present.

 

After opening it up +5V was on all the IC's and none of the IC's get hot so I had a look at the Sally 6502c processor with my scope.

 

I found that there is no activity at all, even on reset, which works ok, on either of the data and address busses or control lines.

 

I found the following:

 

(3) phi0 - permanently LOW

 

(37) phi2 - clock I/P working (coming in from Freddie)

 

(39) phi2 - clock O/P sitting at 1.2V

 

(7) SYNC - permanently low - even with reset no activity at all

 

(34) R/Wbar - permanently low

 

(2) RDY - this is really weird - It starts High on reset but then gradually drops to Low over about 30 seconds

 

(4) IRQbar - this changes state after about 30 seconds

 

This is shown in the scope photo below where yellow trace is RDY and Blue is IRQbar

 

I see from the schematics I have that the RDY pin just goes to the ANTIC. It is also labelled as leaving that sheet but I cannot see it being used anywhere else.

 

There is something amiss here but is it the ANTIC, SALLY or something that could be at fault - could anything else cause the weird RDY signal?

 

As I don't have any spare IC's I wondered if anyone could offer me any advice on what to do next to narrow the location of the fault before I start buying replacement IC's.

 

Thanks for your help

 

 

 

 

[+Nezgar]

If you haven't made any progress with this, I highly recommend posting your question in the Atari 8-Bit computers forum. Lots of very knowledgable hardware people there.