Atari STm fault - gaaargh!

[Spirantho]

Hi people,

Perhaps someone here can help me before I lose what's left of my hair.

I have an Atari 520 STm. When I turn it on, I get no good picture - the screen is white and vertically fills the whole screen. This is because the shifter is just sitting there chucking out 5V on the mono output (tried with colour too, "No Signal" says the TV).

I've already done some detective work... I found that the CO25915-38A glue chip was kaput by putting it into another Atari 520ST (no F or M or anything). With the working chip, the machine is still dead.

I used my Atari ST engineer's test kit to run the diagnosis and got this:
"T4 Video Counter in Memory Controller  Fail".

This apparently means that the MMU isn't generating a correct address on the video counter.

I've tried switching the MMU, Glue, Shifter and DMA chips - no change whatsoever.

The RAM checks out fine in the diagnosis program, and I'd guess the CPU is good otherwise it wouldn't be running the test program at all. The CPU would be tough to change (not socketed!) so

Can anyone help? Any pointers into what should make the counter work? I don't know much about the ST architecture, but I'm learning....!

Thanks to anyone with any advice whatsoever!

[g4ugm]

Well it sounds like an oscillator isn't running but from what I remember almost all signals in the ST are generated from a single clock.

If I remember properley you don't get anything out of the ST Mono when its in color mode and visa versa. Have you grounded the mono detect?

Might be worth dropping barry at keychange an e-mail if really stuck.

http://www.keychange.co.uk/

In the mean time I'll try and remember how this works and see if I can find my diagrams

Dave

Dave

[g4ugm]

I assume you have the Service Guide from here:-

http://dev-docs.atariforge.org/

Browsing it I was wondering if it could be the MFP chip....

[Spirantho]

Thanks for the ideas - yes I do have that service manual, thanks!

Why do you suspect the MFP though? That seems to be quite seperate from the problem bit...?

[Spirantho]

I fixed it!

It was a fault in the modulator! It was lowering the VSYNC signal to 10 Ohms. No idea why, haven't looked at the modulator yet... but the machine works again! Thanks for the suggestions though!

[g4ugm]

Thanks for the ideas - yes I do have that service manual, thanks!

Why do you suspect the MFP though? That seems to be quite seperate from the problem bit...?

It generates timing interupts...