exxos's Atari, Amiga & retro forum

Hi all,

I'm waiting on some parts to arrive for essential maintenance but thought I'd drop a quick post in here describing the symptoms and stuff I've done so far to see if it is indeed a Noob caused problem that could be resolved easily.

Symptom :-

The system is connected to my old CRT TV via a Monitor to Scart lead designed for an ST.

• When powered on, the small green light bottom left of keyboard comes on.
• The TV recognises that the Atari is turned on and switches to the right SCART socket.
• The screen is black. Nothing else happens.
• If I stick a disc in the drive, the drive spins but nothing further.
• No flashing yellow LED on the keyboard.
• The C025913-38 chip (DMA) gets HOT. Too hot to hold my finger on after a minute or so.

What I've done so far:-

• Reseated all socketed chips and given the legs and socket a wipe with Isopropyl Alcohol.
• Replaced all electrolytics on the main board (apart from the modulator section.. I assumed as I'm not using it, it won't matter but maybe it is still live and causing an issue).
• Replaced the caps in my power supply. When plugged in and powered on, the PS is giving 4.9V and 12.1V and looks stable on a multimeter, but not scoped it yet.

I have new ROM and a 4MB memory upgrade ready to fit, but don't want to start that in case my current issue damages these new parts.

It's a c070789-001 rev c1 main board.

Any tips or suggested tests will be gratefully received :D

Cheers

Nick

DMA should not get that hot.. Is it in a socket ? Maybe someone's put it in backwards...

Nope, DMA definitely shouldn't get too hot to touch, as exxos said check orientation if it's in a socket, the chip notch should match the silk screen markings on the board. Also if it is socketed check all the pins are inserted and not bent.

If it is soldered in it will need some work I'm afraid.

If you can, please add photos to this sort of question, as it helps to answer a lot of these sorts of questions.

No, not socketed and doesn't look messed with. Pin1 is at the capacitor end.

Should have said that yes this is an "untested" purchase from an auction site so anything's possible, but it doesn't look like anything has been done to it. The power supply that came with it wasn't giving out any volts on the output side. Not sure if that could have caused or been a symptom of the problem I have now.

I've added some photos below.


Many thanks

Nick

Sounds like the DMA is bad... Also those older floppy drives are better being replaced with a more modern drive. Likely the belt has gone and it may be only single sided.

Not trying to discourage but it might just be easier to get a different machine than spend the time and cash trying to fix it all.

I'd also go with failed DMA, but it's soldered in so that will need a bit of rework to remove and fit a socket.

If you have a multimeter, I'd check the resistance between the ground pin and the 5v pin. If it's a dead short, or really high resistance, that's potentially a bad sign.

cheers.

Yeah, short between 5V and GND with the chip in place. I'll unsolder it with the desoldering station later to check if it's the chip or a short between 5V and GND somewhere else.

Thanks guys

Nick

nick_p wrote: 07 Apr 2021 14:54 cheers.

Yeah, short between 5V and GND with the chip in place. I'll unsolder it with the desoldering station later to check if it's the chip or a short between 5V and GND somewhere else.

Thanks guys

Nick

Yeah, will be good to see if the short is internal. I kind of expect it is.

DMA chip removed.... still got just 43 Ohm resistance between 5V and GND on the board which I'm guessing isn't good.

345 Ohm between pin 20 and 40 on the unsoldered DMA chip so not a dead short or really high....

I think exxos might be right... move on to known working purchase.

rubber_jonnie wrote: 07 Apr 2021 15:09 Yeah, will be good to see if the short is internal. I kind of expect it is.

Seems like it almost has to be. Anywhere else and there wouldn't be any motivation for the current to flow through the DMA chip fast enough to make it burn.