exxos's Atari, Amiga & retro forum

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First things first. Watch that PSU. You don't want a shock from that bad boy! If it doesn't kill you, it's Gona hurt and burn you!

Be aware of a shock risk from the power supply. Mains voltage. Be absolutely careful to check you are diagnosing safely. Disconnect and remove the PSU and remove the mainboard for visual inspection
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Best guess well...

It's bombing out so CPU side may be ok if it's able to show a bomb icon. You could try checking if any of the ram chips are overly hot to touch. As another poster mentioned it could be very likely the cause.

Any short circuits on keyboard and mouse sockets? Any pins bent anywhere else that could potentially short?

Check for cracked traces.

Discolouration or cracked components?

Can you send high res pictures of the board both front and back so we can visually check for obvious segregation.

Any leaking capacitors? The big 4000uf (I think that's what it is) on the main board is a cap that needs to be replaced more often than not.

Power the board up SAFELY using the PSU and use the handle of a screwdriver to tap the board, you may find a dry solder joint by tapping.

Disclaimer: I only have an NVQ2 in micro electronics from over thirty years ago so my advice may be incorrect! And likely is incorrect.

Just a note that this thread is over a year old now and the original poster has not logged in since opening the topic.

So I am locking this thread and assuming it abandoned.

Trip3nite wrote: 01 Sep 2023 20:34 But how sure can I be that a diagnostic cart will find what the problem is?

Well, in certain circumstances, it won't, however given your machine does at least try to boot and then crashes, you stand a good chance of doing so, in addition you can hook up a PC to the ST's serial output and use something like Teraterm to monitor the serial output, it's a good place to find RAM fails.

Also, with regards to PLCC chips, simply removing and reinstalling isn't always enough, you need to do this.

Also when reinstalling PLCC chips it's worthwhile using something like DeOxit or Electrolube once you've cleaned the socket and chip pins with IPA to ensure good electrical contact.

A bad PSU can give strange issues, so if you have a bench PSU, the ST will boot with just 5v connected from a known good supply.

If you don't have a bench PSU then you can use something like a Meanwell RD-50A which has both the necessary 5v and 12v for an ST with the necessary wattage.

It goes without saying, PSUs and mains electricity can give you a bad shock or kill you. Makes sure you are aware of all the risks and understand the necessary safety precautions before doing anything.

I can attest that the heatsinks in ST PSUs have a very nasty bite if you touch them, so be warned and be careful.