exxos's Atari, Amiga & retro forum

Hi folks,

I recently bought a 520STF (motherboard C070789-001 REV.D) with 1MB of RAM and TOS 1.04 on six ROMs. It worked perfectly for about two weeks, but now it has stopped working. Using the Diagnostic Cartridge, I get an "E9 Bad instruction fetch" error after a few seconds of a red and white blinking screen.

Sometimes, I get more information (I have attached a screenshot):
The previous owner upgraded the computer to 1MB from 512KB, and all the memory is beautifully socketed. It also has some obvious modding in resistors and capacitors, I guess.

I've tried changing the power supply and ROM chips, but the problem persists. I tested with and without the floppy drive and still no luck. My next step is to test the RAM banks individually. Not sure if it will work or not.

I have spare parts from other broken ST computers (I guess I'm really good at breaking ST computers) and a decent lab, but I don't know where to start looking. I have explored the Forum, but I did not find any clue.

Any help would be greatly appreciated

Diego

https://www.retromaniek.pl/wp-content/u ... T-MEGA.pdf

This mentions the error, no idea what it means but could be 68000 related?

Have you done some of the basic troubleshooting? Remove the socketed chips (I'm talking system chips not RAM) clean the legs, deoxit the sockets, replace and test.

I'd first start by making sure all the socketed ICs are removed and sockets/IC pins cleaned and reinstalled. You may need to tweak the pins in the sockets too.

After that, if it still gives the same error and won't boot, check the cartridge port connector is good. I've had a similar error when a diag cart wobbled during troubleshooting.

Take a piece of card slightly narrower than the width of the cart port and wrap a pice of paper round. Normal printer paper is fine. Spray IPA into the port and insert/remove the card a few times and the slightly abrasive nature of the paper should give the contacts a clean. Also check none of the contacts are bent and if so use a fine tool like a pick to 'adjust' the contact that is troublesome.

If you still have an issue after this I'd suggest you get the ST connected up to a PC running something like Teraterm using a serial cable and boot the ST, see what you get on the serial output, as there is often additional information you can see before the cart diag menu appears on the ST.

Thanks for the replies!

I discovered the root cause: one or more memory chips in BANK 1 (512KB–1024KB) are faulty. The computer worked after removing all the chips from this bank, but cleaning the sockets and reinserting them caused it to fail again.

Next, I’ll identify the faulty chip by replacing each IC in the lower bank with one from the higher bank until I find the broken chip.

Thank you for your help and support!

Best regards,
Diego

logronoide wrote: 01 Jun 2024 17:14 Thanks for the replies!

I discovered the root cause: one or more memory chips in BANK 1 (512KB–1024KB) are faulty. The computer worked after removing all the chips from this bank, but cleaning the sockets and reinserting them caused it to fail again.

Next, I’ll identify the faulty chip by replacing each IC in the lower bank with one from the higher bank until I find the broken chip.

Thank you for your help and support!

Best regards,
Diego

At least you've found the issue. For reference the serial connection whilst booting with a diag cart will show you RAM issues and can help in diagnosing what bits are bad.