520 STM suffering

[Hanabi]

Hello everyone !

I have been the happy owner of a 520 STM (C070243 Rev B + 512Kb) for several years.
It has been in its box, in my HQ almost since its acquisition.

For what ? Because when I first plugged it in, everything worked fine.
I was able to happily play Defender of the Crown for a few hours.
Then suddenly, bombs !!!! And there, the ST started to do as it wanted.

Once it stays on white screen, once white screen at startup + 4 bombs, once white screen + artifacts...
Often, it arrives at GEM, but I no longer have access to the floppy disk drive, which is nevertheless recognized.

What I have done :
- Changed the PSU + MB electrolytic capacitors ==> this improved the behavior of the floppy drive (the motor stopped turning, did anything)
- I've tested the FDD with my Greaseweazle : fully working
- Cleaning the MB + cleaning the IC legs on supports.


I don't really understand where this could come from.
I don't have a diagnostic cartridge yet. If, with your advice, I can avoid buying one, that would suit me :)

PXL_20240712_005508500.jpg

PXL_20240711_235223080.jpg

Thx in advance :)

Hanabi.

[russellnash]

You can try the standard things like reseat the TOS, MMU and GLUE chips. You have a piggyback RAM installed according to the picture. You should check all the solder joints are in order to the RAM chips, they can be a lot of trouble. If you disconnect one of those grey wires going to the RAM chips you can see if it will boot correctly with just 512K.

[stephen_usher]

Indeed, reseating the PLCC chips with some good quality contact cleaner is the first item on the list. Of course, this means getting a good PLCC chip lifter first.

[Badwolf]

+1 for PLCC socket checking.

So many times on my -FM I think I've broken something in my verilog just to realise I need to tap the MMU a few times and press reset.

Some (most pre-STE?) models shipped with metal clamps over the two main socketed PLCC chips. Mine obviously had them in the past and were removed by a previous owner. I imagine they were there for a reason!

BW

[rubber_jonnie]

+2 for PLCC chip/socket cleaning.

Also +1 for getting a decent PLCC chip puller first, don't be tempted to try lifting out with screwdrivers.

[Hanabi]

Thank you for your answer !
Everything was done except the PLCC's : my shifter is too wide, I ordered another one.

A strange thing happens: since I took the photos, not once have I experienced crashes at startup. I conclude that even my Atari is making efforts to survive :lol:

See you soon for the rest.

[Hanabi]

Hmm. I'm not very proud of myself on this one :roll:
My problems reading floppy disks were due to a too weak FDD PSU :oops:

Everything is not settled yet : I read some floppy disks but not others (commercial games). Despite this, no game launches (although this was the case before).

In short, I will let you know what happens next as soon as I receive the chip puller.

[Hanabi]

Hi there !

The STm is getting better !

To summarize, I have:
- Re-cap PSU and MB (only electrolytics)
- Redid some soldering (a lot, in fact, notably those of the RAM mounted in piggyback, and the PSU, video, mouse/joystick and FDD ports)
- Cleaned with contact spray all chips on support
- Added a 7805 and a filter cap into the floppy drive case. What a beautiful piece of shit, this RF302RC !!! It was really an idiot trap, this messed up PSU story !

I still have to create a floppy disk with some test tools on it (e.g. yaart) to further test the machine.
The rest (and end, I hope !) after the tests :)

520STM_working.jpg

[rubber_jonnie]

:girldance: