520STFM RAM upgrade troubleshooting

[ccuk]

I've just bought a 520stfm which was to be a donor machine (only wanted the case and keyboard) but now that i've opened it up my curiosity has got the better of me and I'd like to see if I can get it running.

It's a C070523-001 Rev.D motherboard but appears to have been upgraded to 3mb???. Apart from the ram upgrade it all looks "stock" and is a tidy looking board so i've decided i'd like to try and save it. It has the original 512kb RAM plus another 512kb which i assume was added later into the empty sockets (different colour caps and different brand of chips) and then there's what appears to be a further 2mb upgrade which was perhaps added later?.

Tested with a known good PSU I initially get black/white verticle bands with a white border which after a few seconds turns to solid black with a white border, which i believe means a RAM issue.

I have removed/cleaned/reseated all socketed chips but no change. Have also checked the board over as best I can and there's no sign of any dry joints, etc...

As it looks likely to be a RAM issue I was going to try removing the 2mb upgrade first and if it still doesn't work then remove the 512kb upgrade, just leaving the original 512kb ram. As you'll see from the photo's the 2mb upgrade is quite "involved" (16 1x1m chips stacked/soldered together with about 35 wires to other components on the board, so my first question is can anyone see a simple way that I could disable that 2mb upgrade without having to snip/desolder the lot?.

For reference R93 and R94 have both had one leg snipped from the board and wired straight to chips on the 2mb upgrade. There is also a single wire which runs under the motherboard from the from the 2mb upgrade to the MMU.

I'm quite new to all this, though I grew up with an Atari, but having recently revived another 520 i'm quite keen to see if I can save this one too. If it can't be saved then I will reuse the case and keyboard with another working motherboard that I already have.

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[rubber_jonnie]

You suspect correctly that it's a RAM issue since a black screen with a white border generally speaking indicates that.

If I'm honest my first step would be to completely remove the RAM upgade and all the wires and rejoin any cut traces and repair the snipped resistor legs.

To be honest, and this is my opinion so others may think differently, but trying to troubleshoot 30+ year old wiring just to disable the additional 2MB is going to be difficult to do and could suck hours of your time.

By all means document everything you remove, but I think going back to stock is the safest option to recover this machine. I'd leave the extra 512KB as that can also be found on stock machines.

It'll be far easier to troubleshoot like that. And of course it may just work once the 2MB ugrade has been removed.

[ccuk]

Many thanks. I'm going to go ahead and remove the 2mb upgrade as it's such a maze of wires. Will report back once i've done it.

[ccuk]

I've now desoldered the 2mb upgrade (512kb upgrade is still in place) and reattached the two resistors that'd been disconnected from the board, i've also looked for any traces that might have been cut but can't see any.

The issue remains the same but i've made a few further observation:
- On a cold boot (machine has been off for at least 15mins) i get the black/white stripes inititally, turning to solid black after several seconds
- On a "warm" boot (either a reset or switching back on within a few minutes of last power up) I just get the black screen (i.e. I don't get the black/white stripes first)
- The ROM chip in position U6 (Chip labelled L1 in photo) appears to have had 3 of its legs snipped off
- positions C120 and C106 are empty, though looking at photo's of other boards this may be normal?

I have also confirmed the MMU is OK by testing it in another 520 where it worked without issue.

I guess the next step might be to rule out an issue with each of the remaining 512kb banks of RAM, but is it possible to disable the banks one at a time to rule each one out as the culprit?. I've seen references to doing this on other board revisions by lifting the respective resistors to 5v but not sure which resistors correspond to each bank on this board.

Any thoughts/suggestions most welcome.

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[rubber_jonnie]

I wouldn't worry about the capacitors, they are probably empty from the factory.

The ROM thing is odd, you may want to pull it to confirm, the legs might just be bent, but TBH you aren't getting to TOS because the RAM is a problem.

However I'd strongly advise to focus on one symptom at a time and for that symptom to be the RAM issue.

In a situation like this it's difficult to know what to suggest since you're new to the forum and we don't know what your capabilities are and what tools you have for diagnosis. Can you elaborate at all?

I'd potentially suggest a diag cart connected to the serial port of the ST and the other end to a PC running terminal software as you will often get diagnostic data out of the serial port even with nothing on the screen.

[ccuk]

Thanks, I've now tried a diagnostic cartridge and had it hooked up to my PC and can see the following error:

I1 RAM/data line error 0000000000000000 0000100000000000 000008

Another post suggests that the position of the "1" might correlate to a specific faulty RAM chip but theirs was a different board revision with different component numbering. From what the other post suggested it seems the faulty chip is in the lower bank (i.e. the factory fitted 512kb) but if I could narrow it down to a specific chip that would be very handy. I have a multimeter and a basic handheld scope but until now my experience has primarily been with auto electrics.

[frank.lukas]

D11 is the problem ...

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[rubber_jonnie]

Thanks, I've now tried a diagnostic cartridge and had it hooked up to my PC and can see the following error:

I1 RAM/data line error 0000000000000000 0000100000000000 000008

Another post suggests that the position of the "1" might correlate to a specific faulty RAM chip but theirs was a different board revision with different component numbering. From what the other post suggested it seems the faulty chip is in the lower bank (i.e. the factory fitted 512kb) but if I could narrow it down to a specific chip that would be very handy. I have a multimeter and a basic handheld scope but until now my experience has primarily been with auto electrics.

I'm in agreement with @frank.lukas .

The block of 16 digits after the word error is what the test is expecting and the next block of 16 digits is what was actually returned.

So counting right to left starting at zero in the 2nd block gets to to the bad bit, in your case 11.

I've attached the STE service manual and whilst not 100% identical, the codes in use found in section 3 are the same and what you would see over a serial connection.

I1 is a stuck data line as Frank stated, and D11 is in the upper RAM bank, so D8-D15 which my best guess says corresponds to U58, though it may depend on your particular board.

I believe D8 = U55 and D15 = U62 with D9 to D14 all the IC numbers in-between.

Youy could leave it on for a period of time and drop some IPA onto U58 and see if it evaporates, failed ICs often get hot. It's also possible you have a solder bridge as it looks like U58 was right next to the old RAM expansion and there were wires soldered nearby.

[rubber_jonnie]

Looking closely at your image I think the upper bank is U54 to U61 for D8-D13.

So I think D13 for you is actually U59.

[ccuk]

*** Fixed ***

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Thanks for all the help and advice - she's fixed.

Turned out there were 4 faulty RAM chips, U29, U32, U34 & U38, all now replaced.

It initially bombed out even even after the RAM replacement but using the diag cart I was able to see a checksum error on one of the ROM chips - fortunately it was just a case of cleaning the pins on the ROM chip and it now boots to the dektop :)


Huge thanks for all the help!