Upgrade non-working 1040STF

[Gitbizy]

I have a 1040STF with a hand-wired 4mb upgrade, and a dual bios mod. See photos. (not my work on either of these...) it did work back in the 90's but hasn't worked since

It is non-working; I get a white screen on power up (see 9994 image). This is the same screen that happens whether the unit is in TOS A or B position.


I would like replace the handwired RAM with the EXXOS 4mb (switchable 1mb/4mb) kit.

Questions:

Thoughts on the white screen?

Do I have to unsolder all the old hand-wired ram?

Any other suggestions?

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[exxos]

In order to diagnose you can only tried the diagnostic cartridge.

If you are fitting in of the RAM upgrade and I would suggest you remove the current RAM as it could potentially damage any other upgrade. Indeed you could have already damaged the MMU at this point anyway.

[Gitbizy]

@exxos thanks for your reply.

I have a diag cart, and I had previously hooked it up and accessed it via RS232 last Christmas when I took my first try at fixing this machine.

The cart would not come up on-screen.

Which check(s) in the diag cart should I run on this to better diagnose?

[exxos]

Which check(s) in the diag cart should I run on this to better diagnose?

See the diagnostic cartridge topics.

https://exxosforum.co.uk/forum/viewforum.php?f=91

Obviously as you have a RAM error would you need to do RAM tests. Likely this will only work over serial with if machine wont boot off the cartridge.

[Gitbizy]

@exxos the ST will not respond at all with the diag cart installed (or without it installed). When I power it up I get a white screen and no output from the RS232. Yes, all my pins are hooked up correct and I'm at 9600.

I also tried this cart with my STacy and it worked fine over RS232.

[stephen_usher]

I think that you're going to have to revert the machine to stock before you get anywhere. That looks like a very hacky "upgrade"

[Gitbizy]

I don't have the original ram. How should I proceed in removing the ram chips?

Desolder from underneath and just remove them all?

what about the tiny jumper wires below the ram and above the TOS? I don't know what to do with those...?

need some guidance on reverting it... because I can remove the ram but I can't install new ram as I don't have it.

Do you all think this is an issue with the RAM itself as opposed to a dead motherboard?

obviously, I am trying to make a decision on whether to scrap it or try and fix it.

[wiwitop]

For the RAM, it's close to what did RMC here :



I can confirm (because I did it 1 month ago) that without the wiring to the MMU, it's still working but only 1 Meg is visible. So at first I would not change the RAM.

[rubber_jonnie]

For me the first thing I noticed is that the white screen has random black dots all over it. This looks very much like a RAM problem.

This differs to the completely white screen that is often present when there is a ROM issue, hence my suspicion you have a RAM problem.

I'd go with what other people have said in that the best way forward is likely going to be reverting back to original spec, so 512KB or 1MB. However you can do some checks first.

I'd start by suggesting that the ROM upgrade is probably OK since it is not connected to the RAM upgrade and just uses a switch to change the address lines in order to switch TOS versions. Reseat the ROMs by firmly pushing down on them and put some tape over the quartz windows to ensure no UV gets in, but otherwise leave that as is for now.

Before doing anything I'd suggest a check and clean of the MMU and GLUE PLCC sockets. You have no Blitter so that's fine.
Use this as a guide to making sure the PLCC sockets are good: PLCC Contact adjustment

For the RAM I would suggest you use your multimeter to check the continuity of every wire in the upgrade to make sure everything goes exactly where its supposed to. It is a pain, but necessary. Also test the decoupling caps to make sure none are shorted. If any problems are found, then repair the wiring or replace the caps as required.

Next thing I'd suggest is that if you have one, use an external known good PSU (I use a bench PSU for this) to provide 5v at 3A. 5v is all that's needed to boot the ST and without a known good PSU you may end up chasing your tail.

Also, with a bench PSU you can set up overcurrent protection and this can help determine if there are any shorts if it trips or power consumption is high. I don't believe this is the case, but worth a check.

Get yourself the data-sheet for the RAM ICs and once powered on, check they all have power as expected.

Unless you have an oscilloscope, at this point there's not a great deal you can do, however 41256 RAM ICs are relatively cheap and easy to come by, so you could buy 16 ICs and 16 sockets, and carefully remove what's installed and fit the new ICs in sockets instead to verify if the machine is working before you jump to the MMU 4MB upgrade from exxos.

There are some possible other options at this point:

1. Leave the machine running and check for hot RAM ICs, replace any that are burning hot and see if it fixes it. Be aware that they can get really hot so try not to burn your fingers!!
2. Just remove the old RAM and go for the exxos upgrade. The old RAM will need to be disabled or removed anyway before installing the exxos adapter. This is a leap of faith though as you don't know if the machine works or not.
3. If you have the tools, do further diagnostics to try and determine the faulty RAM IC or ICs
4. Send it off to somebody for repair.

Hope that helps.

[Badwolf]

Like RJ says, dots on the screen does sound like RAM, but you ought to get *something* from the diagnostic with bad RAM but a working ROM, unless one of the RAM chips is actually driving the databus when it oughtn't be.

The ROM is all socketed and relatively easy to replace. It's also EPROM meaning if you have the kit, you could try, for example, burning DiagROM ST to it (https://github.com/terriblefire/diagromst).

Personally, I'd go down that path as 1) I *do* have the EPROM eraser and burner tools, 2) that RAM upgrade looks like a work of art and I'd prefer to leave it alone and 3) desoldering all that RAM when it might not conclusively be it yet, will be a chore and likely to leave you with a toasted board anyway.

But, as ever, it depends. :)

BW