exxos's Atari, Amiga & retro forum

Long story short: I bought a 520STFM; it doesn't work, and I want to repair it.

Here's what I know so far:

1. On powering the machine up, the motherboard LED comes on. There are no noises from the floppy and the floppy light does not come on.
2. When connected to an old TV with an analogue demodulator, when the computer is switched on the screen immediately goes completely black with a white border and never changes.
3. The largest capacitor (47uF, 400v?) on the PSU board (Mitsumi SR98 model) is pretty swollen. The others all looked OK but I haven't tested them.
4. The voltage from the PSU looks OK, my multimeter shows slightly over 5v and slightly over 12v. I do not (yet?) have anything to measure the ripple.

I don't really want to power it up again without replacing that capacitor (or the whole board). But I don't want to do that if the machine is unfixable...

My plan so far, is to disconnect the PSU and connect a modern one (just splice it into my desktop's ATX supply). Then start looking for failed components on the motherboard.

That's all I have so far. I'd greatly appreciate any pointers and advice. I don't mind spending some cash on test equipment.

Aled

Probably standard fault is that RAM has failed. You can try to feel if any of the RAM chip feels hot (hard since the RAM is under PSU)? Otherwise you can also try and piggback with another ram chip on top. Get yourself a diagnostic cartridge.

The big cap on the PSU probably just looks swollen. It's a plastic sleeve that can be cut off if you want to have a look underneath. But with that said you should still consider changing the cap on the PSU.

If you haven't tried it already, I would give all the socketed chips on the motherboard a good push - both of my STFMs started with black screens and pushing loose chips in enabled them to boot up (and then reveal other problems).

Hi,

Use suggestion given by lilwashu - pull socketed chip from socket (remember chip position as they have corner mark)- square chips first and wipe the pins thoroughly on a sheet of clean printer paper. It works like very delicate sandpaper.

Remote fixing is always a funny riddle

Regards
tOri

Black screen with white border generally means bad RAM. It's worth powering on for a while and then touching each RAM chip in turn to see if any are overheating.

As per the previous suggestions, you could pull any ICs sockets as already suggested, but specifically the ones to focus on are the square PLCCs that contain the MMU and GLUE ICs. Clean the IC legs (I use a fine fibreglass pen, the printer paper also works too) and clean the sockets too, also add some contact cleaner like Servisol, Electrolube or DeOxit before reinserting the ICs.

You may want to try this for the PLCC sockets: PLCC Contact adjustment At least when the ICs are out.

If you can get your hands on a diag cart, then this may also help in that it may boot and give you on screen results that hint at the failure, plus the DIAG cart outputs to the serial port if you can hook up to another machine with a serial port. You need a null modem cable for this and some sort of serial software.

Even if you don't get anything on screen with the diag cart, the serial port output can be a huge help.

Be aware that the ST will run on just 5v, 12v isn't needed for the main functions and you should be looking at 4.75v to 5.25v ideally. Bear in mind these measurements should be made under load and not with the PSU disconnected.

I'd suggest a recap of the PSU too, any failures could mean that whilst the voltages may appear fine, the PSU is unable to deliver the amps needed to power it up.

Just as a sidenote, adding photographs is a big help :)

Thanks for the suggestions. I'll try all these things, and see where I get to.

Aled.

A decent quality photo of the motherboard might be informative.

If, for exampe, there is evidence of a removed memory expansion, there are a few obvious things to check.

BW

Well, I've fallen at the first hurdle. Can't get the metal shielding off.

I've unscrewed every (accessible) screw, and straightened all the tabs that hold it on. Here's what it looks like now:


Three of the corners come up nicely but there is something holding it down near the power connector, at the red arrows.

I've pulled it hard enough to bend it (slightly) as you can see, it's not budging.

OK, I worked it out eventually, need to shimmy the whole motherboard forwards to get the shield around the power switch.

The PSU needs to come out first. It is fasten with 2 screws.

Woohoo!