1040 STFM resurrection project. Need help

[mathias_matschke]

Hi Forum,

I bought a miserable looking 1040stfm with destroyed housing and disconnected floppy in unknown condition. the board is revision 1.1 So want to use this boy to learn how to resurrect him. I know that I will be possibly a marathon but I want to see the GEM desktop. That’s my mission!

Pressing power button shows zero result. No LED on keyboard or So I took the power supply out which is a Taiwan liton PE 2350-2. I replaced the fuse and tried to measure the output. No 5v and no 12v. Normally I would replace the capacitors and then work the other possible issues. However, as I m new to Atari ST repairs, need your insight please what are the correct next steps. I add pics of the main PCB and also the PSU top and solder side. The small IC on the top side is very rusty. The other components look in my layman view at first side ok.

Also a capacitor in the modulator section of the main pcb exploded C221 16v100mf. I tried best to clean up the surroundings and I think once I replace the cap things should be fine there.

Thanks in advance for your help. Appreciated.

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[rubber_jonnie]

First bit of advice would be to get some load resistors from the store, these PSU behave a lot better when they are on load.

Also, they will boot off just 5v if you hook up a bench PSU.

[mathias_matschke]

@rubber_jonnie Hi There,
Thanks for your comment. Appreciated.

Now, as the PSU appears not to be working at all, does anybody of you have any idea looking at the photos what may be the root cause, as I don't think merely replace the caps will bring it to live again. Thanks in advance.

[rubber_jonnie]

@rubber_jonnie Hi There,
Thanks for your comment. Appreciated.

Now, as the PSU appears not to be working at all, does anybody of you have any idea looking at the photos what may be the root cause, as I don't think merely replace the caps will bring it to live again. Thanks in advance.

Do you have a bench PSU? if you have you can connect the 5v only and you should be able to boot.

Unfortunately if the PSU is dead, it's going to be a case of bussing out the connections from the mains input through the switch to the PCB. If that's all good, then check the fuse, after which it's delving into PSU faults, which could be the transistors, but it is a bit of a minefield and electric shocks may await, so care is needed and safety comes first.

If you don't have load resistors then you'll need some. Personally I'd go for a Meanwell RD-50A, those old PSU's are not great.

[mathias_matschke]

Hi @rubber_jonnie Sorry no bench PSU available.

I heard very good things about the meanwell PSU and can buy here in Germany for about 15 Euro, so may give it a shot.

What about the blown capacitor on the mainboard in modulator section please? Wise to recap the whole board as there are almost 20 caps on the mainboard alone.

Thanks for your help!!!

best

[exxos]

It is hard to tell, but it looks like the power supply may have some suspect joints. I would really recommend re-flowing them all to be on the safe side. If the main switching transistor blown it would generally blow the fuse as well. If that is not the case then there should not be really any reason why the power supply is not working.

[rubber_jonnie]

Hi @rubber_jonnie Sorry no bench PSU available.

I heard very good things about the meanwell PSU and can buy here in Germany for about 15 Euro, so may give it a shot.

What about the blown capacitor on the mainboard in modulator section please? Wise to recap the whole board as there are almost 20 caps on the mainboard alone.

Thanks for your help!!!

best

Well, yes, you'll need to replace that. You might want to check the electrodes are not touching as that maybe shorting out 5v.

[mathias_matschke]

Hi @exxos Thanks for your advice. Appreciated. I will reflow all solder points and report back tmrw. When I bought the ST, the fuse WAS actually blown so I hope the main switching transistor was the culprit. I replaced and the fuse and switched on and fuse held, so fingers crossed.
Thanks again :)

Also will try to order the caps in a set here in Germany, So I understand that it is advised to recap the PSU and the main PCB caps,

[exxos]

If you suspect the main switching transistor has blown then I would continuity check it. It would likely fall totally short-circuit. They normally when that happens it will continuously blow the fuse. You will probably have to remove it to check it properly.

Though I wouldn't bother recapping the power supply until it is actually working. Changing the caps is more in the area of servicing and improving life of the power supply. Recapping it when it is faulty is likely not going to do anything but waste your time.

If you can safely power it up without risking killing yourself, measure the voltage across the large 400V capacitor.. you should have voltage there. But also be warned that you will need to discharge the capacitor if it does charge up. Something like a couple watts 10K across it should be fine. Or of anything you could use a screwdriver but it will spark and not do the capacitor much good.. But if it is going to be changed out anyway, then for these types of tests it will probably be fine.

I'm also not sure what the blue capacitor looking parties next to the capacitor and heatsink. It could be a capacitor or a surge resistor (they tend to blow on the SR98) but I am not that familiar with that particular power supply and there doesn't seem to be a schematic for it.

[pixelpusher]

I got my hands on a not-working Liteon PE 2350-2 too, but with several capacitors missing. Seeing the good results Exxos measured for this PSU, I'd like to try rebuilding it.

Has anyone the schematics for this PSU - or at least the original values of the capacitors?