C128 no DC voltage - what can cause this?

Es gibt 11 Antworten in diesem Thema, welches 2.983 mal aufgerufen wurde. Der letzte Beitrag (13. April 2014 um 15:42) ist von Gerrit.

  • Sorry about writing in english, but don't know the German language much - and Lemmon64 is down for the time being :)

    Anyway, just bought a C128 PSU from ebay - but turns out the 5V DC voltage does not seem to work - any idea what might cause this ?.

    I'v opened the PSU up, but the condensators looks fine - no bulging or anything......any tips on what to check would be appreciated :)

    Thanks

  • I don't know the reason but dying PSUs are not really unusual.

    What exact voltage _does_ it supply, btw?

  • Quite a few C128 PSUs that I have seen did have shorted electrolytic caps. Defective electrolytics don't always show visible signs of damage so you would need to desolder and test them.

  • Here's what I did when I had a dead C128 PSU: Stripped out everything after the rectifier, replaced the capacitor and used an adjustable Regulator based on the LM2596 (from Ebay) instead. Then adjusted the regulator to supply 5.3V on its output which translates to about 5.1-5.0V once it gets to the C128.

    You can now get even smaller and more efficient adjustable DC/DC-converters on ebay cheaply. Those no longer need a heatsink.

  • Something I just remembered... The original C128 PSU has 2 resistors that are just wires. Depending on how the PSU was stored, their solder joints might have rotted away. I had 2 of those and it was a real pain to fix.

  • How would one go about to "discharge" it so it's safe to work on ?.

    I'v attatched some images of it to show how it looks.


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  • After a bit more measuring it seem that the number 4 pin (5v DC pin) is "shorting" itself (1.5 ohm between the chassis ground and the 5v DC - not sure if this is normal ?) to the square metal surrounding the connector - I presume this one is supposed to be a ground and should not happen......

    any idea what might cause this ?.

    It does not seem to be shorting itself to the ground pin (pin 4) in the connector

  • This is a PSU where the step down to a save voltage (20-30V) is handled by the Transformer. The switching regulator then takes this intermediate voltage to generate the +5V. So once you disconnect it from the power outlet, there won't be any dangerous voltages lurking anywhere.

    You should remove the 3 screws (2 on the transformer and one on the PCB) that hold the PCB in place and take it out of the case. Then take a look at the underside of it and upload a photo if possible.

    What kind of square plug do you have? Can it be disassembled or is it molded?

  • Attatched images of the bottom of circuit board - seem to be some corroded solderings at least.

    Also I removed the screws in the bottom and a nut fell out - I presume this is only to attatch the ground wire coming from the socket ?

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  • The solder joints look good... the brown stuff is flux residue and can be ignored.

    The plug you have can be disassembled, there is a metal tab somewhere in the middle of the case that you have to push in slightly, then you can slide the case back.

    You should reattach the ground wire... Good for your health. ;)

  • Is there any schematic for this PSU anywhere - with info about what one should measure at different points ?.

    I can only find some versions that doesn't seem to have much info on them......

    I'l open up the connector to have a look :)