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Yesterday I received a donation of hardware from a childhood friend, including this lightly used in its original box, with perfectly preserved manuals.

Bernie

In spite of its excellent conditions, this does not power up at all 😭

The AC fuse in the PSU and the DC one on the motherboard aren't blown, so my next suspect is the "wedge" power-supply.

These PSUs made by in the '80s are known to fail often. Sometimes, they even output higher voltages on the +5V line which powers ALL the chips on the board! 😱

But this isn't the case: the power LED is steady off, and I measured near 0 volts on VCC pins.

Last night I carefully opened the wedge PSU with flat screwdrivers, but the interior is filled with a potting resin which is stuck to the box.

I measured +5V after the regulator, but it drops as soon as I connect the board.

If the board was shorted, the fuse would have blown. So I guess the regulator is unable to source more than a few milliamps.

I couldn't make further progress without tools. I'm literally working from my mother's kitchen! 🤓

I detached the keyboard to test the power LED with a battery: it works.

I cleaned the contacts with isopropyl and plugged the flat cable back on the board and now... IT WORKS! 🎉

Was it just an oxidized contact? I'm not sure.

Anyway, the tuner video signal is very noisy. I wrote this short program to test sound output.

First time I play with the BASIC V3.5 multimedia commands, and it's awesome! :commodore:

@codewiz what do you measure on the power plug? Careful not to short out the pins! :)

@codewiz another candidate is a corroded power switch. Can lead to „no power“ symptoms.

@root42 Aha!

By the way, I found schematics and service manuals for the Plus 4, but couldn't find gerbers or pictures to help follow traces on the board. Know where to look?

@codewiz no, you go by the silkscreen on the board. :)

@codewiz yeah I guess the regulator is borked. Ditch the old PSU, buy a round DIN socket for the Plus/4 and use a modern C64 PSU. Sad that the PSUs are all potted. Otherwise one could easily repair them.

@root42 Even these C64 -> Plus4 adapters cost a lot, like 15 EUR + shipping.

Ok ok, I guess spending 2 hours repairing a PSU would cost more, but... isn't it more fun? 😂

@codewiz if the PSU weren‘t potted: go for it! That’s why I love the original 1541. You can repair the whole PSU section easily.

@codewiz Check for anything in the Plus4 that's hot - it's possible the PSU is delivering the 5v to something that isn't quite a short; I had that recently in a PoE switch.

@codewiz what exactly did you clean now? Power supply socket / plug?