Hi there,

I have a third party C64 PSU (Micro Mart).

I was given this with a C64c which had fried RAM (hmm...)

Having replaced the RAM I've tried the PSU quickly and it seems to power the C64 up but the PSU is making a fairly loud buzzing noise.

I've posted some photos below of the inside and of the connection - assuming that this pse caused the RAM to fry and is faulty, how can I test it and repair it?

I have a multimeter but have no idea what I should be doing to test it.

Pics:-
Pic 1
Pic 2


Many thanks
Lee

Having a failed DRAM from a C64C is something which happen rarely than on breadbin, so I'd suspect the PSU as well.

The design of that Micro Mart brick is surprisingly simple, I thought it was something of higher quality given the available space. I would say that the design is the same of a breadbin Power Supply.

No need to diagnose it, to my opinion: just replace Diodes/Capacitor/Regulator and check if the buzzing noise goes away. Is the regulator a 7805 or a 7805CV?


Cheers,
Oge
_________________

Hi,

Just checked, 7805CV,

So would this one be a direct replacement?

As for the diodes, not sure what I'm after?

Thanks
Lee

Diodes are usually marked 1N4007 or something like that.
You could check here:
http://www.ebay.co.uk/itm/200737932079
I'd go for at least 2A ones, maybe the 1n5408 - if the pins will fit the original holes.


One of the 9VAC pins in the connector looks very bent, does it go in correctly?


The iron core transformer itself can make a buzzing sound, only way around that is to put put a bunch of epoxy on it, wait for it to harden and then check again and add more if it's not quiet...

You should measure close to 5V DC on the closest together pins in the DIN-connector. Multimeter set to DC, then probably 10VAC on the other two pins with the multimeter set to AC.

If that's OK I'd measure the 5V when turning computer on, if it stays at 4.8-5.2 everything is cool. Goes over 5.5V shut it down right away!

The user port is an easy place to measure, or over a logic IC if the computer is open. The large capacitors on the right is an easy place as well to attach a clamp. GND for measuring 5V is everywhere...

If it's on and you start hearing the buzzing noise, try pushing the iron core transformer with an isolated material, to check if the sound changes. Not uncommon that the caps can make noise as well... next thing to press.

Hi,

thanks for the replies,

I have changed the regulator and the loud buzzing noise has now gone,I need to put my ear next to the PSU to hear it now. Of course this could be because having removed / replaced the iron core it has been screwed in tightly?

The pins do go in okay, I've now straightened them a bit with plyers.

I have multimeter but when I connect to the 2 close together pins or the other two pins I get nothing when measuring voltage, I do however get something when measuring current. Either I'm doing something wrong or my multimeter can only pickup voltage above a certain amount?

Here is a pic the exact multimeter I have, any help / guidance appreciate
http://www.whiterosetools.com/..._3CG9V.jpg

Cheers
Lee

Don't measure current on the pins, you're shorting the PSU and can blow components (fuse, if any, should have blown).

To measure voltage put one test pin connector in the black hole and the other in the middle hole. Turn knob to 20 on the V= scale, measure the two close pins in the DIN connector. 5.23V, nice... (maybe)

Turn knob on Multimeter to 200 at the V~ scale, check the other two, should probably be around 10.

Yes, tightening those screws may have helped.

Whoops okay, thanks

Still wasn't finding any voltage, thought might have blown fuse so changed that. no luck so tried a 9v battery, nothing showing on that either so maybe multimeter fried or just duff?

Plugged PSU quickly into a battered C64 and it powered up okay so power is obviously there.

Maybe I just need to get another multimeter?

Thanks both for all the help.

Cheers
Lee

I bought one of these as an extra, has worked well.
http://www.ebay.com/itm/280799777718

Check your test leads... maybe turn the knob a few times see if it starts working.