ST / STE power supply works but outputs high voltage on 5V and 12V rail - why?

[Chandler]

I've seen a few power supplies like this in the past and would like to try and find the cause of this problem

I've seen it on both the STF / STFM Atari ASP34-2 and STFM / STE DSP 508A power supplies

The power supply works BUT outputs a higher or very high voltage on the 5V rail, eg: i have one that outputs 10V on the 5V rail !! the same PSU outputs 22V on the 12V rail !!

But what components have failed? to cause this

[mikro]

Is it under load?

[Chandler]

Yes

[sety]

Q3 TL431 or something in that area?

[mpattonm]

But what components have failed? to cause this

Elyte capacitors.

[Chandler]

I should have mentioned that I have changed all can type capacitors and on this psu also the TL431

[Altstar]

Have you checked the resistors for reference / feedback for the TL431?

[siriushardware]

Also possibly the feedback optocoupler?

[Chandler]

OK i found R9 (120R) resistor dead

[siriushardware]

I think I'll write that on my circuit diagram, quite a deadly fault, in terms of the damage it could do to the ST, STe.

[Chandler]

I think I'll write that on my circuit diagram, quite a deadly fault, in terms of the damage it could do to the ST, STe.

So do you think it would be safe to test the PSU now? obviously not connected to an ST but under load

[siriushardware]

If you have replaced the resistor, yes, test it with loads (but definitely not in an ST or STe yet). What are you using for loads for the +5V and +12V? chunky resistors? Light bulbs?

[Chandler]

Light bulb

[siriushardware]

OK then, give it a go and see what output voltages it delivers (a) with no load and (b) with a lamp load. Is this one of the versions which has an adjustment preset on it, or one of the ones where the 'adjustment' is set by fixed resistors?

[Altstar]

An old CDROM drive (with an adapter) works well too

[Chandler]

OK then, give it a go and see what output voltages it delivers (a) with no load and (b) with a lamp load. Is this one of the versions which has an adjustment preset on it, or one of the ones where the 'adjustment' is set by fixed resistors?

It's the Atari PSU ASP34-2 with fixed resistors (yes i have tested those also)

[siriushardware]

An old CDROM drive (with an adapter) works well too

Not in this case, I presume you read that the PSU in question was originally producing gross overvoltage on both rails? If it is still doing so after replacement of the failed resistor then any 'smart' load, like a CD-ROM drive or HDD would be instant toast.

That's why the process is to try it with no load, then a 'dumb' load like a 6V motororcyle lamp (for the 5V output). It shouldn't be alllowed near any kind of computer hardware until it is definitely producing the right output voltages first.