Arp Omni repairs

[sneakthief]

Currently repairing an Arp Omni for a client.

This Omni is in really, really rough visual shape with some strange button replacements. The power supply has a charred current-limiting resistor and the board is burnt.



I believe this happened because the psu screws were loose on the left and the PSU heatsink was no longer able to dissipate heat to the case.

Considering how flakey these PSU's are, I ordered this instead - which I'm sure will make the rest of the job easier and prevent another notorious Omni PSU blow-up

http://de.mouser.com/Search/ProductDeta ... 7-BB15-1.5

I just wonder if any other damage was caused by the PSU blowing. The other rail wasn't putting out voltage.

I'll find out soon enough when the new PSU is delivered.

[yorgatron]

I believe this happened because the psu screws were loose on the left and the PSU heatsink was no longer able to dissipate heat to the case.

is that paint on those screws?

you might want to use some Loctite when you put it back together.

something to look for when I get mine opened up.

[sneakthief]

I imagine ARP used something similar to Loctite. I guess it doesn't last 40 years

Instead of Loctite I'll use bolts with nylon thread-locks with the new PSU.

[schmidtc]

Arp put a dab of paint on most internal bolts to know when to void the warranty. Good luck, omni's can be a total pain.

[Dr. Phibes]

I'm not sure if that heat damage can be contributed to a bad heat sink connection (although I'm sure it didn't help)

Maybe that uA723 regulator has failed and the pass transistor is constantly saturated? It just seems that current limiting resistor is the element that's been generating the most heat, not the pass element itself. Or maybe it's the +15V trimpot on the sense input that's out of whack?

[HideawayStudio]

I'm not sure if that heat damage can be contributed to a bad heat sink connection (although I'm sure it didn't help)

Maybe that uA723 regulator has failed and the pass transistor is constantly saturated? It just seems that current limiting resistor is the element that's been generating the most heat, not the pass element itself. Or maybe it's the +15V trimpot on the sense input that's out of whack?

Yes, exactly what I'm thinking - yes the Omni PSU is a little sketchy at times but it's not as if it runs at 180C and needs the heatsink from h**l. My guess is that something much further downstream has short circuited - this is depressingly common in Omni's that haven't at least had their tantalum caps across the rails changed. It's either this or, as Dr.Phibes suggests, the pass transistor has gone awol and the PSU has dumped full whack into the rest of the instrument - in this scenario, if you are lucky, the synth has been retrofitted with crowbar diodes as I seem to recall this is one of the more unfortunate failure modes in the Omni.