Both SSL 661 PSU Tripping on both rails - SOLVED

[mike_signo]

Hello everyone.
I'm got an SSL 4056 G+ ultimation that keeps tripping.
We are running 2 661 PSUs with 1 Changeover unit as specified by SSL back in the day.
Recently it started tripping in a way that I've never experienced before.
Both rails on both power supplies trip and the console goes down with it.
Does anyone have any suggestion on what it could be?
In the past I had one of the rails go on one of the PSUs, I swapped the 661 and I was fine for a while.
Since it's both the PSUs tripping could it be the Changeover unit?
It happened every month or so but now it's daily so it has become a massive problem.

any help or suggestion is very welcome!
in the meantime I'll take apart the spare 661 we have and replace the heat compound.
All the best
Michele

[horizonsound]

I would suspect the bridge rectifiers in the changeover unit. There is not much else going on in the changeover unit.

Can you test it without the console on (buckets turned off, or power cables removed) and see if it still trips?

Anthony.

[mike_signo]

Hi Anthony!

Should I just turn everything off on the console side and see if it still trips?

Thank you
Michele

[horizonsound]

Yes, turn everything off at the console end.

Anthony.

[mike_signo]

Hi,
I ended up having to open up the PSUs and check the +20V and -20V output transistors.
With horror I discovered the previous tech didn't bother with matching them at all!!!!
The beta ranged from 51 to 463 which is awful.
I actually very surprised that they managed to work at all for so long.
Now I've matched the transistors taking them from a big pool of spares I had around and they are +-2% from each other.
I've alligned the +20V and -20V under a 15A load with a load box that I made exactly for the job, 3A for the 7V.
I've put them back in service last week and the console has behaved very well since then.

[radardoug2]

A power transistor with a 463 beta??? I dont think so, that sucker was already dead.