Universal Audio LA-610 schem

[Swampy]

Hey, does anyone have a schem for this lying around? I have an elusive short I'm on the hunt for.

Ta
Swamp

[Enzo]

Only thing I have is the old original LA2A that it is based on.

Urei LA 2 Schematic

[Swampy]

Thanks, but it looks quite a bit different to the board I have in front of me.

I'm reading 0 ohms resistance on pin one of each tube socket with the tubes yanked...surely I'm on the right path here??

[Swampy]

I lie...I'm looking at the sockets backwards. Duh

[Enzo]

What seems to be shorted to what? What does it do wrong?


And have you contacted UA and asked for schematic?

[Swampy]

It's popping the fuse immediately on powerup. Power tranny is good, rectifier diodes are good and filter caps are good. That's where I'm at on it currently.

I messaged UE but yet to hear from them. Naturally, the owner wants it in the morning.

[Enzo]

I don't know the unit, is it ALL tube, or tube and solid state or straight solid state? If the PT is OK with nothing on its secondaries, try connecting the secondaries one at a time to see which one is causing the excess current. Use a variac or bulb limiter to save fuses.

And measure where the secondaries go before connecting them back up. You could have four good rectifier diodes in a bridge format, but a cap across the incoming AC could be shorted without upsetting the diode testing. And if you tested filter caps with a meter for shorts, a cap might not look shorted to a meter that uses 1 volt for testing ,but still be a dead short when it sees 400v. Disconnect the filters to see.

[Swampy]

It's tube and solid state - tube preamp and opto compressor section.

The secondary taps connect by way of a plastic plug type connecter ( I don't know what name these are given?) but I've managed to ascertain when I lift the bridge rectifier diodes on the filter cap side of the unit stays powered up, so the short is obviously in this part of the circuit.

[Enzo]

B+ or SS supply? Even though they may test on a meter, try getting rid of the filter caps. Just unsolder one end and lift from the circuit if possible. We don;t care if that makes things hummy during test, all we care about is not blowing fuses.

[Swampy]

Ok, I've been experimenting with this unit alot over the past two days and come to the following conclusion.

When I d/c the HV connection (leaving just the heater tap and phantom power tap operational) and power up the fuse still blows, and when I d/c the phantom tap she still blows, so there's something going on in the heater section causing an overcurrent.

So I took out a couple of filter caps (it's a DC heater supply) and replaced a voltage regulator and the unit holds now, but when I reconnect the HV powersupply to the bridge rectifier it blows it's fuse again.

Powering up via a lightbulb current limiter, the bulb glows bright and dims over the course of 30 seconds. Then, if I unplug the unit and plug into a wall outlet it holds its fuse.

So it's inrush overcurrent that's blowing the fuse. Once the caps are charged it holds.

I've been testing with fast blows but perhaps it needs slow blows? I've put considerable effort into tracing this 'short' and all efforts to date have resulted in utter confusion and dismay due to the elusive nature of this problem.

Chur,
Matt

[diydidi]

Swampy. did you manage to get it s(h)sorted?
I too need a schematic.