Sovtek MIG 50 blowing fuses

[goldtop5]

This sovtek mig 50 I'm working on blows fuses, even with all the tubes removed. I disconnected the center tap of the output transformer and everything seem normal. Could the output transformer be bad or is it something else in the circuit that could be causing it?

Here is a schematic on it that I found online:

[The Dude]

The transformer could be shorted primary to secondary (easily tested). More likely one or both of the flyback diodes is shorted. For some reason, I can't download the schematic you posted, but looking elsewhere I found a schemo that shows this unit does have the diodes. If you're not familiar, they are the diodes off of the OT primary's outer windings.

[Jazz P Bass]

Sovtek Mig-50 Schematic

Most probably the transformer is toast.
But yeah, check the diodes first.
Here is a resized file.
Courtesy of Bruce @ Mission Amps.
Thanks Bruce.

[soundguruman]

Yeah, it usually blows the diodes first.
Keep in mind that Russian diodes in this amp are labeled opposite of American diodes.
The Anode is marked with a line on the Russian diode.
The cathode is marked with a line on the American Diode...
so don't put in American diodes backwards by mistake. You gotta reverse it...

[goldtop5]

Ok thanks, I'll give it a shot. If the OT happens to be bad, what other OTs can I use in its place? Someone told me that an OT from a Fender Bassman would be comparable.

[soundguruman]

Ok thanks, I'll give it a shot. If the OT happens to be bad, what other OTs can I use in its place? Someone told me that an OT from a Fender Bassman would be comparable.

Bassman, Marshall 50, hammond, etc...is all OK, there is all kinds.
you will tweak the bias rectifier voltage resistor to make it compatible.

[goldtop5]

I replaced the flyback diodes. There were a pair of 1n5401s. One of them tested fine and the other read about 33 ohms. I had 5402s in stock that were the closest match. I soldered them in and the amp was brought back to life. Now all I need is a set of control potentiometers so I can kick this thing out the door! Thanks again guys!!!

[Bruce / Mission Amps]

Most probably the transformer is toast.
But yeah, check the diodes first.
Here is a resized file.
Courtesy of Bruce @ Mission Amps.
Thanks Bruce.

NOTE:
Somebody texted me, quite a while back, that I have the center tap node of the filament supply connected to the wrong part of the B+ rail in that drawing.
It has been a while since I've had one of those amps to check that out!!
But there is no way a 25v capacitor could take screen voltage!!! LOL

[52 Bill]

1N5402 is a 200 volt part? How did you wire it in, single or multiples in series?

[The Dude]

I replaced the flyback diodes. There were a pair of 1n5401s. One of them tested fine and the other read about 33 ohms. I had 5402s in stock that were the closest match. I soldered them in and the amp was brought back to life. Now all I need is a set of control potentiometers so I can kick this thing out the door! Thanks again guys!!!

HOLD ON! The 1N5401 is only rated at 100V PRV which is probably why one of them shorted. The 1N5402 is only rated at 200V, so not a vast improvement in this case. You'll see that the schematic calls for a 3KV diode. I would use 6 1N4007's (3 per side in series like some of the old Ampeg schematics) to be safe, or clip them out all together. The 1N5402's may cause more damage if one shorts and somebody over fuses the amp.

That said, glad you found the problem! Indeed, the pots in those amps are crap. I just got done replacing all of the pots in two of those amps with CTS pots and it was a huge improvement.

[km6xz]

The ring can be on either end, it depends on the class of diode and color of the ring in Russian domestic produced diodes. If you can't read the ID markings, measure to make sure. The id markings either with text or color codes give the basic specifications, such as PIV, semiconductor type etc.