Univox bass head repair. Puzzled

[Bob-I]

A friend found a Univox bass head cheep, only it has no volume.

I opened it up and found the impedance switch completely fried, almost melted down. I disconnected it and wired in a jack directly and it sounded great.

So I bought a replacement switch, and while I was at it I replaced the 4 speaker jacks. Now we're back to the orignal problem. No volume, and the output tubes current is running away. All voltages check out, plate, screen and grid.

The preamp puts out the expected signal so it's in the output stages.

Any thoughts?

Here's the schematic.

http://schematicheaven.com/bargainbi...ox1050bass.pdf

[MarkB]

A friend found a Univox bass head cheep, only it has no volume.

I opened it up and found the impedance switch completely fried, almost melted down. I disconnected it and wired in a jack directly and it sounded great.

So I bought a replacement switch, and while I was at it I replaced the 4 speaker jacks. Now we're back to the orignal problem. No volume, and the output tubes current is running away. All voltages check out, plate, screen and grid.

The preamp puts out the expected signal so it's in the output stages.

Any thoughts?

Here's the schematic.

http://schematicheaven.com/bargainbi...ox1050bass.pdf

When you wired it directly, which tap did you use? Did you try them all indvidually? How were the jacks grounded originally?

[skyboltone]

Hi Bob:
I know you've done this already so I'm cautious to ask; did you disconnect the switch and jacks and check for output at the OT taps?

I kinda think that the switch got between a short (like a cable) and the OT. Now the OT has a ground, perhaps intermittent on it. Have you got a megger? Now's the time to check for insulation integrity in the OT. You might even replace the OT for good measure. If it put out enough power to fry things I doubt it's in tip top shape.

Dan

[Bob-I]

When you wired it directly, which tap did you use? Did you try them all indvidually? How were the jacks grounded originally?

I wired up the 4 ohms tap. After I wired in the new jacks and it didn't work, I removed the jacks from the chassis so that they weren't grounded. Same issue.

I'm getting the same response on all taps.

[skyboltone]

The Output Transformer is internally shorted. The output tubes, through the primary winding, are working into a shorted OT secondary.

Dan

[Bob-I]

The Output Transformer is internally shorted. The output tubes, through the primary winding, are working into a shorted OT secondary.

Dan

That's what I was thinking. Thx for the confident diagnosis.

[Bob-I]

The Output Transformer is internally shorted. The output tubes, through the primary winding, are working into a shorted OT secondary.

Dan

That's what I was thinking. Thx for the confident diagnosis.

Hot damn, you hit it right on the nose, only it's not an internal short, it's on the ground wire as it enters the xformer. I heatshrinked it and the amp's working great!!!

[dehughes]

The Output Transformer is internally shorted. The output tubes, through the primary winding, are working into a shorted OT secondary.

Dan

That's what I was thinking. Thx for the confident diagnosis.

Hot damn, you hit it right on the nose, only it's not an internal short, it's on the ground wire as it enters the xformer. I heatshrinked it and the amp's working great!!!

Sweet! That's a nice feeling, when you get something up and running after it has been "mysteriously" down...

[skyboltone]

Hot damn, you hit it right on the nose, only it's not an internal short, it's on the ground wire as it enters the xformer. I heatshrinked it and the amp's working great!!!

Good on ya! I think bad trannys are repairable quite often.

DAn

[Ron]

Xsistors ferever.

[Ron]

But why did the switch fry?

I'm guessing the amp had a shorted load, which fried the switch and heated the hot lead into the OT. The hot lead's insulation melted enough to short to the grounded lead.

Gimme high voltage over high current anyday. I'd rather jump than burn.

[Bob-I]

But why did the switch fry?

I'm guessing the amp had a shorted load, which fried the switch and heated the hot lead into the OT. The hot lead's insulation melted enough to short to the grounded lead.

Gimme high voltage over high current anyday. I'd rather jump than burn.

Either that or one of the output tubes shorted. Either way, I'm just happy for my friend, a $75 amp that sounds great. It's got 3 orignal Tung-Sol 12AX7's that sound great. The Sovtek 5881s are brand new so I'm guessing the prevous owner replaced them for a reason.