Yaqin mc-100 v3 bias trimmer smoking/heating up

[mastajax]

Hello friends,

I have a yaqin MC-100b that ran fine for months then one day it was turned on and smoke poured out of v3 bias trimpot area so I pulled it apart (SEE PHOTO) and found v3 bias trimpot had blown open!

I replaced this trimmer and energized it but same issue happened with new trimmer!

Can someone please advise on what to check?

[anneo]

On the schematics I have there is a W1 and W2 to adjust the bias.
No V3
Without more details,impossible to help.
Anne

[mastajax]

Hi Anne...

V3 is the tube socket.... it is the only bias trimmers for the amp on the power tubes if you look at the schematic.

You are right it is W1.

Thanks for your help!

[anneo]

If W1 is the only one for adjusting the bias,clairly your amp is different from what I have here.Do you have a schematic of that one ?
Anne

This is what I have,

[anneo]

Well,well on the french forum there suddenly is a second bias trimpot
So my schematic could be correct,exept the crossing left of C6 is no connection(the point shoudnt be there)
The only way I see W1 go up in smoke is a shorted R19.
If the -70V was the culprit W2 goes too.
The other connection to W1 is a 100k resistor ,not burned? then nothing wrong comes from there.
Anne'