GK400B DC offset / Blowing Fuses

[Mr_bibbles]

Got a purdy lil' vintage GK400B, however it is giving me some trouble. I've worked some solid state gear before but I'm at a bit of a loss on how to proceed with these symptoms...

1. Fuse was blown when I got it, put it on a limiter and installed a new fuse. Powered up with no load, LIghtbulb flashes very bright!

2. Disconnected board from the Bridge rectifier, amplifier no longer pulls current. I can measure 55 / -55 off the cap leads. This means the in-line, transformer, rectifier and caps are all good, yes?

3. Measured -2.1 VDC on the output, way too high of a DC offset. Note though, that the fuse is blowing with no load attached.


So, would this point us towards a failed driver rather than a failed output? I don't really understand solid state outputs as well as I could, but if the Q4 driver wasn't supplying the 2.1VDC, could that account for the offset and in turn the blowing fuses? Should I try removing the driver to see if that stops the current pull?

Thanks

[g1]

Yes, transformer and supply should be good. As rectifier is good, number one suspect is output transistors, so check Q12 to Q15 first.
Also check for any indication of previous repair attempt as sometimes power transistor insulators get damaged or removed and not replaced.
Measure the +/-55 rails again when bulb is lit. If one is much lower than the other it canl point to the side that is over-conducting. It can also be the cause of the DC offset on the output.