On 12/13/2011 8:07 PM, Phil Allison wrote:
** Hi,

I have a VOX AC100cp ( Chines made) on the bench - it is about 6 years old.
It has all its original parts but is in a very bad way. The known faults
a

1. One (Ruby) EL34 has lost vacuum.

2. Two scorched 5W screen resistors, one detached from the PCB.

3. One blown HT fuse, a 1 amp slow blow - which may well not be the only
one.

4. Output tranny OPEN on one side of the primary and exhibiting symptoms of
shorted turns.

5. Bridge rectifier only conducting on one half cycle - giving 50Hz
ripple.

The scorched 5W resistors are on the side with the open tranny winding.

The EL34 with no vacuum is on the other side.

Wanna have a go at the sequence of failures ?



... Phil





Is this the output stage in question, on page 5 (assuming the head and
the combo are constructed the same)?

http://www.ampix.org/albums/userpics...x_AC100CPH.pdf

It looks like there are a few bridge rectifiers in the circuit, one for
the solid state preamp stuff, one for the output tube bias, and one for
the preamp heaters. Which one failed?

It would seem that a loss of bias on the output tubes might account for
some of the problems, but I'm guessing that you've already considered
that and that the failure mode may actually be more subtle.