Unused for a year and then switching on , nasty magnetisation type noise
and "B" 1.2 amp thermal breaker trips out.
This disengages mains Tx secondary middle from ground , via a triac
system crowbar fashion.
Selecting DDT disabled, then amp runs ok. I tried running on a variac at
60% mains, with DDT enabled , to see if it would run and then probe
difference between A and B channels. Ran ok,so turned up variac
progressively, but now even at 100% , amp runs normally.
So presumably a cap somewhere got reformed, how to track down which one?
Other than ESR meter on caps/replacing all,probing for A/B differences,
anyone aware of where a failing cap or more generally zener , opamp or
whatever may lie, in the DDT processing section?

N_Cook wrote:

Unused for a year and then switching on , nasty magnetisation type noise
and "B" 1.2 amp thermal breaker trips out.
This disengages mains Tx secondary middle from ground , via a triac
system crowbar fashion.


** Nonsense - the triacs are used in a speaker protection crow bar.

The power amps use PSU centre taps as speaker outputs - like some QSC amps.


Selecting DDT disabled,
Ran ok,so turned up variac
progressively, but now even at 100% , amp runs normally.
So presumably a cap somewhere got reformed, how to track down which one?
Other than ESR meter on caps/replacing all,probing for A/B differences,
anyone aware of where a failing cap or more generally zener , opamp or
whatever may lie, in the DDT processing section?

** What happened is that ch B produced a large DC offset when switched on, probably due to bad contacts on the many push fit connectors OR maybe a leaky coupling electro somewhere - THEN the triac crow bar activated and shorted the output.


.... Phil

no bad ESR'd electros found. 2 bad spade cons on the A channel. All TO3
bolts needed tightening, 2 on different TO3 about 1/2 turn loose.
Perhaps that and cleaning/reseating opamps will have trapped the
problem. Reassemble and powerup again and see.