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