Dynacord Powermate 1000MK1

[Aloon26]

Hi all,

 

I am new to post here but have been using the forum for years !

 

I have been repairing Dynacord desks for many many years and have come across a fault that I cannot rectify .......any ideas ??

 

The desk came to me with the protect only light on (not limit), on inspection all of the output stage was in good order and running, I found two diodes D828,D518 dead short and an A92 Q535 shorted, these are in the ‘time_in’ circuit.

 

I replaced all faulty components and the desk fired up and ran, Happy days I thought .......I rebuilt the desk and connected up for a full system test and on switch on the fans continued to run past the time delay and a small pop came from the desk !, on inspection the same diode,D518, had poped !, I replaced again and it seems the -11 ref voltage on D518 was very low, 3v and slowly rose to the correct -11, until the voltage reached the ref -11 it would always blown the diodes with the HT connected, I have replaced all the transistors in the right hand amplifier protect circuit and several capacitors with no joy, my next step is to replace the TL074 IC, does anyone know the theory of operation for the protect circuit on a Dynacord desk ?

 

Any help would be much appreciated