Hiya
I am repairing a Dynacord Powermate 1000-2 mixer/amp and have a question about DC blocking capacitors in the signal path.

I am getting a slowly dropping DC on the DISP1 node (output of I6AA op-amp) on switch-on. This is causing the green "signal present" LED to come on on that channel which takes about 30 seconds to turn off. It seems to be caused by a charge on C35A that dissipates slowly through R54A (1M Ohm).

It seems to me, it would have been better to put the C35A on the output of I5AA with a resistor to ground. Is there a reason why this would not work.

I have checked all relevant components and power supplies and I cannot find any faults but I'm sure this is not normal behaviour.

Any help greatly appreciated.

 

If you have multiple channels, compare signals using an oscilloscope, for example, the signal at TL072 I6AA pin-1.

 

I did compare the signals on the other mixer channels and they all get the DC on pin 1 of I6AA. It jumps up to about a volt then slowly drops back to zero. The mixer works but all the signal present leds stay on for up to 30 seconds. At first, I thought it was a power rail issue as it hapens to some degree on all channels.

 

Just as a test, try putting a 10kΩ resistor at I6AA from pin-2 to GND.