dbx 160SL problem

[psych60s]

Hello

I'm currently servicing a dbx 160SL with a "motorboating" problem on both channels. I got the schematics from dbx.

I made a quick test with a 1kHz tone :
It seems that the motorboating problem is present even at the input stage. The VU-meter when set to "Input" or "Output" display the motorboat sound. All controls are ineffective, and the meter doesn't react when controls are tweaked.

I've also played with the "THRESHOLD" control, which is ineffective too. However, threshold LEDs are working, and correctly display what's happening (motorboat sound peaks light the red LED).
If I set the meter to "Gain Reduction", I can SEE that controls are tweaked but I still can't HEAR any change. The meter display the gain reduction that should take place, as motorboat peaks logically make the needle dancing.

When bypassed, the unit outputs the 1kHz tone without problem.

Any idea about what I could be? I suspect a PSU problem but I don't know exactly how the 160SL has been designed so I may miss something. Could a faulty condenser in the PSU allowing signal to be shorted to ground and create kind of a feedback loop?

Thanks for your help!

Quentin

 

[PRR]

Motorboating on both channels almost HAS to be the common power supply.

 

[psych60s]

Seems that the PSU card is all OK... There is a header dedicated to supplying the different power voltages to the main board, with decoupling caps around it, one of these caps has a slightly bulged top.

I've also seen a very ugly leak at the bottom of an heatsink in channel 1. Two caps around it are the usual suspects. Could be a problem as they are also coupling caps.


Thanks for you help anyway!

Regards

Quentin

 

[pucho812]

bulging caps and leaky caps need to be replaced. it's a sign of a problem in a cap.