Hi all,

Sorry to hijack this thread. I am a bit of a forum noob so not familiar with the etiquette.

I have a Mackie thump 12A with the th-15a power supply and amplifier. I got it for free off facebook and I don't know its history, all I know is that it wasn't working. Visual inspection reveals some brown discharge on some of the PCB's. There was also no R108, all that remained was a radial lead sticking up. I have replaced with a 5W equivalent.
With everything connected (except speakers) the power supply appears to cycle and just recently has starting giving an audible chirp intermittently. (this is only recent)
If I isolate the DSP board ribbon cables the cycling appears to stop.
With the DSP board connected the voltages fluctuate as I expect.
With the DSP board disconnected the voltages up to the amplifier board appear to be stable. Although at one point I recall seeing 59V at R200 and 0V at R201. However I can't recall what configuration of connectors I had at the time.
Can I assume a faulty DSP with a short somewhere?
I am a student and have not had a great deal of experience in repairs but I am keen to learn. I can read a schematic and I am a licensed electrician so I am aware of the safety aspect.
I have a multi and an old single channel oscilloscope (that needs a good de-gaussing).

 

Further to the above. Please ignore the 60V and 0V comment above. I must have been trying to reference to supply earth and not ground.
With preamp and DSP boards disconnected. Everything appears to behave.
As soon as preamp
Board is connected using either/or/both ribbon cables the power supply cycles.
If disconnect the small XLR input board from the preamp, everything behaves again.
Very strange.
There are no visible signs of damage with the input board.