Akai AMX10 losing B+ Voltage

[average joe strummer]

Hello all.

New here. Glad I found this forum looking at the knowledge here. I usually work on guitar tube amps but took on the repair of a small Akai PA head. Symptoms were speakers cutting out. I looked at the speaker relay, suspecting it was the culprit but it tested fine. I tested a few points on the PCB where voltages were printed and notices the B+ wasn't stable. It was in-fact disappearing until so low that the relay opened and cut speaker power. The B+ then spiked back up, closed the relay and speaker functioned again.

So, I measured across the main filter cap. Voltage was rock solid, but if I place the negative probe on chassis ground, and positive on the cap, the voltage started dropping towards zero again. I'm at a loss as to where the voltage could be going. Would a leaky cap send voltage to ground? Any help would be massively appreciated.

AJS.

[benj38]

Can you upload a schematic we can all look at?

Your description hints at the possibility of some loose ground connection. Some instruments transmit the negative side of the voltage from the filter capacitor (which is connected to ground) to other boards not through a wire (like the plus side), but through the chassis, and screws that clamp the ground PCB trace to the chassis.

[andy3055]

Can you upload a schematic we can all look at?

Your description hints at the possibility of some loose ground connection. Some instruments transmit the negative side of the voltage from the filter capacitor (which is connected to ground) to other boards not through a wire (like the plus side), but through the chassis, and screws that clamp the ground PCB trace to the chassis.

Like benj38 has mentioned, there should be a bad connection. Re-solder the filter cap terminals-especially the negative.

[average joe strummer]

Thanks for the replies benj38 & andy3055. 

I couldn't find a schematic anywhere. If you know a secret site where rare schematics are located....I'm all ears 

I'll have to strip the power board out to get to the solder lugs, but will do that today and get back to you.

Thanks again gents.