Acoustic 470 Hum Help!

[misterc57]

This amp looks like it has had a ton of work done in the past. This amp has what I think is a 120 hz hum.

I noticed the 4700uf 50V cap starting to fizzle/spew smoke on its plus side. This cap is adjacent to the huge 7800uf 100V cap.

I measured 78 VDC on the positive side of the 4700uf cap. which is rated at 50V. On the board someone wrote 2000uf 50V (perhaps that was the original value of this cap).

Guessing I have an issue before this cap since the voltage is too high.

I have attached images and what I have for a schematic. Having a hard time finding this cap in question on the schematic. Would like to back trace and check components and try to figure out why voltage is too high.

Thank you!

Acoustic470PA (1).pdf

[Enzo]

It is on the right near the speaker jack. it is the speaker cap. And the schematic calls for a 2000uf 50v cap there.

If you have +78v on the positive end, that means your amp circuit is failed.

[misterc57]

Looks like a lot has been done in the past; perhaps all output transistors, bunch of caps and resistors. I can see a burn area on the pre-amp board that has been repaired also.

Is that C409 and does it say there should be +46V there?

Any suggestions on where to start? Start taking readings across the units test points on the schematic?

This cap will keep burning up while I analyze with power on.

Thank you

[Enzo]

So disconnect it.

It is only there to block the DC from the speaker.

It says +46 because this is a single supply amp with an 92v supply, so the "output" will normally sit at half the supply voltage. So approach it like any solid state amp with V+ on the output. COuld be shorted output transisor, could be any one of several smaller transistors further back, could be an open resistor.

[misterc57]

I still struggle with solid state concepts and diagnostics. I am more comfortable understanding tube amps.

Checked a bunch of resistors on the amp board. There are (2) 3 watt 1 ohm resistors, R424 and R425. I am not sure which is which but one of them is reading 62M ohms. I snipped it out and jumped a .33 ohm (3 watt) in there, between the resistors and jumper cables I am at about 1 ohm total resistance. Now when I dial up the variac to the amp my current starts to peg very fast with less than 10 volts being read at C409 (which is still in circuit at this time).

[Enzo]

And something is pulling a lot of current through that resistor, probably a shorted output transistor.