Sunn Concert Bass (Blackface) Restore Help

[misterc57]

I bought this amp head knowing it had undescribed issues needing attention.

At idle I see around -34 VDC at the output jack with no load, and around -25 VDC with a 8 ohm dummy load

This amp has two 6800 uF filter caps and one bridge rectifier. I have attached the schematic which I think may be the correct one.

Still trying to learn solid state amplification workings.

I can see work had been done before on the amp board. See pics.

This schematic shows a + and - 34 VDC rails at the output jack. Does this mean there should be 0 VDC at the output jack? And it looks like the 5W 22R at the output was a replaced component along with some messy work on the underside. Don't care for all the glue on the top and bottom side.

I have plus and minus DC on the relative rails.

All four 0.33R 10 watt are showing negative VDC (in the -27 to -30 range).

Looks like one of the final 5 output transistors has been replaced. The other four are Toshiba Japan

The entire amp is showing a current draw of about 1.25A at the variac meter.

Need guidance. Thank you, Mark

[Jon Snell]

Q1 is a driver transistor, not an output transistor.
There should be 0volts between the speaker out and ground.
The 0volts is determined by the two resistors; R112 & R114 providing a centre voltage equating to 0volts.
There should be no more than 60mA draw when sitting with no signal and no load.

There are a few burnt resistors and bad repair work possibly from the previous repair. That will be a fun project.

[loudthud]

Although this appears not to be the original version with the floating power supply, I would suspect a short within the driver transformer between the primary and one of the secondaries. The only available replacement is from Mercury Magnetic$ unless you can find a used amp with a good transformer. The way to verify this is to remove the four output transistors from their sockets, then check for DC on the output. Check and replace any resistors that are out of tolerance.

[g1]

This schematic shows a + and - 34 VDC rails at the output jack.

Not sure exactly what you mean, but the wiggly line just shows what is located on that board, it does not show connections.

[Jon Snell]

I have never experienced a winding issue with any of these transformers and unless the transformer has been physically damaged, seriously doubt it is faulty.

The most likely causes are;
1/ Centre ground from transformer secondary floating and not grounded.
2/ Output transistor fitted incorrectly no/damaged insulators.
3/ Output transistor leaky.
4/ Balance resistor open circuit.
5/ Errors causing incorrect replacement of pcb track, (wires connecting resistors etc) from unskilled personnel trying to fix it.

This is an extremely simple design and should be straightforward to repair.


Edit. Part schematic added.

[misterc57]

Not sure exactly what you mean, but the wiggly line just shows what is located on that board, it does not show connections.

Yes I thought those were connections.