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.

[misterc57]

Not sure if this means anything but...

R103 has 25 VDC on the Q102 (BC141) C side and 4 VDC on the Q102 B side. Adjusting the bias pot changes the V only a couple V up or down at the base of Q102.

R103 measures 15K in circuit.

[Tom Phillips]

...the wiggly line just shows what is located on that board, it does not show connections.

That was a really goofy drawing style using the bold wiggly line with the terminations sitting right on the line.

[J M Fahey]

Some of what I want is redundant but the idea is to double confirm some values.
The old Carpenter´s rule: "measure twice and cut once"

¨Please post:

Q101 Emitter, Base and Collector voltages to ground with R104 bias pot set to halfway.

Q101 voltage Base to Emitter

Q101 adjustment range (say, from +3V to +5V or whatever) relative to ground.

Does Q101 emitter voltage track its base voltage?
It "should" stay about 0.65V below but here we have damaged amp, so ....

Q1 B, C, and E voltage to ground.

Q1 Vbe

Do Q1 B and E voltages rise/fall following R104 adjustment?

As of power amp offset:

Voltage across R112+R113 must be same (or VERY close) to voltage across R114+R115 ... how much?

Voltage across R113 and R115, which is power transistor bias voltage.
I expect some 0.53V each, which is underbiasing, which means some crossover distortion present but it is inevitable.

They must be biased cold since there is no thermal compensation included.

Acceptable in an amp played LOUD in a NOISY environment, he he.

[misterc57]

Appreciate the guidance! I've had to put this one aside for a while but hope to be back to it soon.