Kustom Lead 3 s - repair

[Mathias]

I have a vintage Kustom Lead 3 s on the bench and need some advice on the repair. Buddy said the amp started smoking on startup.

It had a blown output transistor and a burnt resistor (R20) which I replaced. Attached is a schematic of the power section.
Without a load and on the bulb limiter the amp has no shorts. But when I connect the speaker the amp seems to short out. Bulb is fully lit.
I checked with another cab and same issue. Speaker is fine, the jack too.

I checked the driver (Q5) if it had blown out too but it reads out fine on a component tester.

I would like to hear some suggestions as to what is going on or what to check. Should I check all the other drivers too for shorts?

Thanks in advance.

 

[radardoug]

With parts replaced, do you read dc on the output? Then other parts are blown.

 

[Voyager10]

To expand on that - can you (safely) measure the voltage between ground (at the input jack) and (a) the output, (b) the +40V supply, and (c) the -40V supply (without a load connected)?


if connecting a load causes an immediate problem, my initial guesses would be either a fault which causes a large DC offset on the output, or the amp bursting into oscillation.

 

[Mathias]

With parts replaced, do you read dc on the output? Then other parts are blown.

Yes, there is about 90mv DC on the jack. I'd had the same suspicion. Something else is blown and the jack has DC voltage.

 

[Mathias]

To expand on that - can you (safely) measure the voltage between ground (at the input jack) and (a) the output, (b) the +40V supply, and (c) the -40V supply (without a load connected)?


if connecting a load causes an immediate problem, my initial guesses would be either a fault which causes a large DC offset on the output, or the amp bursting into oscillation.

I've metered the following DC voltages:

a) 90mv
b) 17.65V
c) -17.65V

My guess on how to proceed would be to start lifting components around the drivers. Any suggestions as to what would be likely to fail/check first?

 

[radardoug]

90mV is not the problem. That is typical for a working stage. As others have said, it may be oscillating. You should put it on a dummy load with a good scope across it.

 

[CJ]

plug it in with no load and measure dc volts on Q1 and Q2 2N4249

EBC for each to ground, and also between E and B for each, set meter on millivolts dc for E-B,
do not short B to C with probes,

report back immediately...

 

[Voyager10]

OK, +/- 17V on the power supply, when there should be +/-40V, is an immediate red flag.

Was this with a light bulb in series with the mains (for current limiting)? If so something is causing the amp to draw way too much current.

One possibility it that the capacitors C3, C5, C7 or C8 have failed. There should be several volts across each of them. If any read 0V or close, then it probably means they need replacing.

As CJ mentioned, measuring the voltage between base and emitter of all the transistors is useful. Should be 0.6-0.65V for all of them, except Q6/Q7 which are for protection - Vbe should be close to 0 normally. (Wrong voltages doesn't necessarily mean the transistor itself is faulty but it's useful for diagnosis).

 

[Mathias]

plug it in with no load and measure dc volts on Q1 and Q2 2N4249

EBC for each to ground, and also between E and B for each, set meter on millivolts dc for E-B,
do not short B to C with probes,

report back immediately...

Measured the following:

Q1
E: 665mV
B: 31mV
C: -18,75V
E-B: 626mV

Q2
E: 659mV
B: 23mV
C: -18,13V
E-B: 617mV

 

[Mathias]

OK, +/- 17V on the power supply, when there should be +/-40V, is an immediate red flag.

Was this with a light bulb in series with the mains (for current limiting)? If so something is causing the amp to draw way too much current.

One possibility it that the capacitors C3, C5, C7 or C8 have failed. There should be several volts across each of them. If any read 0V or close, then it probably means they need replacing.

As CJ mentioned, measuring the voltage between base and emitter of all the transistors is useful. Should be 0.6-0.65V for all of them, except Q6/Q7 which are for protection - Vbe should be close to 0 normally. (Wrong voltages doesn't necessarily mean the transistor itself is faulty but it's useful for diagnosis).

C3: 23V
C5: 26mV
C7: 25V
C8: 2,5mV

Seems like C8 (and maybe C5) is gone too.

 

[CJ]

Diff pair looks good, move on down the line,


Measure e-b voltage on q3,4,5...

 

[Voyager10]

With those readings, it's most likely C8 has failed short-circuit and needs replacing. (It should be easy to check with a meter).

I would also replace R13 and R16, as they will have been cooked if C8 is shorted.

 

[Mathias]

With those readings, it's most likely C8 has failed short-circuit and needs replacing. (It should be easy to check with a meter).

I would also replace R13 and R16, as they will have been cooked if C8 is shorted.

C8 was indeed shorted. I've replaced C7/C8 to check. Still a short on the speaker. I suspect C5 is gone too and I'll check those resistors.
Rail voltage is correct now by the way. Sitting at + and - 40V.