Kustom 400, burned 100 ohm resistor

[52 Bill]

Glad to hear that it's working. Whenever I mount a power transistor, as soon as it is tightened down, I test for continuity from chassis to case. Because the collector is wired to the power supply there will often be some sort of reading, but except for a few rare amps, there will never be a constant low reading.

On Kustom amps the collector wire is separate from the mounting screws, so as soon as the transistor is mounted, you can test for continuity between the case and the chassis before connecting it to the circuit.

Now get to work on the preamp problem!

[davohilts]

Well, I had some time today to delve into the preamp problem. In this amp there are two power amp boards, the 704a (the one I was working on) and the 704B which are connected to another board, the PC 504 which has two 36892 power transistors as well as a bunch of other components. The 504 board is then connected to the PC 303 board which in turn connected to the PC403 board. The 403 board is the one that the input doesn't produce sound, just hum which is unchanged by volume.
I thought I'd poke around and see if there was any clear reason for this but everything looks fine, no burned components or loose wires.
I did to try and trouble shoot things by running a 1KHz signal into the input of the 403 board and trace it through the board. I got signal at the volume, bass and treble pots and they controlled the signal, making it louder, more treble and bass. At the output from the board there was no signal.
I followed the wires (green and red) that run to the 303 board and then to the 504 board and then out to the power amps. Voltage was present the whole way.
Now for my first discovery.
I tested the voltage on the big caps and measured 40 VDC.
I then measured the voltages on PC704A at the red and green wires entering the board and measured 39.8 VDC for the red and -39.8VDC for the green, the other red and green wires leaving the board tested 8.33 VCD for red and -8.09 VDC for the green. These match fairly closely to the schematic. I tested the voltages at the collector (red), Base (blue) and emitter (yellow) of Q1-4 and got the following:
Q1 & Q2: Base=-26.8 VDC, Emitter=27.4 VDC, Collector=39.8 VDC, Q3 & Q4: Base=39.8 VDC, Emitter=39.4VDC, Collector=-28V VDC.
I thought I'd compare these measurements to those of PC704B at the red and green wires entering the board and measured 39.8 VDC for the red and -39.8 for the green, the other red and green wires leaving the board tested 8.33 VCD for red and -8.09 VDC for the green. These match fairly closely to the schematic. I tested the voltages at the collector (red), Base (blue) and emitter (yellow) of Q1-4 and got a big surprise!
Q1: Base=32.8 VAC, Emitter=32.8 VAC, Collector=39.8 VDC, Q2: Base=30.8 VAC, Emitter=30.8 VAC, Collector=39.8 VDC, Q3 & Q4: Base=-39.3 VDC, Emitter=-39.6 VDC, Collector= 28V VAC (note that when stereo switch is engaged this value goes to 41 VAC).
I wasn't expecting to find AC current. The DC measurements were difficult to measure because they were changing constantly.
Note that the measurements were taken with all effects off and turned down, volume, treble and bass turned up all the way, no speaker load or input signal.
Now I'm really confused.
Is this normal?

[Enzo]

I am glazing over with numbers. My first reaction to your dead preap channel was DC offset somewhere. I don't know what meter you have, but those AC voltages might be meter artifacts. Stick with DC readings. If you have two identical circuits, then use the god one as a model. Are not the 704 A and B boards the same circuit but mirror image layouts? Or am I thinking of something else?

I suspect you have one or more failed transistors putting large DC power supply voltages on base pins and such.

[52 Bill]

Well, I had some time today to delve into the preamp problem. In this amp there are two power amp boards, the 704a (the one I was working on) and the 704B which are connected to another board, the PC 504 which has two 36892 power transistors as well as a bunch of other components. The 504 board is then connected to the PC 303 board which in turn connected to the PC403 board. The 403 board is the one that the input doesn't produce sound, just hum which is unchanged by volume. match fairly closely to the schematic.

The PC504 is the low voltage regulator board that reduces the 40 volt supplies down to plus and minus 8 volts. All of the red and green wires that connect the boards are the power supply lines, red is positive and green is negative.

It seems that you have the correct power supply voltages, the power amps are working and one of the preamps is working. That means that your problem is only with the dead preamp board. If the first few stages are working then continue following the signal path to see where it stops. If the transistors test okay and there are no loose parts on the board, then my first guess would be signal caps.

[davohilts]

Thanks 52 Bill! Am I worrying about the AC Voltage on the Power Board transistor needlessly? I don't have a way to test the capacitors for leakage so with tube amps what I usually do is clip a known good cap in parallel to test one that is in place. Is this useful on solid state circuits too?

[52 Bill]

Am I worrying about the AC Voltage on the Power Board transistor needlessly?

I'm with Enzo here, your readings don't make any sense. If there was that much ac on the power supply, the amp would be humming like mad, if it was working at all. Make sure that you have a good ground connection with you meter lead when making voltage readings. And if you use the chassis as your grounding point, be sure that the chassis is directly connected to the power supply ground. On the Kustom the power supply ground is directly connected to the chassis, but on some other brands you will find that this is not always the case.

I don't have a way to test the capacitors for leakage so with tube amps what I usually do is clip a known good cap in parallel to test one that is in place. Is this useful on solid state circuits too?

Adding a cap in parallel will find a cap that has gone open or has gone down in value, it will not do anything about a leaky cap. Taking voltage measurements is the usual method of testing for leakage.

Tube or solid state circuitry, a cap is a cap. They will act and test the same in either application.

[davohilts]

I have gone back to trace the preamp circuit and have found that the preamp is in fact working. I get a strong signal which I can control using the volume and tone controls at the blue wire leaving the board, I must not have got good contact with my tracer when I checked it as it is closely attached to the boards. The blue wire along with a red and green run from the 403 preamp to the 703a power amp and I get signal there. I can also trace the signal back out to the stereo switch and back to the Power amp 703a and I believe these are shown on the schematic as A, B, C, and D.
C and D are the stereo switch feed I think as the copper trace on the board has been removed and is fed from each side by these. I can follow the signal thru the two transistors Q701 and 702. As I was tracing the signal I noticed that the 470 ohm resistors R719 and 720 were getting hot, I could feel the heat being given off, then the 100 ohm resistor R724 began to smoke. Shut the amp off and pulled the power board and Q1 and 4 had blown as well as Q707 ,Q704, 705 and 709, CR704. To say I'm frustrated is an understatement. The loss of the drive transistors is a real blow as they were new old stock.
I'm now back to square one. I guess I need to find replacement drive transistors which are NTE 128 and 129 transistors but need to have a clip on heat sink. Sometimes I wonder why I like to do this! I thought I was getting close but alas, I'm now farther away from solving this problem.