Kustom k-200B - 4 No Output

[1WoodHippie]

Hi guys. I'm new to the forum and needing some help. I'm mostly a tube amp guy and not really worked on solid state stuff. I've got a guy that's headed to Arlington for the show next weekend and was desperate to try and get this amp working so I told him I'd give it a shot. I'm very technical and will be good to go once I'm walked through 'shooting a solid state once.

On the bench is a Kustom k-200B-4 head with no output. Turns on and holds the light and thumps the speaker but no output. The power board is the PC703. I can trace a signal through the "bright" channel all the way to one of the blue Inputs on 703 but that's as far as I can trace. It can't trace it on the other side of R700 or 701. And I don't have a signal at the "monitor" output either.

Another weird thing is if I input a signal on the other "normal" channel I can't get a sig trace on either Blue input wires.

I do have +-8v and and the +-40v for the 703.

Where should I start first?

Thanks again to all in advance for your help!

--eron

[52 Bill]

The obvious answer is to check Q700.

If the power amp thumps, that's a good sign. What happens if you inject a signal into the monitor jack?

[1WoodHippie]

Inject Signal into the Monitor and i get sound. I'm guessing that Q700 is bad. Now to find a replacement.

As a side note (and learning point for me) How do you check to see if it's bad for sure. Remove it from the board and run a diode check across the Gate/Source/Drain....??

[Enzo]

It is a bipolar, it has base, emitter and collector.

The schematic is chock full of voltage readings. I see at least a couple there around Q700. Are they remotely close? An open R703 for example would shut Q700 off completely.

[1WoodHippie]

Yea, my voltages are almost right on the money on Q700.

[1WoodHippie]

Replaced Q700 and it brought it back to life. But my normal channel is still dead. (you can barely hear it at all) Does anybody have a schematic for the PC403 board?

[52 Bill]

What controls does the normal channel have? I can't find a PC403 anywhere.

[1WoodHippie]

I'm not in front of it now but it's got like Vol, Bass, Treb, Bright, Boost, Harmonic Clipper. It's the only one I've seen so far with that harmonic Clipper on it. I can detach that board from the chasis and it comes back to life. But if you ground out the Vol, Bass, or Clipper pot it kills the signal. And those pots all have 2VDC riding on them. But without a schematic it's gonna make life a LOT harder to find where that DC is leaking from....

[Enzo]

Maybe.

Look down the length of the board. Are there any small electrolytics? Like small value 1, 2, 5uf types coupling stages? Look on the PC703 board, see cap C702 right after Q700? Like that. Replace them. In old transistor preamps like that, that is THE one most common repair I have to do.

[1WoodHippie]

Replaced a couple of 1's and same result. If you ground out the Vol, Bass, Har Clipper or the Low Input jack you loose it.....

[Enzo]

What do you mean by "grounding out" the pots, and by "2v riding on them"? Thw bodies of the controls? some solder terminal of the control?

[Enzo]

Here:

[1WoodHippie]

thanks enzo!! I mean if you touch the pot shafts of just those certain pots and input jack to the chasis it kills the signal. And if you read the DC on them when they are not grounded to the chasis you get 2VDC on them. If you disconnect the board and the second input from the chasis so none of those pots are touching the chasis the signal comes back fine. I'll pour over the schematic today....

[Enzo]

Since the body and shaft are not connected to the resistive elements in the pots, I don't see how the schematic will help locate the source of stray voltage.

Is there a ground trace under the nuts for the pots?

However, the schematic also lacks a discernable ground connection other than the FS jack as far as I can see, connection wise. SO why not connect it all up like it should be, then track down the loss of signal. The impression I get is that your signal with nothing grounded is a red herring. break the ground on a guitar cord and plug it in and you get a lot of "signal" in an amp too.

With everything bolted together, how do the DC voltages look all over compared to those on the schematic?