Troubleshooting an old Kustom 100

[Neal]

Hey Gents!
I'm trying desperately to finish up a few projects here (so I can move on to other projects!).
I've been sitting on a non-functioning Kustom 100 guitar amp for a couple of decades now, but now someone wants to buy it, so I've gotta get it fixed.
I'm pretty comfortable working old tube amps, but I haven't done much with solid state stuff.
I found the schematics on this site (thanks).
Symptoms: Amp lights up, reverb crashes when bumped, so I know the power section is okay. When I turn up the volume, I get a little volume at first, the the volume rapidly tapers off to nothing at about 2 on the dial.
I pulled the volume pot and tested it, and the sweep is normal and the resistance value is good.
If you were in a hurry and wanted to troubleshoot a solid state amp, where would you start? My first thought is to use a signal tracer, but if there's a quicker method, I'd sure like to hear about it.
Thanks, guys!

[Jazz P Bass]

First of all, slow down.
There is no surer way to wreck a piece of equipment than ripping through it.
There are a number of voltages marked on the schematic.(rare occurrance)
These are static voltages. No signal.
Check the transistor voltages near the volume pot.
You may simply have a capacitor that is leaking voltage, or pulling it down.

[Enzo]

Desparate is clinging to a rope danglong over the abyss. You just have an amp you want fixed.

Suggestion, any time you want to discuss Kustom stuff, include the board numbers. I can look up the schematic for PC5033, but Kustom 100 is kinda vague, at least to me.

Do what Jazz says, look at DC voltages through the board. Me, I'd look at any and all small electrolytic caps in the signal path as needing replacement. They are like 40 years old now.

And procedurally, look at the schematic. Apply a signal to the input, then trace it stage by stage through the amp.

[Neal]

Thanks for the replies, gents!
I checked the voltages on the transistors on the input section, and a few were off. I started checking component values on the associated parts, and found some 1% resistors that were way way out of tolerance. I'll have to order some parts from Mouser.
Since I have to order parts anyway, I'm going to go through the circuit thoroughly and check everything. I'd hate to replace a few parts only to find more things downstream that need to be fixed.
It may be a good few days before I get the parts in and get back to it, so if I need to post back, I'll be sure to include the board numbers.
Thanks again, Jazz P and Enzo!

[Enzo]

Don't worry too much about resistor values. I mean if a resistor should be 10k and it reads 11k, no one cares. If your 470 ohm resistor comes up 22k, well, yeah, that's no good. And of course, if you are concerned with resistor values, they MUST be removed from the circuit before measurement.


This is not rocket science, just a basic amplifier, so if a resistor is off by 20%, aside from it might still be within tolerance, it shouldn;t silence the amp, it would usually just affect tone or gain.


GO step by step, FIND the actual problem. THEN go back and list a million parts you want to "upgrade." There is going to be a thing responsible for the dead channel, finding it might be educational. And a lesson techs have to learn is that all failures are not parts. A failed connection is just as effective at killing a circuit as a bad part.