Kustom 250 Fuzz issue

[lowell]

This amp has volume issues that come and go when the fuzz is engaged. It swells when it happens so I figured maybe a cap. I've checked the voltages in the fuzz schematic and all seem good except for Q11 collector. It should be 3.2v but it is 1.8v. The 11.4v supply test point on top of the 47k is 11.1 so I figure that's close enough. There is no dc after c39, Q11 base is .5v, C38 has no dc on the left side... so they all seem fine. I diode drop tested Q11 and it seems fine at .7v CB and EB. I'm not sure what Q11 is as the schem does not say.

[lowell]

Ok I replaced Q11 just to see if that collector voltage came back and it's much better at 3.4v.

Now I'm realizing that the issue with the volume is this: When the fuzz is TURNED OFF it takes about 4 seconds for the CLEAN sound to rise back. I'll investigate further... Enzo if you have schems for these boards I'd like 'em. 000-5065, and the power supply if ya got it.

[lowell]

Can someone tell me what AC voltage is acceptable in a solid state amp as this? I'm getting 500mvAC on the + supply, and 800mvAC on the - supply and this is right on the main filter caps.

[lowell]

Enzo do you have the other boards for this amp? 000-5065 and the power supply board if ya got it. Can someone please help me w/ this? I've checked DC voltages in the fuzz circuit when the problem happens as I turn the fuzz OFF, and don't see any rising/lowering voltages. When the problem happens as I switch off the fuzz, the clean sound takes about 5 seconds to rise back to full volume. The sound is also a bit distorted and thin during that 5 seconds. I'm pretty lost in diagnosing this issue. The + and - supplies are fine too, even when switching.

[Enzo]

OK, so the amp works OK in clean, at least after a little time? And the amp works OK in fuzz, at least once it gets there? Then your power supply must be OK.

PC5065 is the power amp, and won;t be involved with this. And power supply? A whole board for that? If there is no board number, I have no schematic. Besides, what is it, +/- something for the power amp, and a couple 12v zener supplies for the preamps? PLus filter caps for each.

Look at the fuzz switching. The way I see it, on the 5069 board where the fuzz is, the signal comes in lower left J1 pin 2, goes over to R78, R77, then back off the board via J1 pin 1. If Q8,Q9 are turned on, they short across the two resistors, so the signal passes right on through - CLEAN. But if those transistors are off, then the high resistance of R77, R78 mostly blocks the signal, so it then goes through Q10-12, which is the fuzz circuit.

Note that when Q8,Q9 are on - the clean mode - Q10 is also on, and it grounds off the input to the fuzz. That keeps fuzz noise out of the clean.

Since clean works eventually, I have to assume R77,R78 are ultimately being shorted across. And since the fuzz works, I have to assume Q10-12 are working.

Now imagine this. You are in fuzz, so Q8-10 are all off. Q8,9 make the signal go through the fuzz, and Q10 off means the fuzz is not grounded off. Now to go to clean, you either footswitch or panel switch the function of S3. This grounds off the -12v coming through R89. That removes the negative turn-off voltage from the FET gates. R85 helps by pulling up to insure no turn-off voltage.

Remember JFETS are normally on unless turned off at their gates.

And here we are. either that control voltage is snapping up and down with the switch, or not. Either CR8,CR9 are working or not. The gate voltages of Q8,9,10 are snappin up and down with the switch or not. And lastly, either Q8,9,10 are turning off and on or not.

Those JFETs have four legs, pins 2,3 are the gates. The bottom of the drawing shows which legs are which. The control voltage is on pins 2,3. Is the control voltage changing smartly, or does it lag?

Pins 1,4 of the JFETs are the current path. They should go from a high resistance between them to a low resistance and back with the switch. Either get right on each one or measure resistance across R77, R78. Yes, while the circuit is live. While in fuzz mode, the two resistors should measure a high value. I don;t know what is off board connected to them, so I don;t know what resistance, but lets say over 200k. Check each one - does it transition from low resistance to whatever high resistance and back promptly with the switch or does it take the 5 seconds.


And it only takes one of them. If either Q8 or Q9 has a hard time turning on, then the clean will have a hard time coming back on.

SO that is my vote - Q8,9. Either one or both of them is bad, or they are not being turned off smartly.

A quick test - during that 5 seconds it takes the clean to return, quickly short across R77 with a test lead. And the same for R78. or the pair of them. That will tell you if it is them blocking your signal or not.

[Enzo]

Kustom PC5065 power amp board

[ReginaldBisquet]

I was working on one of these earlier this year:



The only issue (at the time) was that someone had sliced the power cord. I soldered on a nice thick 300v cable with the existing power cable stub and shielded (and grounded) the cable.

Worked great for a few days and then I noticed a harsh squeal coming from either channel. Going to open it up again and take another look. Also, are the blue buttons for power supposed to light up?

The schem for this amp is located on top of the chassis box. Super nice! Should I take hi-res photos and post them somewhere?

[lowell]

Ok cool so I'm glad I was right about how this circuit works. At least how the FETs are functioning. I tested across R77 and R78: R78 had 300ohms/500kohms in CLEAN/FUZZ modes. R77 for some reason is 300/88k. I took the board out to remove those resistors and measure them and they are both good. I then saw that Q8 pin1 was not quite making it through the board and didn't seem connected. I resoldered it and put it back together and it seems the issue is fixed. Thanks Enzo.

[lowell]

Yes that'd be great.

[52 Bill]

The power and polarity switches are supposed to light up. The replacement bulbs are #335.

The blue cap must be carefully removed and the bulb unscrewed from the switch. A small piece of rubber tubing will help to remove the bulb.

[ReginaldBisquet]

Where do I post the photos of the schem for this amp?

Yes that'd be great.

[lowell]

can you attach them to this thread? See the "manage attachments" below your reply while you're writing it.

[ReginaldBisquet]

Kustom 250
Model 1
1972

Schematics as per the inside of the amp's case. Hopefully, you guys can clearly read everything on these schems. Let me know if you need anything else!

[ReginaldBisquet]

I think I have a local source for a #335. Cool!

I have a question in regards to this amp. After I took a photo of the schematics, I decided to go ahead and fire it up. Everything was sounding good until I turned up the volume and struck a chord. A LOUD buzz emitted from the speakers! This occurs on all four input jacks.... effects or no effects. Seems to occur only when I give the guitar a good keraaaang.

Since the issue is occuring on all four inputs, I can only imagine that it's a capacitor issue on the main board.

True?

[pontiacpete]

perhaps a speaker, ??