Kustom K100C-6 schem?

[Garydean]

I'm trying to locate the schematic for a Kustom K100C-6, please. The preamp boards on the unit I'm working on have no power.

Thanks,
Gary

[Enzo]

What are the numbers on your circuit boards? Look for a number like PC5033.

[Garydean]

The main board is a PC900.

[Enzo]

Lemme go look...

[Garydean]

thought I would bump this up. I'm still looking. OR could I get some advice on what I should be looking for? I have correct voltage to the main board but even tracing it through the circuit I can't figure out why it's not making it to the wires leading to the preamp board. Where typically could this get lost? A bad component or a broken trace. I've inspected the whole thing for breaks. I do see one empty pad on the main board. That's why a schematic may help me figure out what was in there. Looks like someone has been in there trying to fix it.

thank you,
Gary

[Enzo]

There is a split supply, +/-8VDC for the preamps.

I posted the PC900 schematic and also the layout over at www.ampix.org in the Enzo gallery

What preamp boards do you have, if the problem is not on the PA board.

[Garydean]

Thank you so much Enzo. The preamp board ID is KE1 1 1700B. Yup, figured the supply was split...both are down, thank you.

Thanks again, this should help unfold this mystery.

Gary

[Enzo]

Before you go nuts over that, since both derive from the main rails, are the main rails both present?

[Garydean]

Yes I have 39v and -39v present on the board.

[Enzo]

Just checking.

[Garydean]

while working on it, one of the leads from the clip on diode snapped at the case. I have to track one of those down before I can continue. The 1 ohm sand resistors started sizzling.... was my only indication. I couldn't even see the break till I moved the leads on the diode. Good thing I wasn't listening to music, never would have heard it in time to shut things down.

I disconnected the rails going to the preamp boards to see if the voltages changed on the main board....no changes.... is that step correct to help divide?

Gary

[52 Bill]

while working on it, one of the leads from the clip on diode snapped at the case. I have to track one of those down before I can continue. The 1 ohm sand resistors started sizzling.... was my only indication.

Gary:
The diode on the heatsink is in the bias string for the output transistors. The generic number is 1N3754, which has long been out of production. You can replace it with almost any silicon diode, but you will have to come up with some way to mount it down to the heatsink.

Check all the power amp transistors and the 1 ohm emitter resistors for any damage.

[Garydean]

Thanks Bill,

The only reason it's on the output transistor board is to dissipate heat in the clip? The metal casing of that diode has no relevance to ground?

Thanks,
Gary

[52 Bill]

The only reason it's on the output transistor board is to dissipate heat in the clip? The metal casing of that diode has no relevance to ground?

Just the opposite. The diode senses the heat from the output transistors and reduces the bias to the output stage. The metal case is insulated from the diode itself, unlike the axial versions of diodes in that style case.

[Garydean]

ah ha! Very cool.

I replaced Q913, Q914, Q915 and Q916 and I still get -.7V at the green lead off the main board. I checked every resistor and cap around it...

Gary

[Enzo]

Well, fix the +8 side first. I think the -8 tracks off it.

[Garydean]

I've been at this sucker way too long. I've had almost every component off and tested.

Almost all the readings are off....way off. For example the only place I have normal readings are at the collectors of Q910 and Q911 that's the positive rail for the preamp. All the other readings are in the negative mV range on that rail.

It's about the same with the neg rail. -38.5 on the emitter of Q915 and -37.6 on the collector of Q914. everything else reads in the negative mV range. I've replaced almost all the transistors on the main board 10 to be exact. Two resistors that were 20% over the limit and one busted up diode. Reworked 80% of the main board including all wires to and from the main board. I've played he continuity game on all the traces several times. I also tested all the transistors on both preamp boards.

I have yet to try removing the preamp boards and it's wires, apply an external power supply and load.

I figured by now the faulty component would have exposed itself thru my primitive shotgun method, which is all I know at this point with solid state circuits.

I am welcoming new suggestions or even old ones. I've put the project aside and fixed a few amps since I started this problem child..... nothing has jumped out at me.

Thank you,
Gary

[Enzo]

This is not a large circuit. On somehting like this I often just set the meter to diode test adn run down the length of the board. I check every diode and transistor. Once I have weeded out any shorted ones or obvious stinkers, then I shift my focus to resistors - specifically looking for open ones. Don't focus only on transistors, they are not the only parts that can fail.

In that power supply circuit, I see five small electrolytics. They are immediately suspicious for their age if nothing else, especially if they are tantalums. A shorted C912 could shut them down. The values are not critical, a 10uf will replace those 6.8 just fine if need be.

There should be +/-40v on R929/938, so what is on caps C909/913?

And there is a few grounded points in that circuit, is there continuity from that ground back to the main filter ground?

[Garydean]

I lifted every resistor and replaced 2. I'm after those electrolytics now, even though the ESR readings were good. All diodes tested fine as well.

Thanks for sticking with me on this one Enzo.

Gary

[Garydean]

bingo!!!! there were 2 bad tantalums on the main board totally shutting everything down. After that, one of the preamp boards was totally dead. Keeping in step with what happened, I replaced 2 tantalums and that channel is now up and working.

thank you Enzo and everyone else who stuck with me,

Gary