Kustom K25c-2

[lowell]

This amp has -28v on the output. The output transistors don't seem to be shorted. The diode "thing" mounted on the heatsink has a broken lead. I believe this is the CR5 8v 2W to ground in the splitter tail.

Anyone know what part that is? It's too oxidized/rusty to read. And I think it must reduce the bias current of the splitter pair when the heatsink temp rises, but not sure. Care to explain?

https://www.google.com/url?sa=t&rct=...V9XuXFSnz-C1HQ

[Enzo]

Look at the circuit. -25v flows through R65 330 ohms to this device. Continue to the left to see this becomes the -8v rail. So this is an 8v 2 watt zener diode.

Your differential pair instead of referencing to -8v now references -25. SO I would expect the output to go way negative too.

[lowell]

Ahhhhh....yes. thanks!

Is that a special device because of heatsinking or any 8v 2w zener will do?

[Jazz P Bass]

Can you compare it to CR4 (+8V zener diode).

1N5923.

(8.2 V 3 watt is easier to obtain)

https://www.digikey.com/product-deta...SCT-ND/2120505

http://www.mouser.com/ProductDetail/...bonm%252bsRbO5

[g1]

I believe this is the CR5 8v 2W to ground in the splitter tail.

Don't guess, check.
The notes on the schematic say it is CR6 (bias string diode) that is mounted on the heatsink. That makes more sense to me, for thermal tracking.

[52 Bill]

Don't guess, check.
The notes on the schematic say it is CR6 (bias string diode) that is mounted on the heatsink. That makes more sense to me, for thermal tracking.

Agree, the only diode that is mounted to the heatsink is the bias diode, a 1N3754.

Replace it with any small signal diode like a 1N914/1N4148, etc. Try and find a way to mount it in the clip on the heatsink. I used to epoxy a diode inside a small piece of aluminum tubing to use as a replacement. If you don't mount the diode on the heatsink the amp may go into thermal runaway and shut down when the heatsink gets too hot.

[lowell]

Replaced it with a 1N4148. Yes it indeed was CR6. Made a little aluminum clip and bolted it with bird pooh to the heatsink.

Still -28v on output. I've checked a bunch of things and believe the input transistor to the splitter is shorted C to E. I have -28v on the splitter emitters. And am getting 0v drop from C/E of that input transistor Q16.

[lowell]

SO WEIRD!! The 2N3567 splitter transistors say 2N3657 on them. Very very odd.

[lowell]

Is 38737 pnp driver an 80v 1A? Can't find a datasheet.

Both splitters were shorted and the + rail driver is not conducting.

[g1]

SO WEIRD!! The 2N3567 splitter transistors say 2N3657 on them. Very very odd.

How about all the other ones the schematic is calling 2N3567 ?

[52 Bill]

Did this amp ever work like this?

Those numbers are SCR's not transistors.

[lowell]

SCRs?? What's that, schematic component " " something? Ha.

I looked up 2n3567 and they "look" the same. Looked up 2n3657 and found nothing. Replaced them both with MPSA06, and the 38737 with a KSA1013 and the amp is now working. (Adjusted for different lead/pinout of course) Played 7watts of 200hz for about 5 minutes and didn't see any terrible temperatures with my gun.

[nosaj]

SCRs?? What's that, schematic component " " something? Ha.

I looked up 2n3567 and they "look" the same. Looked up 2n3657 and found nothing. Replaced them both with MPSA06, and the 38737 with a KSA1013 and the amp is now working. (Adjusted for different lead/pinout of course) Played 7watts of 200hz for about 5 minutes and didn't see any terrible temperatures with my gun.

Silicon Controlled Rectifier
https://www.allaboutcircuits.com/tex...rectifier-scr/

[Enzo]

2N3657 is a typo then. Small Bear even sells the 2N3567.

[lowell]

After all was said and done, fixed, I found a 2n3567 in the back of the amp. Someone musta replaced one already...seemingly with transistors labelled 2n3657. Odd to see typos ON the device itself.