Kustom TRB400 schematic?

[Dave Curtis, dB AudioTech]

Would be greatly appreciated.

[Enzo]

What are the board numbers?

is this an old product, or is this contemporary Kustom product?

[Dave Curtis, dB AudioTech]

Hi Enzo,

Thanks for the quick reply.

It's fairly recent. Made in China.

The solder side of the preamp board says:

Kustom Music Inc.
Copyright 2KYEAR
TR400B
Preamp bd

On the rear jack board:

280-120400-110

Power board:

280-120400-120


Loud buzz. Diodes on the power bd checkout OK; R314 & 337 are burnt. I haven't pulled it apart, yet.

[Enzo]

Not familiar with the unit, but burnt resistors and loud buzz - look for DC on the speaker - sounds like shorted output transistors offhand.

My fairly extensive Kustom file covers the old stuff, not the recent stuff. But have you contacted Kustom and asked for the info?

[Dave Curtis, dB AudioTech]

Thanks, Enzo. Yes, Mark Sharp from Kustom sent me them yesterday, along with an offer to become one of their service centers (I'll have to mull that one over; I'm already too busy). I haven't checked for DC on the output, but I did check the outputs for shorts, which came up negative. I still haven't pulled the PA assembly; I have to get the "cutting-out Crate" done first.

[Dave Curtis, dB AudioTech]

Any I idea why these 2 resistors are heating up (R314 & 337; upper left on p3)? I've replaced them with 1/2W parts. There's no buzz now, and and the amp works, but these 4.7K are getting very hot. It looks to be part of that Vactrol circuit (mute?). I'll post some voltage measurements L8R... (IIRC, 70-something at the top of R314)

Thanks,
Dave

[Enzo]

I have no idea, those along with 317 and 318 are in series across +V and -V. They seem to be setting some thresholds for the limiter opto. I think...

Check your zobel at the power amp output. COnceivably if the amp was unstable and oscillation above hearing, a ton of signal could be coning through those diodes... Nah, I am not happy with where that theory is leading.

Whatever is +V and -V, calculate Ohms Law what those resistors have to dissipate and see if the original part wattage was sufficient. At 70v, I get about .6 watts. Of course I haven;t had my dinner yet.

[Dave Curtis, dB AudioTech]

Originally Posted by Enzo

I have no idea, those along with 317 and 318 are in series across +V and -V. They seem to be setting some thresholds for the limiter opto. I think...

Check your zobel at the power amp output. Conceivably if the amp was unstable and oscillation above hearing, a ton of signal could be coming through those diodes... Nah, I am not happy with where that theory is leading.

I haven't scoped it yet... Been pulling long hours at the day job.

Originally Posted by Enzo

Whatever is +V and -V, calculate Ohms Law what those resistors have to dissipate and see if the original part wattage was sufficient. At 70v, I get about .6 watts. Of course I haven;t had my dinner yet.

I measure 117V from the top of R314 to the bottom of R337, for about a 1.5W; the OEM parts were cooking at .75W each. 317 & 318 both have about 20V, putting them near their .25W limit.

[Dave Curtis, dB AudioTech]

Update on the hot resistors:

A tech at Kustom advised replacing the 4.7K resistors with 1 watt parts. I'll be doing the 1.5K resistors also.