Roland KC-350 Preamp Problem

[The Dude]

Measure the resistance from the negative rail to ground on the preamp board. If it's low, which it probably is, you are going to have to find the component that is loading that rail. It's not easy. There are lots of things tied to that rail. If there are things getting hot, try unsoldering those first. You can often unsolder just one leg of a component instead of completely removing it to save time.

[RWood]

Spent another session with the amp today. First off, to The Dude, I've already checked the resistance to ground on the negative rail and that led to a shorted cap which has been replaced (see post on 8/19). Now, resistance to ground from the negative rail initially measures over 2Meg and climbs, as it does on the positive rail. So something else is causing trouble. Here's what else I did.

I removed IC13 which as reported a few days ago was registering a little hot. I also removed C223 and C213, right next to it, also showing hot. Curiously, one of these caps appears to be open (no reading on the Fluke) while the other one measures 89n, out of circuit. With these components out now, I again applied enough voltage to get -6vdc on the negative rail with the board disconnected. Then, connecting the board, the negative rail loaded down as it did before, to about -4v (positive rail went down to 5v under load). So apparently this IC could be part of the problem but is not the sole culprit.

What I did next was to test again for temperature but at higher wall voltage, about 66VAC on the variac. I was hoping this would make more obvious the components that were heating up and sure enough, IC4 and IC5 got very hot. Where ambient temp was 26°C, and ICs were running 34° to 46°, IC4 was 68° and IC5 was a toasty 72°. So I removed them as well, tried it out and I am still seeing the negative rail voltage loading down when the preamp board connection is made.

I did not remove any of the caps around those two ICs but I can do that. At the minimum I will be needing new ICs, and thankfully they are available and cheap. And maybe several more of them are fritzed. But I have still not gotten to the bottom of the problem. Maybe it is multiple problems. Any ideas what direction I should take from here?

Thanks as always.

[Enzo]

Just keep at it, if all the ICs are bad, then all of them are bad. A power supply problem at some point could have damaged every chip on a rail, it happens.

[RWood]

Thanks, Enzo. You enabled me to have a V8 moment and just pull the rest of them. Now with all eleven of the 4580s removed the voltages look fine, with only a slight bog down when I plug in the preamp board,from 6V to 5.5, on both the negative and positive rails.

I've got a dozen of the opamps coming from Mouser; in the mean time there were a few .1 ceramic caps that gave me weird readings so I'll pull those as well. I've got my fingers crossed! Thanks everybody for sticking with me on this one. I can hardly wait to get back into some tube amp terminal strips!

[RWood]

And.....it works. All new opamps, negative rail is rock solid on -15v and it plays fine. Not sure what it was that caused all these components to fritz but I hope it doesn't happen again! Thanks, everybody who replied, for all the tips and assistance. I learned a lot about solid state amps, pcbs etc working on this.

RWood

[Enzo]

My main suspect is that the 25v supply came through a failed 15v regulator circuit, putting 25v on the ICs.