Peavey KB60 problem - possibly power supply

[Zipslack]

I am trying to repair a early to mid-90's Peavey KB60 solid-state keyboard amp that is driving me crazy. It came in with the repair ticket description of "Won't turn on - no power". After removing the chassis and doing a visual, I noticed the 2A fuse was blown, so I started looking at the power supply section and output transistors. Output transistors tested okay with DVM in-circuit, so I concentrated on power supply. Found one of the negative rectifying diodes was shorted and the 2200u/50V filter cap on the negative side was leaky (electrically and figuratively). I went ahead and replaced all 4 diodes (just to be safe) and the filter cap. Unit powered-on without blowing the fuse - so far, so good.

Now, here is the problem. As soon as it turns on, I get a buzzing/crackling noise near power supply section and the (new) negative filter cap starts frying and gets warm. I can only power-on for a split-second, so I cannot do typical measurements. I suspect that there is AC showing up where it should not, or positive DC feeding back where it should not be. Just to be sure, I have tested without the preamp circuit (opened the Power Amp In jack) and have tested without the power transistors and op-amps, as well as mid-gain transistors - still same problem. All diodes check okay, including the bias-string. Caps and zeners all check okay - still exhibits problem with them out-of-circuit. Checked for shorted traces and defective solder joint - nothing. Re-flowed joints on control pots and jacks just for good measure - nothing. Transformer reads proper with resistance checks (no insulation tester, though).

At this point, I have checked and re-checked just about every component on this board and STILL cannot find the problem. I feel that I'm overlooking something very simple and/or stupid, but cannot find it. I have tried it with the speaker connected and get a loud hum with speaker extended forward (positive DC?) A couple of quick measurement showed 25VAC across the negative filter cap and no DC at speaker outputs (?!) I cannot find a short to ground or chassis anywhere. Does anybody have any suggestions? Does Peavey-Master Enzo have any sage advice?

[Enzo]

My tech senses are tingling that your filter cap is in backwards. Check that out.

Or one of your diodes is in backwards?

There is nothing downstream that will heat up a cap. pretty much overvoltage or reverse voltage will do that.

[Zipslack]

Thanks for the advice

Thanks for replying. I'm pretty sure I got the caps and diodes back in proper orientation (I penciled in orientation on the board when I removed them), but I could have made a boneheaded error. I'll double-check this evening when I get back to it. I don't think any other tech has been inside this amp, but I'll double-check the orientation against the schematic/layout diagrams just to be sure.

[Zipslack]

Well, I was 50 percent correct and Enzo was 100 percent correct. I was getting +DC where it didn't belong -- because I put the negative diode pair in backwards! Well, at least now I can fire it up and start doing troubleshooting for other problems. Thanks for reminding me how simple things can trip you up. I looked over those diode 100 times without noticing my mistake.

[Enzo]

Of course. They look like perfectly normal diodes.

believe me, I have made my share of bonehead mistakes.

If that cap seeing reverse V got hot, I'd replace it again, even if it seems to be working.

[Jag]

I once spent a week chasing hum in a '65 Bassman before I realised I'd put the bias supply cap in backwards. It really didn't make much mess when it vented so it didn't look bad. We all do it!

[Zipslack]

Yeah, I already switched-out the bad filter-cap....now I'll have to find out what else I might have messed-up with the +DC....

[Enzo]

A couple months ago I replaced the bias cap in some Fender. Nice new cap, very neatly and carefully soldered in backwards. Powered up and BANG, the cap innards shot past my ear and across the shop. I felt the little breeze.

perhaps I'd best install another cap.

[Steve Conner]

Happens to us all!
EEVblog #42 – Exploding Capacitors in High Speed @ Electronics Engineering Video Blog Podcast

[Robert M. Martinelli]

Happened to me too!

However, the most spectacular "polarity inversion" issue I have witnessed was when I saw ( luckily I was some 30 ft away ) a big AC drive explode upon powering up after being installed. A guy who was close to the open cabinet was thrown several feet away ( luckily he wasn't seriously injured, but his face's color resembled a well cooked lobster, giving new meaning to the word "sunburst" ).

The flash has been so bright ( not to mention how loud the "big bang" has been, making my ears whistle for a while ) that my eyes needed half an hour to get back to normality.

When we investigated the problem we found the drive's cover had been thrown away, and the DC rail caps ( working in the 550 VDC range in a 400 V AC drive ) had exploded, and, by examining their remains, we found they had been connected BACKWARDS at the factory. I've been installing and programming ( literally ) thousands of drives, and I'm still wondering how the d@mn thing managed to slip through the factory QC system unnoticed.

Since then, I've become even more cautious than I was before about polarity.

Cheers

Bob

[Zipslack]

Mysteriously Disappearing Voltages...

Okay, same amp...still driving me crazy. I think I've been looking at this thing too long and overlooking something obvious (once again). With the diodes in the proper orientation and filter caps replaced (and in proper direction), I am not getting my +/- 16VDC voltages. I get +/- 40VDC out of the rectifiers (measured with reference to chassis), at the filter caps (no AC on the scope - just full-wave rectification) and at the power transistors and elsewhere where it should be. After the filter caps, there is a filter network of 2 power resistors, 2 caps, and a zener on each rail. I measure 20V dropped across each resistor with no voltage left for the final zener/cap section! I find no shorts or breaks anywhere, nothing is getting hot like it's sucking current away, and I've tested it without zeners and without caps just to make sure the filtering isn't shorting to ground. This is happening for both rails. Any ideas where my voltages are hiding?

Here's the schematic and layout files.

[Enzo]

OK, no +/-16v. R158,159 drop +40 down the zener CR113 for +15. If there is no voltage at CR113, then either it is shorting to ground - the voltage - or it is not getting there in the first place. Feel R158,159. If they are hot, then something is grounding off the +15. If they are cold, then one of them is probably open.

The negative side checks the same way.

If the resistors are both hot, and voltage at the zeners is pretty much zero, it is possible both sides are shorted to ground, but it is also possible something like an op amp is shorted V+ to V- inside. That op amp would then be getting very hot. ANother possibility would be a protective diode. At various points in the amp, ther are pairs of protective diodes - clamping diodes - one to each 15v rail. If both diodes in a pair were to short, then that is a dead short across +/-15.

SO start with R158,159, and R178,179. What is happening with them??

[Zipslack]

All 4 of the dropping resistors are hot (5W each, so I figured they should be warm). None of the op-amps are even warm - everything on the board is cold except the 250 Ohm/5W resistors, which are pretty warm. I like to do "touch" checks for excessive current before I start measuring -- much easier to check for excessive heat than to de-solder and measure current. No sense in putting excessive wear and tear on the pads.

I'll check the diodes for voltage drop, if there is voltage present, and forward/reverse conduction. I have a scope-meter that does a component test that is really nice for that - it displays a conduction graph for forward and reverse conduction, very much like a textbook diagram. Also works well for quick-checking transistor junctions.

I hadn't really considered the clamping diodes - like I said, I think I've spent too much time over-thinking this thing or getting too distracted by the power supply issues from earlier. Think I'll go fix a Masters AC30 and then come back to it. Tube amps just seem to be much easier for me to deal with and maybe that'll clear my head to get back into this thing. Of course, it would probably be better if I could get more than 15-30 minutes at a time to devote to troubleshooting this thing (I just do repairs as a part-time contract job around my regular job and personal life). I need to go ahead and build my workshop out in the garage storage room so my wife and daughter won't be continually interrupting me while I'm trying to work.

[Zipslack]

Solved !

Okay, I finally got this thing finished. I just wanted to say thanks for all the input and suggestions. I still don't understand exactly why it happened. It turned out to be an op-amp problem, but not what I would normally expect. It wasn't shorted internally, it just wasn't making proper connection in the socket. I removed and checked each op-amp and powered-up after each one. None of them tested bad, but when i replaced U6 and powered-up, my low voltages appeared! Checked voltages around the circuit and then connected a keyboard and tested - everything was "go"! Still don't understand how the IC not being seated properly would drain off the current/voltage like that without anything getting warm, but there it is. Anyway, just throwing this out for future reference. It's all packed-up and ready to return and I'm going to do my best to forget I ever saw it!

[J M Fahey]

Hi Zipslack
*Maybe* there's an explanation for

Still don't understand how the IC not being seated properly would drain off the current/voltage like that without anything getting warm, but there it is.

You received it already not working.
Maybe a ham-fisted "tech" or the owner itself did a couple little things here and there, among them, pulling that IC and reseating it the wrong way= short across +/-15V.
You pulled it, (and others), left it on the table, and when reseating it *you* took notice (of course) of those funny notches or embossed dots that these "bugs" carry and seated it properly.
I guess that might have happened, because the alternative, it losing contact because of insufficient seating would place not short anywhere.
Anyway, I'm only guessing, what's important is that it works now.
I very often get amps where the owner swears on his dead mother's grave "I touched *nothing* at all", yet it has obviously been tampered with, there are sloppy solder blobs, non-original parts, backwards electrolytics (that one is very common), whatever.