I may have killed my PA head...ideas?

[zissou]

Hi all,
I think I may have killed my new PA head...maybe not? Please?? I'm a full-on novice with electronics, so please excuse my poor lingo.

I recently bought a cheap ($100) 8 channel KMD PBM8-300a PA head. It had noisy gain pots though, and so I decided to take it apart and spray them with Deoxit. That all went fine but when I was putting it back together in the cabinet there is a 5 pronged connection between the front side of the head (which has all of the inputs and knobs( and the back side: power, etc) and I accidentally connected it the wrong way without realizing it. I turned it on and tried testing with a microphone and wasn't getting anything and then started smelling something...a terrible (and depressing) smell. I unplugged it and took it apart again and realized what I had done...rechecked that connection and sure enough it fit better the other way and the parts clicked together, so I definitely had it wrong. I connected it properly and tested again and still got only a background "on" sound but no functionality. Turning knobs produced no change in sound.

My question is this: what have I done? Is it fried? Is it repairable? Ugh...

Any help would be greatly appreciated.

[Chuck H]

what have I done?

It's really impossible to diagnose this sort of thing remotely.

Is it fried? Is it repairable?

Probably not by a novice and probably not for under $100

[Steve Conner]

Well, it might be impossible to diagnose remotely, but let's try anyway. The preamp is probably on the front board, and the power amp/power supply on the back. The cable between the two boards will supply power, say + and -15V, to the preamp.

By plugging it in backwards, you probably shorted the power supply and burnt out the voltage regulators. Or, you swapped the + and - rails, frying all of the chips on the preamp board. Or, you sent 15V where the audio signal should have gone, which might just burn out one or two ICs in the preamp.

So there you go, diagnosing the problem remotely is easy, it's fixing it remotely that's impossible. I'd open it up and follow my nose. You smelt burning, so look for obviously crispy and stinky components. Maybe you'll get lucky and it'll just be a voltage regulator, resistor or whatever.

If you don't have basic equipment like a multimeter and soldering iron, probably best to cut your losses and quit right now, you're only down $100.

[zissou]

Thanks for your responses. I took it apart again and had a close look and "followed my nose" and found two burnt resistors on a small board on the power amp side. I'm going to try replacing them. I think I can handle the soldering and the resistors are cheap, so I don't have much to lose. My biggest concern with this is figuring out what the colors the bands were, since they are pretty well scorched...I guess this is where a schematic would come in handy! Thanks again.

[Enzo]

Unless you are incredibly lucky, resistors usually burn out because transistors shorted out. The bad transistors won;t look any different, you have to check them with meters. resistors are cheap, so go ahead and see.

And schematic? Try contacting Kaman and ask if they can provide it. Don't tell them what you did, just ask for the schematic for repair and give them your serial number from the amp if they ask for it. (have it written down)

[zissou]

thanks, Enzo. I'll hope I'm incredibly lucky but just in case I'm going to pick up a multimeter tomorrow. I guess if the resister isn't the problem I'll check the transistors and see if I can find a bad one. I found the schematic on ebay for $5...the only other spot that had it was asking $20, so $5 sounds pretty great. Fingers crossed...
Thanks again, all.

[Enzo]

Did you contact Kaman and ASK them for it? They are the K in KMD. Free is even better than $5.

[zissou]

No, I called but they had closed for the day and since they didn't have ANY support section on their site I didn't feel confident that they would be much help, so I just ordered the one on ebay...I guess I should have tried that route first.

[Enzo]

Well, 5 bucks won;t kill the wallet.

But in general, never assume the web site is all a company has.

[zissou]

Update: the schematics arrived and were bad photocopies that were virtually unreadable...
So, (Enzo) I took your advice and contacted Kaman and the guy I talked to searched a file cabinet and said he'll scan and email me everything he has. He was totally cool about it and glad to help. Thanks for the suggestion- wish I'd taken that route 1st.
Thanks again!

[Chuck H]

You paid for a schematic and since an illegible one is useless you should ask for a refund. Probably end up more trouble than it's worth and you may just have to drop it but you surely don't get anything if you don't at least ask. If you paid for a book and all the printing was smeared you would return it. Just because copying schematics is a less common industry doesn't make that kind of thing OK. The schematic you paid for is strictly about information. You got no information for your money. All you got was something in an envelope so the guy could say he sent it to you. Sorry for the rant but that kind crap really opitomizes a lot of what I think is wrong with our culture. He knew the thing was illegible, he stuck it the envelope. He didn't care. Send me five bucks, at least I won't leave you with unrealized expectations and hand you my garbage to throw away. That seems like a better deal, eh?