PA Mixer/ Amp PCB Schematic

My son is constantly tearing down and repairing mixers, amps, and whatever. He hardly ever replaces entire boards but sometimes it is necessary. No reason you can't just replace the capacitors if you can identify them. If not try searching online for places that sell the schematics.

perhaps this place http://www.schematicconnection.com/store/asp/prodtype.asp?prodtype=91&ph=cat

There is a good chance the cap is still good and the only trouble is its connection to the board. Clean the board and connection well with circuit board cleaner then re-flow the solder around the connection and try it. you don't have much to loose by trying and you may save $$ it if cures the trouble with the amp.

What kind of active devices does it have on it (transistors/small ICs/big ICs)? Are the components marked with ID? What's the type of construction (surface mount/multilayer/double- or single-sided)?

The answers to these will give you an idea of whether you stand a decent chance of fixing it without documentation - unless it's something as straightforward as North of 60 suggested.

Upon further Deconstruction of the PA unit, The source of the problem became apparent to me. (see photo)

Fount the burnt spot on the board. Two of the Diodes connected near/to the "speaker outs" decided to self destruct. I will be snagging a couple new Diodes from the shop tomorrow to replace the burnt ones with.

Anyone have any concerns of possible down/up stream components that may have been put in harms way when the Diode failed? As far as I can tell, this looks to be a "standard style" audio amp. (i know what your thinking.."standard style, Pheh... whats this guy on... Eh? am i right?)

Thanks for everything!

(its a poor picture captured with my poor camera phone)

The crispy circuit board beneath those diodes can easily be a red herring. The long term partial heating these power diodes create could have over the years discolored the circuit board.

Judging by the vintage of the part assembly, you have a well aged piece of equipment. While this is can be a great quality in wine, frequently this doesn't apply to electronics. However, I will say that designs of this era were not as optimized for component longevity failures to occur simultaneously as today's designs frequently are. So repairing this unit may produce a reasonable return.

Without a schematic, identifying the sole component failure can be difficult. I recommend a shotgun repair. (No, this does not involve pump action or any use of gunpowder.) Instead this will involve replacing all likely to have excessively aged components, specifically the electrolytic capacitors and power semiconductors. The capacitors hot glued to circuit board are where I would start. (Lots of fun ) If you have the replacement diodes stand off the circuit board, just like the power resistors in the background, further toasing of the circuit board can be averted. While these parts are out of the circuit you might test with a VOM to see if any of the biasing resistors have opened.

If this approach fails, now bring out the Mossberg.

Checkout the:

Musician's Friend, Inc.
PO Box 4370
Medford, OR 97501-0168 Toll Free: (800) 391-8762 http://www.musiciansfriend.com/

If all else fails you might try these guys

Yes.

Indeed you can repair this yourself.Need soldering iron,dvm,and if possible borrow,rent (come up with)ESR meter for the caps.Caps can be checked in circuit quite reliably,unless two or more are paralleled.

Remove one end of diode to check w/dvm.

Remove ANY sticky,(manufacturing residue)resembles dried,glue,REMOVE all of this as it conducts when aged.

Look carefully for bad solder joints,better yet reflow entire board.

Finally clean all contacts at insertion end and also output end.

You can do it.

Joe.

Thank you all very much for the insight and support. Tonight I will be replacing a few of the burned up components. If all goes well, I'll have a PA tonight for rehearsal... If not, you will be hearing from me again... well, actually i suppose you'll hear from me either way, as I will post results, regardless of the outcome .

Thanks again.

-Russ

Any ideas or suggestions on what would have caused the fault to begin with??

Wrong Ohm rated speakers?

Direct short at speakers?

Someone plugged an input into the output by accident?

Under rated amp to speaker?

Etc Etc...

any ideas?

At this point, its just more of a curiosity, and something to keep in ming for the future.

thanks

Certainly a shorted output could have caused the problem. Along with excessive intended or unintended drop testing. (That happens to be why those capacitors were glued to the board. Excessive vibrations will induce metal fatigues of the wires on the capacitors. Use RTV to glue any capacitors back in place.) Since PA equipment frequently operate around "happy people" you may even have inappropriately placed, alcohol laced fluids at one time, somewhere. My prime candidate though is simple age related failure. Science call this phenomena that all things eventually fail, entropy. Likely the insulating fluid in one of the electrolytic capacitors leaked enough that a DC current was passed into part of the amplifier that damaged an output transistor.

I do think you maybe a little too optimistic that your new PA will be working by tonight. Locating the parts may take more time than doing the work.

"I do think you maybe a little too optimistic that your new PA will be working by tonight. Locating the parts may take more time than doing the work."

Under normal circumstances i would agree... I have a full Electrical Lab to snag parts from here at work. Have everything I need secured in a ziplock, ready for placement/solder. I even grabbed a bunch of parts that I dont currently need in anticipation. I was able to get everything with the exception of the 2 largest cap's which are not shown in the pic. Now I wait til 5, drive home, solder away and prepare for practice/recording tonight...with my fingers crossed of course.

I enjoy optimism

As capacitors age and are routinely heated they dry out then short. It is a very small and isolated fire if that's any consolation