Speaker wire short

[tat]

I had a make shift extension speaker connected to a Peavey bandit 75. Unfortunately he wire was tripped over and shorted. What damage could this cause? The preamp and speaker seem OK but the main power amp is producing unusual distortion. 

[Roly]

Output shorts, no matter how brief, are something that many solid-state output stage really object to.  There is a good chance that you have boofed one (or more) of the semiconductors in the output stage, and this has to be found and fixed before it causes more damage (such as to your loudspeakers).

Assuming that you have powered the amp up since this event and it isn't actually smoking or blowing fuses, power it up with no speakers connected, and measure the DC voltage coming out of the amp speaker connection.  This should normally be only a small fraction of a volt, but if you have more then this, +ve or -ve, then you have a dead device somewhere.

Can't find the 75, but the 65 should be quite similar.

[tat]

The output DC voltage measures around 200mv at power up and then stabilizes to 3.1mv after a few minutes.  There doesn't appear to be any unusual smell, heat or visual signs of damage. I don't own a signal generator or scope but am familiar with testing individual components and can use a multimeter and LCR meter. If someone could point me in the right direction I was hoping to find the problem. I have the drawings for the amp   

[tat]

Found collector and emitter leads loose - can't imagine a current surge could blow the solder away? Re-solder and it sounds good to go. I hope this is my only problem and quite embarrassed I didn't spot this sooner. Now on to the next project - a Peavey Special 212 Transtube . Looks right off the bat like a power supply issue. The solder is blown away from 2 leads of the bridge rectifier.

This Forum is just fantastic for the information people have shared. As I gain more experience and knowledge in amplifier repair I hope to contribute in a helpful way.

Thank you

[J M Fahey]

Well, you were *very* lucky, that's for sure.

can't imagine a current surge could blow the solder away?

Well, I can  :lmao:
Or, to be more precise, have seen such many times.
But I'm thinking that in this particular case, *maybe* the wire "pull" (when somebody tripped it) flexed the board enough to crack those solderings.
Modern lead free solder is , to sayit succintly, pure cr*p.
So much so, that it is *forbidden* to useit on Space, Military and Medical stuff.
How's that? :duh
MOTE: never ever again add extension speakers to an amp which does not have an extension hack.
Designers know that and rate parts *just* for the task they expect, simply can't take a heavier load.
Being a Peavey, it's well built, and can survive abuse for some time, but don't count on it forever.
If you want to crumble walls, get a used rack type power amp, drive it from a line out jack and add extra speakers to it, not the combo.

[Roly]

Current surges can do a lot of damage all right, but as JM says, a physical cause is much more likely in this case; wire tripped over; and no melted semicons.

[tat]

You are absolutely right J M the Bandit 75 has its place. I have a new respect for an amplifiers output.

The Special 212 has an external speaker jack and gives me 130 W RMS into 8 ohms I just need to get it up for the job. I re-soldered the bridge back into the board but no cigar. I'll pull and test it. The solder on this looks to have been actually splattered from the hole in the board.   

[Roly]

Next time; damaged component?  Pull and test first.

Any suspect components really need to be isolated from their surrounding circuit to be properly tested.

Any suspected damage around the o/p stage then you should be using a limiting lamp, see;

http://www.ssguitar.com/index.php?topic=2093.0

Things may have gone open circuit initially, limiting damage, but now you have started reconnecting things there is a real risk of a short, and a limiting lamp allows some power through for diagnosis with little risk of further damage.


6.5mm connectors were never designed for the sort of current implied by 130 watts into 8 ohms

P = I^2 * R

I = (P/R)^0.5

I = (130/8)^0.5 = 4ARMS, 4*1.4 = 5.6Apk

These were actually designed for 1mW in 600 ohms;

I = (0.001/600)^0.5 = 0.001290A or a couple of milliamps

... maximum, and have a very small contact area leading to very high contact current density and little spot arcs.

In contrast, for example, XLR connectors have three spiral-cut wraps that grip the pin and make a large low resistance contact with low current density.

This Forum is just fantastic for the information people have shared. As I gain more experience and knowledge in amplifier repair I hope to contribute in a helpful way.

Very serious protips/philosophy mode=on:

There is a lot of forensic/CSI/Sherlock Holmes about fixing stuff - above all being observant, and suspicious, aware and cautions of your assumptions (because its normally the bit that you assume is okay that is the actual cause of your trouble, you just haven't found it yet).

My wife is a crossword and suduku nut.  I'm not.  But repairing stuff is a different form of similar problem-solving; there is a crime scene, there are clues, there is a right answer, you will be rewarded when you find it (and most techs get a real charge out of knocking over a hard one, a satisfaction of nailing jelly to a wall and making it stick), but nobody knows what it is yet - and it won't be in tomorrow's paper.  With every fault, you're a pioneer.

It can be very bruising to the ego, but also a more exciting adventure than the best book, movie, or mystery detective story.

The thing itself is just what it is, a lump of stuff.  It has no wish to drive you nuts, and it is totally honest answering your questions; you just have to ask the right questions, and find a viewpoint that makes those answers fit together in a logical and coherent way.  The key point is that you have to shift your view/perspective/understanding until it matches the physical reality you have in front of you on the bench.  The real problem isn't the equipment, it's you.

I use the term "clairvoyant" in the literal sense to mean a clear view, unencumbered by any pre-formed idea.  This is entirely a problem of personal perception and the sooner you get to a point of seeing a mechanism for what it really is, the quicker you can tell very quickly what sort of thing might be wrong, and what will be needed to fix it; to tell at each point if it is this, or that, (or perhaps both).

An important question with all faults (which sometimes can't be answered) is "what caused this"?  A fuse blowing or a resistor smoking isn't the fault, it's a symptom of the fault.  The better you understand how a fault came about, the better you can repair it in way that it won't recur.

There is a huge amount of detail (and they keep inventing new stuff every day to try and keep up with), but generally, be observant "your best service instrument is your eyes", pay attention to detail, be suspicious, be curious, be inquisitive.

Being patient, methodical, and persistent are important, but so is taking a break, particularly if you feel stuck and going around in circles.  Often a new idea will strike while having a meal or coffee break.