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Peavey 45W PA-100 Tech Says Junk It !!

Started by Andy54, April 07, 2013, 01:36:30 AM

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Andy54

I've had this 1980's SS Peavey PA 100 for longer than I can remember.

It started to develop a lot of loud static ie pops & crackles even with all pots at zero. This static makes the PA useless for say a low volume level vocal / guitar etc. Increase in PA volume doesn't affect the static volume so it's ok for high volume performance. Not every wants the musicians blowing them away ;)

Took it to a tech who says that he'd have to start at replacing all the transistors, which would add up to many hrs and cost $400 or so.
His advice was to junk it and buy another PA.

Thought I could do the transistors myself. The problem I have is as a newbie to involved electronics [ I can do guitar electrics etc ] when I started to check out
how and which transistors to replace I realised I don't know enough. I found that Peavey has a list of replacement transistors but I found that the original and the suggested replacement have different Base wires eg existing transistor has Base as middle but replacement has Base as left side wire. See attached diagram.

A number of replacement transistors have different base wires to the original so that's got me really confused.

Any help would be great as I don't want to junk this PA. Also haven't got the money for another better one.



J M Fahey

Your "tech" diagnostic is flawed.
"Crackling noises" are not repaired by "replacing all transistors".
The Peavey equivalence is correct (of course), you are mis interpreting your readings.
Don't "shotgun" the amp, meaning don't replace parts at random trying to repair it by chance, because it does not work that way.
Start by googling or asking Peavey for its schematic.
Write them providing exact model and if possible serial number.

phatt

#2
x2 what JMF said.
Just like to add that I recently emailed Peavey for an old model Amp schemo and they did reply with what was asked. (took about a week)
Make sure you quote the exact model and I added the serial number for clarity as models often change mid production.

Even for the novice a close visual inspection can often reveal some clues as to what might be wrong.
At the age capacitors may have leaked, look for goo oozing out of the can or bulging ends. (not to be mistaken for glue)
And don't forget big heavy parts can stress the solder joins often I find hairline cracks in the solder joins of big Can Capacitors.

FWIW ,, I've found older Peavey gear to be very well built so yep might be worth keeping.
Phil.

Roly

Welcome!

Quote from: Andy54Took it to a tech who says that he'd have to start at replacing all the transistors, which would add up to many hrs and cost $400 or so.
His advice was to junk it and buy another PA.

I've got a better idea - junk your "tech" and find somebody who actually knows what they are doing; better still, come here and have a go yourself.

"Replace all the transistors" my  :grr.  This makes me mad.  :trouble

"Mr Mechanic, my car is going, but there is a bit of a miss in the engine"
"Oh, I will have to replace the whole engine, transmission, and back seat - $4000".

If one of the techs who worked for me came out with this sort of garbage they wouldn't be for much longer.

a) he's a total know-nothing nong     :crazy2:
b) he was blowing you off because he doesn't want the job for some reason
c) he wants to sell you a new PA

...or all three.

Get the schematic/circuit, scan and post here, take crisp well lit pix of the amp internals, overview and a few section views, and post those too.

When we have that information we will suggest things to test for and try to locate the source of the problem.  You report back your results, and we proceed from there.  You will need a multimeter, some common sense, and possibly a soldering iron.
If you say theory and practice don't agree you haven't applied enough theory.

g1

  Have you got the schematic yet?  Enzo showed you where to get it from in the other forum:
http://forums.peavey.com/viewtopic.php?f=27&t=30657
  He posts here also, it is probably best to choose one forum and stick with it.
The schematic does not seem to be available, so until you can get it from Peavey and post it here, it will be hard for anyone to help you.

Andy54

#5
Thanks for the positive support guys. I really appreciate the welcome. :dbtu: 

Being a newly retired high school teacher [ workshop / manual arts / materials design etc] I have time on my hands. I can solder and know some basics of using a multimeter. I've been building electric and acoustic guitars for the last ten years in my spare time. 

I live in rural Australia with the closest tech a 1hr drive away. He had the PA for 2mths and charged me $110 for "servicing" ie replacing four capacitors. When I got it home it had the same problem so I rang him and that's when he told me he would have to "do a blanket replacement of the transistors".

Replaced caps are:   1 x 100v 220uF      2 x 63v 100uF   1 x 35v 68uF 

Original caps seem to be in good condition from outer appearances ie no leaking or discolouration.  The two big 50v 2200uF  caps seem in good condition but should I resolder them in case of poor joint ?

I've attached the schematic and some general view photos of this amp.



balaboo

Get yourself a freeze-spray can - in a pinch, auto refrigerant.
Spray the driver transistor and the output transistors, and the emitter resistors. Also, use a magnifying glass to check the PCB tracks, perhaps wash the whole board with alcohol.

Andy54

Thanks for the info but I'm such a novice that I wouldn't know where to find those components at this stage.

Maybe you could give me a rough idea what section of the board they're on from the photos ?

What does freezing these components do ?


Enzo

Freezing components does a number of things.   Some problems are thermal related, ie they don;t start until the amp warms up to full operating temperature.  Hiting a thermally sensitive part with freeze-it will chill it , so that identifies thermally sensitive parts.

But also the instant tempoerature change causes a mechanical shock through the part, so even if it isnlt a thermal problem, it still can sometimes reveal what parts are bad.

As a teacher, if you knew some student was stealing from the class shop, you COULD just throw all the students out of the class.  That would be the "replace all the transistors" approach.   OR, you could take steps to determine WHICH kid was stealing, and then replace just him.   That is troubleshooting, which is what we do here.   ANother problem with all the transistors is, what if it is a noisy resistor?   In other words we threw out ALL the students only to find it was the other shop teacher doing the stealing.

Use something like a wooden chopstick - something insulating - and push on each part on the circuit board.   Do any of them respond with the crackle?  And for that matter, just press down on the center of the board a little to give it a touch of flex.  ANy reaction?  Those indicate a cracked solder joint or a cracked copper trace.

Sure, resolder those large caps, can't hurt.  Those are old and may be worn out, but crackling is not what old worn out caps do.   Certainly leaking and bulging caps are bad, but 99% of bad caps look no different from good ones.


The base is NOT the middle leg of the EP430, why do you think that?  The 430 is just an old Peavey house number.  The TIP31 drops right into its place.  That size and shape is called TO220.  TO for transistor outline.   As far as I know ALL TO220 bipolar transistors have the same pinout (leg arrangement), which is B C E.

Consider your troubleshooting goals:  Your amp works, but makes crackling noises.   SO I tend to doubt you will find some part that measures as BAD on a meter.   Of course one of my Enzo-isms is to never think up reasons not to check something.  SO chek them, but don;t expect to find readings WAY off the schematic unless there are parallel circuits to confuse the meter.

You isoloated the problem to the power amp by turning the master volume to zero and the noise remained.  There are only four transistors in that circuit.  In fact ther are only 14 transistors in the whole thing.  $400?????

The small transformer is to drive the output transistors.  If you removed that 430 transistor and powered up without it, you would hear ONLY any noise the two large output transistors make.  The transformer isolates them.  The transformer itself could have a cracked lead, but I think it would do more than crckle.  Still, grasp it and give it a little wiggle, any noise?  If the noise continues without the 430, the your outputs are somehow noisy or the transformer is funky.   If that stops the noise, then it is coming from the 430 or the small transistor wired to it most likely.


Much as I dislike the shotgun approach, it probably would not hurt my feelings to just replace the 430 and the small guy with it.

Roly

(crossed with Enzo, wotesaid, plus...)

Quote from: Andy54I live in rural Australia

Where?  I'm at Creswick near Ballarat Vic, but I also run Ozvalveamps, am on AGGH, and may be able to find someone to give you a hand.  Your background sounds like you have a good fighting chance with this one.

In a tighter pinch some metho on a cotton bud dabbed on suspect components, followed by a gentle blow also works as poor mans freeze-it.

Quote from: Andy54Maybe you could give me a rough idea what section of the board they're on from the photos ?

We will, just have to digest the circuit a bit first.

Quote from: Andy54What does freezing these components do ?

This is based on the guess that noise such as you describe is coming from inside a failing component, most likely a transistor, and by chilling it you often get a change in its behavior.

Quote from: Andy54It started to develop a lot of loud static ie pops & crackles even with all pots at zero.

We just need you to confirm that this is the case because if it is it means that the fault is in the "back end" of the amp, the main power amp that drives the speakers, and not in the preamps, which eliminates quite a large lump of the circuit to start with and gets us a lot closer to the fault.

Simple test; does thumping the case make any difference to the noise?  If it does it suggests that it may only be a broken solder joint or similar, if not it is more likely to be one of the components itself, and in particular a transistor, but overall this is not generally a failure mode of larger power transistors (the big ones bolted to the heatsink).

Normally I would say persist with a tech who has missed a fault, but in this "blanket transistor replacement" case, give him a big miss.  His shotgun or blunderbuss method of servicing always ends in tears (and in this case a badly depleted wallet).  It is not how real tech's repair things.

Which leads to; first we find the fault, then we take Berlin.  Diving in and randomly doing stuff actually muddies the waters, so first we find out what is happening, then we repair that, and we know it is actually repaired and not just papered over to reappear at the worst possible moment.

Assuming that the fault is not changed by the Master volume control, the areas we need to look at are the driver stage which is on the end of the main PCB nearest the pilot light (that transformer is the driver transformer) and perhaps the output stage itself which seems to be mostly on the back panel, so we need a pic of that from the inside.  It also looks like the output transistors are connected via sockets, and these must also be suspect at this age, but should be cleanable if needed.

Just next to the driver transformer there are two transistors, a small pre-driver and the flat pack driver itself.  What you can try at this stage is to take a cotton bud wet with (fresh, dry) metho and dab a few drops on the small transistor with the yellow top (circled in the attach), and see if that produces any change at all.  If not, try the flatpack transistor next to it.  This may cure the problem for a few moments, or it may suddenly make it worse, but what we are looking for is any change, something that indicated we are close to the fault.  {even if it responds it still may not actually be the transistor but an accumulation of dirt on the bottom of its case - I had one just like this that got noisy when you breathed on it, and a good clean where the leads came out cured it completely.}

You are a bit lucky in that this output stage is one of the older transformer-coupled style and should be somewhat easier to fix than a modern direct-coupled one.

- confirm it's there with the Master down
- dab the transistors and report results
- post pic of inside rear


Quote from: EnzoOf course one of my Enzo-isms is to never think up reasons not to check something.

:lmao:  :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.

Andy54

#10
Thanks Enzo, Roly et al all very helpful. I'm certainly going to have to read and digest this info.

Roly, I live south of Murwillumbah Nth Coast NSW near one of those villages that consist of only a post office/ general store/ garage called Burringbar. I've gone into hiding to escape former students  8|

Over the next while I'll try some fault finding.

When the master volume is at zero the amp has static. Static starts as amp warms up
 

Attached are a pic of the back of the amp. Also a component [ approx 7mm x 30mm, brown with one grey & one black stripe ] I found under the two large capacitors in what I now know to be the driver stage. It looks like it's leaking a fine machine oil.

Could this be contributing to the problem ?

phatt

Hi Andy,
Man I love equipment like that,, so obvious and you can see the parts and room to work on it all. <3)

As to Enzo's thinking up reason not to not check something,,  :cheesy:
And Also
"When the master volume is at zero the amp has static. Static starts as amp warms up"   

The ol grey matter triggered a distant memory. Ding!

There is the very outside chance that it could be the main power switch failing.

I picked up an old Sansui HIFI unit a few years back with same symptoms, drove me nuts for ages. Amp worked,, no overheating,, just an annoying crackle as it warmed up and stayed there,, WTF??  :grr :grr

Finally had a moment of brilliance and indeed the spring loaded terminals inside the mains switch where heating up the spring causing loss of contact.

Replaced switch,,The Amp now works perfectly.
Just thought I'd mention it,,You never know?

Cheers from a fellow Aussie mate,, I'm in Nambour and I'm likely even more rural than You,, Heck apparently anything north of the NSW/Queensland border was once considered outback rural by some clown polly in Canberra.  :lmao:

Phil.

joecool85

Wow, we're becoming quite the Aussie hang-out here! 

Welcome aboard and good luck fixing your PA.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Roly

I actually know of Burringbar - I lived at Elands near Wingham/Taree for about 15 years; same PO/general store/pub all in one. (for the benefit of our international friends, we are now about 1000 miles apart).

Quote from: Andy54Also a component [ approx 7mm x 30mm, brown with one grey & one black stripe ] I found under the two large capacitors in what I now know to be the driver stage. It looks like it's leaking a fine machine oil.

Could this be contributing to the problem ?

That will be one of four rectifier diodes, and since they don't contain oil I doubt this has anything to do with your problem (really great macro BTW).

Interesting thought @Phil, but my money is still on the pre-driver (the little transistor with the yellow top).

A simple way to test Phil's idea would be to unplug the amp, solder a wire link across the power switch (provided it only has two connections) and switch it on at the power point.  (If it has four connections please don't try that at this stage, post a pic first).  Also pull the mains fuse, give the ends a good clean and restore and test.

Investigate where the wires on the driver side of the driver transformer go.  (one side will go to the flat pack transistor and power rail, the other towards the output transistors on the back.  If chilling the pre-driver and driver come up zip it might be helpful to lift one of the driver side transformer connections (if it's not too difficult/impossible), that way we can confirm (or not) that it's coming from the driver stage and not the output stage.

You can also try holding your soldering iron on the top of each of the driver transistors for a few seconds in turn to see if heating changes anything.  My bet is that when you heat the pre-driver it will get worse, but we'll see.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

Quote from: joecool85 on April 08, 2013, 09:07:35 AM
Wow, we're becoming quite the Aussie hang-out here! 

Welcome aboard and good luck fixing your PA.
Rather than an Aussie thing, we might call it an Antarctic area. :duh
Argentina and Australia/NZ are *much* closer than conventionally drawn maps show. :o

Just look at our AerolĂ­neas Argentinas Buenos Aires-Auckland/Sydney direct flight path:



For me, shorter than flying to Europe or USA.

Yes, of course, it will be a "chilling" experience flying so close to the South Pole, and the plane should better *not* make a crash landing.
Rescue would be *very* difficult, if at all possible.