swr workingman 15 catastophe !

[ducatiman]

gentlemen! this noob begs your help. playing very loud in very warm ambient conditions and my swr WM15 bass amp blows the main fuse (of correct value).
upon later inspection , i find that capacitor C11 has failed (vented). i replaced with new (of correct value)
plugged in with no load or signal and 20-30 seconds later POOF again! same cap blows!
obviously more than meets the eye going on here.

i'll try to attach schematic to my 1 st post here.

any help or opinions are greatly appreciated.

thanks, ducatiman

[Mars Amp Repair]

Well,
Knowing these swr amps, most probably you have a blow output section. There are probably a few transistors shorted in a way that is bringing the + & - voltage supply rails across that cap & exceeding its voltage rating. like Q-10 & Q-3, but if any of them is shorted, there are others also shorted.

If you've never worked on a transitorized power stage, my suggestion (esp with the SWR's) would be to refer it to a reputable repair guy in your area. They can be very tricky as well as frustrating if you don't have the proper equipment to test with.
What is your level of experience? glen

[Enzo]

Right. Those output devices are too expensive to just flip the power on and see if it works. Either bring the thing up on a variac or at least use the lightbulb trick.

Check the power transistors for shorted condition. And the drivers.

And work without a load until you are sure the amp is not putting DC out on the speakers.

Wouldn't hurt my feelings to replace that cap with a higher voltage rating.

[ducatiman]

off to the tech.....

i'm strickly a parts replacer....off to a tech she goes. no time to learn a new trade now.

would it be unreasonable to pay 1 hour of bench rate for a "quotation" of repair cost? enzo and glen, how do you guys work it?

both you guys, thanks much for your input, with just a DMM to my name, i'm well aware its over my head.

------------------duc

[Enzo]

I try to avoid estimates, I prefer to establish a budget. I can work on an amp a while and find a lot of bad parts, but until I fire it up and check it out, I don't know what else might be bad. Basically to do an estimate you have to find oout what is wrong in the circuit. So you are basically doing the repair to make the estimate.

My experience is that 99% of people who ask for an estimate "before you do any work," and I tell them $80-100, they respond with "Oh, I just didn't want to get a $300 bill surprise." If we just discuss that we don't want to pay more than $150 for a repair, then the tech can say, Oh we will be well below that, or alternatively, both these transformers are smoking, it will be more than that. Whatever.

On the other hand, if the tech takes it apart and spends time diagnosing it, he deserves to be paid for his time.

In my shop, I gave everyone my once-over for free. I'll take the lid off and in a few seconds I can determine if the output transistors are shorted, or the main rectifier, or if necessary, look for clues to a transformer failure. Xfmr failures are rare, really, so it doesn't come up that much. Once I know the nature of the repair, I can quote close enough.

[Mars Amp Repair]

I agreed with Enzo.
The times I end up wasting a bunch of time esimating a unit & having it turned down, is the time I don't give the guy a ballpark figure to expect up front.
I do have a fairly minimal estimate charge & let folks know that up front. I helps as I'm not always there when the units come in. I've never had anyone complain about it either. Of course it's only charged if they refuse the estimate.

As Enzo said, most folks are cool with what it commonly takes to repair most amps, but just didn't want the $300 surprize. glen

[Rob Mercure]

I'll "third" the estimate bugaboo and give a very "fresh" example. Had a JCM 2000 in until yesterday morning which, according to the customer, popped the HT fuse a few hours after being dropped. Got into it and it had a sorry quartet of mismatched old EL 34s - some of which seemed to have come with the amp in 1998. Told the customer that "if nothing else was wrong" the cost would be the bench fee if he supplied the tubes (hey, who makes any money on tubes these days? I'd rather the customer order them if I don't have 'em in stock and take me out of the loop - but I do sell NOS tubes for about 80% of the going rate.) So, got the amp in and after rebiasing it with a new set of SED's there was still a hum in the output section that I determined was coming from the bias supply - 120 Hz hum on g1 on the 'scope and full wave bias supply in these puppies. The huffy customer didn't want to pay to have the bias hum repaired and seemed to feel that my "estimate" of just the bench fee should take care of the issue! (I did diagnose the problem within the bench fee time but I can't repair the problem within the 1/2 hour the bench fee covers). I did try to establish an initial "budget" but after you buy the tubes and pay for the fuses it's hard to hold "$100" hard and fast! Guess they think that the unit can be repaired at Walmart.

Rob

[Enzo]

Even when the customer is going to supply the tubes - and I agree with being out of the tube loop - I always install my tubes to see how the rest of the amp is working. FOr exactly that sort of reason.

Try explaining to the customer when you quote a job at your rate expecting to charge an hour labor for example, and he knows another shop that charges less per hour, though the tech there would probably charge two hours. He hears teh rate and nothing else.

[Satamax]

So, got the amp in and after rebiasing it with a new set of SED's there was still a hum in the output section that I determined was coming from the bias supply - 120 Hz hum on g1 on the 'scope and full wave bias supply in these puppies. The huffy customer didn't want to pay to have the bias hum repaired and seemed to feel that my "estimate" of just the bench fee should take care of the issue! Guess they think that the unit can be repaired at Walmart.

Rob

How many times have i heard that new tubes should take care of all the isues