Ampeg SVT4-Pro Problem/Question

Flawless!

 

I fix a lot of these that come through as returns. Unfortunately, Ampeg doesn't have a 'Repair Shop', per se, for you to send it to. Repairing a channel is a lot of time and a lot of components. At a normal electronics shop you'll be looking at a whole mess of $$$$, and if you're REALLY lucky the guy might even be able to fix it.
I know this isn't helpful at all.

 

If you were in my area, I'd send you to the best Ampeg tech I know(me).

 

I love modesty!!

On all of the MOSFET amps I've worked on the output devices were the problem. They are not that expensive individually BUT they do have to be matched to each other in order to work properly. Again not a difficult task BUT you'll need a lot of them to produce a matched set. IIRC Ampeg should have these sets available. Unless you have the knowledge I would advise against trying to do the repair yourself.

 

So...PM hodgy for a shipping address, start making a good double box to ship it in and get-r-done.

 

If you have to ship it......For the best service, since it is old enough to be off warranty, LS Electronics in St Louis. Most folks there built, repaired , or even (the owner) designed Ampeg amps. For SLM amps, if THEY don't know the unit, NOBODY does.

The 4 PRO is just more stuff, of the same type as most amps. Yes, it is Mosfet, they are just output devices, and actually more rugged than "bipolar" transistors.

Since the other channel works, the mosfets are much less likely to be the problem. Usually they will fail in a way that causes a short across the power supply and pops the breaker. obviously that isn't happening. Can still be outputs, of course, but no use just assuming the most expensive problem is the issue.

So....... a technician would first determine if the bad channel has a DC offset.... that will make the output relay open to protect speakers. Can be from bad outputs, but also can be from a host of other issues, mostly less expensive.

Bottom line... have your tube tech look at it.... he should, if he is ANY good , be able to tell generally what is wrong. if he can get there, he can probably fix it too.

 

There is much to be learned from Jerrold and the guys that worked at SLM.
I can barely open one of these up without putting between 3-5 hours, depending on how badly things are messed up. So you're looking at shipping, plus whatever the bench charge is ($60-$80/hr. is not crazy for some places). If the tech knows what they're doing, mulitply that by 3 or 5. If they don't, then you'll get something a bit higher.
I fixed one last month that had both channels blown, it took me around 6 hours and I replaced 68 components. On a really good day with one channel blown (or half a channel), you can get away with replacing a mere 18 parts. That's as a minimum.
And no offense to Jerrold's experience with the 4Pros, in the 100+ of these that I've fixed, the Mosfets are nearly ALWAYS the problem (or part of the problem). But that's just my experience.

 

You need to find out!

otherwise we may read about the one someone bought cheap from "a guy" because it had a "blown channel"................... that turned out to be bad solder........

We don't want YOU to be "that guy" that sold it.
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Hodgy..... you betcha....Mosfets can surely be the problem...... especially if they are not matched....... a number of early amps did not have the best matching system used, and they surely could kill mosfets if stressed. And the power amp does get stressed, so it's a prime target for failures.

So can 10 years of dust buildup kill the mosfets, and/or a dead fan, bad rack ventilation, component failure....the list goes on.

To be fair, there's also a fair amount of bad solder, dirty switches, failed op-amps, etc, etc, etc that isn't anything to do with mosfets...... Lot-o-stuff to check through. Which is why I hope the OP gets it looked at to make a smart decision.

Then also....Maybe you see the ones others couldn't or didn't want to fix...... The price of working for the manufacturer.... you get the hard ones.

 

You got that right! I get plenty of units back that have obviously had some other tech's hands in there and were sadly given up on.
Kranahan, there's a pretty simple thing you can check out to see if there's a problem with the Mosfets and you don't even need to be a tech- pop the top open and look for blackend/scorched components! A more subtle approach would be to take an ohmmeter and look at the 1.5k resistors that are coming off of the mosfets. Chances are good that if those are gone, so are the mosfets.

 

For under $30 at Radio Shack you can pick up a functional multi-meter and you might be suprised how much you use it once you get the hang of a few functions on it. If you can, get one with a diode checker on it, that's my #1 secret weapon when fixing amps.
I don't see why you couldn't use this massive power amp for headphones. Unless someone else has some experience saying otherwise. I've never tried it, but since it's a solid state amp you don't need to have a load- you could just run your A and B channel line outs into your mixer.

 

Not exactly like putting your headphones into the plug on the back, but just run the line out or effect send into your computer soundcard (might need adapters), stick the headphones in the jack, and viola...headphone amp. Watch your volume and turn it down all the way when starting it up and gradually bring it up, though. It could easily blow headphones.

 

yes, that is a decent first check. Often the mosfet will have a cracked case if bad (but not always), and if cracked, it will have spewed smoke over the area around it.

And, those 1.5K (in the protection) usually open if the 0.47 ohm 5watt resistors have opened.... so you could alternatively check the 0.47 ohm parts instead of the 1.5k.... you find an open one, the associated mosfet is almost absolutely going to be shorted (blown).

If the amp lights up, and any mosfets are blown, it's odds that some of the 0.47 ohms have blown open, because otherwise it would pop the breaker. There are no fuses in the individual amplifiers, so if the amp is shorted, it will load the power supply and pop the breaker unless either the mosfet has opened up or the associated 0.47 ohm has.

If the mosfet is "open", you'll notice, because the epoxy plastic case will almost certainly have exploded or at least cracked open, with smoke and fire damage.

The 0.47 ohm often won't look any different whether blown or good, you need an ohmmeter.

 

So I fund this by google searching, and thankfully it led me here. I signed up and all. So I have an Ampeg (crate apparently) SVT 4 Pro, with channel B dead, I got it all apart, swapped a few small burnt up resistors, and tested it, and still nothing, then read this thread, and tested the .47ohm 5W resistors. I have 5 of them that appear to be dead (half of channel b all in a row), and thus I assume the 1.5K ones are as well, and likely the mosfets. Now for my questions... How in the world do I get them out... The mosfets appear to have glued themselves to the head sink and the insulator strip. Everything is unscrewed and out, but they wont budge free. Also, once those are free of the heatsink, then what else should I be testing and looking for... are they more of victims, or causes? Anything I should be modding, like larger parts, more heatsinking, hot-glue to reenforce for vibration... etc? Where do I get the parts I need to fix this? What kind of prices will I be looking at? Thanks guys. So far, of all the forums I have be at and read thru, over the many years, this thread is one of the most helpful and useful yet. Good job. Thanks Again.