Ampeg SVT4-Pro Problem/Question

[kranahan]

Hey guys!

I picked up a MIA 4Pro last week for $300. The only problem is that power amp A does not work (mono bridge does not work as well). Power amp B works great! I have read old forums on problems very similar to this with the 4Pro. I have tried contact cleaner and inserting instrument cables in all of the jacks. I have played with all the buttons to see if that was the problem. Nope. Still no sound!

So... I am wondering where to go from here. Should I send it back to Ampeg to see if they can fix/replace the power amp or is that too expensive?

I have a tech but he specializes in tube amps. I'm not too sure about his knowledge on mosfet power amps...

I'm just looking for someone who has been in a similar situation and what you did to resolve the problem.

Thank you all and have a great day! GO SHARKS!


*Sidenote: There are no certified Ampeg service centers in the SF Bay Area.

 

[Dave W]

I picked up a flawless MIA 4Pro last week

power amp A does not work (mono bridge does not work as well).

Flawless!

 

[kranahan]

Flawless!

Correction: Aesthetics are flawless!

 

[hodgy]

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.

 

[kranahan]

You are right. When I sent my 3pro back about 5 years ago, Ampeg just gave me a new unit because it was still covered under warranty. No attempt at repairing it as far as I was aware of. Hmm... What to do?

 

[hodgy]

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

 

[BassmanPaul]

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.

 

[B-string]

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

 

[Jerrold Tiers]

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.

 

[kranahan]

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.

Thank you Jerrold!

The tube tech said no on the amp because he does not do work on mosfets/solid state amps.

The closest certified Ampeg technician is in Sacramento. I'm going to give them a call today. If they won't do it, then I will consider shipping it to LS.

Does anyone know how much the repair should cost?

 

[hodgy]

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.

 

[kranahan]

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.

Maybe it's not worth it to keep it with those expenses. I could just buy a used one for cheaper than the total expense of the 4pro and repairing. I need to think this out...

 

[Jerrold Tiers]

Maybe it's not worth it to keep it with those expenses. I could just buy a used one for cheaper than the total expense of the 4pro and repairing. I need to think this out...

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.

 

[hodgy]

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.
*
*
*
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.

 

[kranahan]

Jerrold: I am taking it up to Sacto next week. I'll keep everyone posted.

Hodgy: The first thing I do when I purchase used amps is take a look inside. Then clean it! I did not see any blackened/scorched components. I do not have an ohmmeter but I will look into investing in one.

Side question:

I was originally looking into purchasing a 7pro as a practice amp to use with headphones. My new living situation does not allow me to play through speakers. When this deal came along I felt I couldn't pass it up. Anyways... I am wondering if the 4pro (when healthy) can be run through a mixer or headphone amp and still use the stereo output through the headphones? For example, 4pro (stereo output) -> mixer/headphone amp -> headphones. I know it sounds like a dumb question but I want to make sure before I spend more money on this rig.

I know it seems a waste to use the 4pro as my "headphone amp"...

I really appreciate all your help this far.

 

[hodgy]

Jerrold: I am taking it up to Sacto next week. I'll keep everyone posted.

Hodgy: The first thing I do when I purchase used amps is take a look inside. Then clean it! I did not see any blackened/scorched components. I do not have an ohmmeter but I will look into investing in one.

Side question:

I was originally looking into purchasing a 7pro as a practice amp to use with headphones. My new living situation does not allow me to play through speakers. When this deal came along I felt I couldn't pass it up. Anyways... I am wondering if the 4pro (when healthy) can be run through a mixer or headphone amp and still use the stereo output through the headphones? For example, 4pro (stereo output) -> mixer/headphone amp -> headphones. I know it sounds like a dumb question but I want to make sure before I spend more money on this rig.

I know it seems a waste to use the 4pro as my "headphone amp"...

I really appreciate all your help this far.

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.

 

[JimmyM]

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.

 

[Jerrold Tiers]

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.

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.

 

[kranahan]

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.

Wow. Thank you. It's a little bit too much information for my basic amplifier knowledge. I'll start with the ohmmeter. Then check the resistors. The 4pro goes to the tech on Tuesday no matter what!

 

[vintagegroove]

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.