SVT-4 Pro issue

Tyme · #1 04-05-2011, 04:34 PM

Problem starts after amp is powered up for about 5 to 30 minutes. I'll hear some cracking followed by a loud pop.Then no audio produced on both A or B outputs. This happens when playing, or idle. I can turn the compression to the off position and it will produce very low audio, muddy and distorted (peak led flashes while adjusting compression back and forth). No other button/knob makes a difference. I can then power down, then back on works great..

then the process starts all over again.

Wired stereo into 2 Ampeg HLF cabs. Bridge button set to stereo mode.

After reading other threads here on SVT-4 problems I preformed the following.

Replaced pre-amp tubes
Re-soldered every connection
Cleaned every input, output & all switches
Disconnected BBE channeled through effect sends/return
Looked for any visible issues (none seen)
Moved head from top of cabinets
Added fan outside cabinet to help with cooling

I brought to a local "Amp guy" prior to me doing any of the above, and he stated that he re-soldered a couple bad solder joints and it tested fine, producing full power both channels, and in bridge mode. but was never able to produce the same problem.

If someone could recommend any other possibilities or a good solid state amp guy in the Springfield, MA. area, it would be greatly appreciated.

Thanks.

craig.p · #2 04-05-2011, 06:38 PM

In between the last time the amp was ok and now, was the amp transported? If so, did it take a beating?

Next time the problem manifests, pound the top of the amp, but softly, and see if you can make the problem intermittent rather than steady. Or at least see if a crackling noise happens. That would point to a bad solder joint or bad etch (aka trace).

Your amp guy may have resoldered a couple of bad joints but that doesn't necessarily mean he found them all. Also, did he check the tube sockets?

Check internal jumper cables (if any) for tight pin fit.

See if the amp takes longer to manifest the problem when you run it in the garage. It's only April, and Springfield is still cold. I'm thinking the problem may be temperature-related.

Tyme · #3 04-06-2011, 03:31 PM

Hey Craig, Thanks for the reply.

No beatings/drops that I'm aware of..I've tried to reproduce the problem by banging on the top prior to... & while the issue exists. no difference/change either case. I personally re-soldered every connection and cleaned every connection to include the tube sockets as well as internal jumper connections/pins. My guess is that it's temperature related too..that's why I added a second fan on the outside of case. I was thinking it might be a internal temp device/component going bad or something, but my abilities end with soldering. I would love to find a good amp guy that has solid state troubleshooting abilities.. seems they all major in tube amps, and minor in solid state around here.

craig.p · #4 04-06-2011, 04:09 PM

Wow, when you said every connection, you meant it!

I went to Ampeg's support area. They have a page where you can type in a zip code and they'll show a ton of repair places. Not sure how close any of 'em are to you, but I think Extra Effort Electronics is less than 30 mins from you. They're an Ampeg warranty center, too, so they must at least halfway know what they're doing.

EXTRA EFFORT ELECTRONICS
498 BRITTON ST
CHICOPEE, MA 01020
Phone: (413) 533-1208

Tyme · #5 04-06-2011, 04:36 PM

Yes, I too went to the Ampeg site and contacted Extra effort. They were not happy with there dealings with "Loud" and were in the process of terminating their service agreement, and not taking in any more Ampeg product for repair. Orange Sound was the "Amp guy" that soldered a couple bad solder joints and returned amp with the same problem. the others are out of business, with the exception of the big box stores. Maybe "Extra Effort" is worth a second call... seeing it isn't a warranty repair.

craig.p · #6 04-06-2011, 05:05 PM

Yeah, they probably meant they're not taking in warranty work any more.

Tyme · #7 04-06-2011, 05:10 PM

No, He was pretty unhappy with Loud.. He was very clear that he wanted no more dealing with them, even for parts. I originally told him it was non warranty.

craig.p · #8 04-06-2011, 05:19 PM

Oh. Ok, gotcha. I guess they hosed him & he went (justifiably) nonlinear.

Bright side of this is that it's not 80 pounds but only 40. I mean in terms of having to ship it somewhere.

Hey, just wondering, have you a) tried another speaker cable, b) tried another cab, c) tried running the amp with no load & plugging a cab in only after an hour has gone by? Also d) connected fx in to fx out with a cable? I know they're long shots, but at least if you try them you can knock them off your list of possibilities.

jphilauren · #9 04-06-2011, 05:27 PM

Someone from my hometown!
Try Aztech Electronics, Inc., Watertown, Massachusetts they are in Watertown you may not find anyone in western mass, these guys are great. You might also have some issues with the various 1/4" inputs, might want to try either cleaning them or take a plug with emery cloth wrapped it.

Tyme · #10 04-06-2011, 06:30 PM

Thanks guys, I'll give your suggestions a try. Would a bad/failing LM35 temperature sensor react this way? Thermal shut down?

craig.p · #11 04-06-2011, 06:57 PM

I don't have a schematic for that amp, so it's tough to say. Depends on how they're using the part, where it's mounted, etc.

You could pop the cover and feel whether the heat sink(s) or any of the output transistors get excessively hot when just sitting idle. You might have an output transistor poorly thermally coupled to its heat sink, but this is a total swag, so take it for what it's worth, i.e. as close to zero as you're going to come before the day's out.

The problem could be almost anything, really. The amp needs to cook into a hefty dummy load (with an audio generator driving the input) until the problem surfaces. If that doesn't reproduce the problem ("no fault found," same as what the other tech saw), then it's got to be your speaker cable or one or more speakers in your cab. Even a rubbing voice coil or partially shorted voice coil could do it, driving the impedance the amp sees down to where the amp gets too hot, or changing the impedance's reactive component enough to make the amp unstable and go into oscillation. This is why I suggested what I did in the previous post. But again, these are just SWAGs+imagination, feeding off a bad dinner about an hour ago.

Jerrold Tiers · #12 04-07-2011, 08:47 PM

It sounds like something is screwy in the compression circuit.....

The control for that is directly in the grid circuit for the second tube stage. If some DC from, for instance a leaky cap (C2) gets in, it might do something similar, although I suspect the compression amplifier IC as much as anything else.....

The peak light flashes indicate a "signal" coming from the control, which should not happen, unless maybe some DC is on it... That would mess up the grid bias of following tube.

Does seem to be localized there, if that is the onlt control that affects it , and there are other effects of turning it (the peak light).

Loose solder, maybe.... temperature sensitive part, either cap, or more likely the IC, very possible.

The good news is you have a decent indication os WHERE the problem is, for anyone with access to schematic.

I can send you one, if, for instance, the tech doesn't have it, or you just want it "in case".... PM me an email.

fenderphil · #13 04-07-2011, 08:56 PM

have you tried it with a different cab? or removing one cab? maybe a short in one of the speaker wires is causing the amp to overload.

Tyme · #14 04-08-2011, 02:03 PM

Craig.. Thanks again.

Jerrold,
Great info, thanks. I do have a schematic, but have limited skills using them to troubleshoot components.

Phil,
Yes, I've tried swapping cables and speakers etc. Everything works great with my backup amp. No problems.

I have the amp in the professional hands

of another amp repair guy. I'll report back with his findings.

Thanks for the reply's!

Tyme · #15 05-26-2011, 02:10 PM

Hello Again,
Well the new repair guys had my amp for 2 months now and the following is his e-mail to me.

"Hi Mike. The output transistors are the most likely culprit. They break down under load when they get warm enough but they test good on the testers. Substitution is the only way to eliminate them but there are alot of them and they would all need to be swapped out and they are costly. That's why most folks don't want to work on the high wattage solid state equipment. If you don't replace the faulty parts you can instantly fry the brand new ones you just installed. The factory would be able to fix it and ensure that everything is 100% with a guarantee. They would probably just replace the output boards with brand new ones instead of going to component level. Gary"

Should I have him replace all the output transistors and hope for the best?

Jerrold Tiers · #16 05-26-2011, 09:43 PM

Quote:

Originally Posted by Tyme

Hello Again,
Well the new repair guys had my amp for 2 months now and the following is his e-mail to me.

"Hi Mike. The output transistors are the most likely culprit. They break down under load when they get warm enough but they test good on the testers.

Should I have him replace all the output transistors and hope for the best?

I wouldn't suggest it...........

While it IS at least possible for there to be a problem of that sort with *an* output mosfet, it is essentially impossible for there to be a problem with *all* of them.

I suspect he wants you to give up and go away.

The way he describes the problem he "suspects" is not a description of a problem that can happen more than once.... if the devices "break down" they have "failed", and probably will be damaged and not work thereafter. I suspect that is just careless use of terms...... in the attempt to get you to go away by scaring you off with a predicted big bill.

Your initial issue with the control would not be in any way associated with the output devices.... you said the compression control would light the preamp peak light when turned, which is not supposed to happen.

That, as I think I mentioned, can be caused by a leaky capacitor , which is not impossible, would likely cause the limiter to be "on" full, cutting the volume, not to mention causing a strong positive bias on the next grid, shutting down that tubeand the sound. It could even produce loud "cracking" noises. The suspect part in that case would be C2.

Am I saying that is the problem? Nope, I am suggesting that if someone presented me with the description you gave, I'd probably be looking for a problem in that area first.... I might end up elsewhere, of course.... stuff happens.

Tyme · #17 05-27-2011, 04:48 AM

Thanks for the reply Jerrold. I got the impression he gave up, and is scaring me off as well.

Tyme · #18 07-13-2011, 02:44 PM

Quote:

Originally Posted by Jerrold Tiers

I wouldn't suggest it...........

While it IS at least possible for there to be a problem of that sort with *an* output mosfet, it is essentially impossible for there to be a problem with *all* of them.

I suspect he wants you to give up and go away.

The way he describes the problem he "suspects" is not a description of a problem that can happen more than once.... if the devices "break down" they have "failed", and probably will be damaged and not work thereafter. I suspect that is just careless use of terms...... in the attempt to get you to go away by scaring you off with a predicted big bill.

Your initial issue with the control would not be in any way associated with the output devices.... you said the compression control would light the preamp peak light when turned, which is not supposed to happen.

That, as I think I mentioned, can be caused by a leaky capacitor , which is not impossible, would likely cause the limiter to be "on" full, cutting the volume, not to mention causing a strong positive bias on the next grid, shutting down that tubeand the sound. It could even produce loud "cracking" noises. The suspect part in that case would be C2.

Am I saying that is the problem? Nope, I am suggesting that if someone presented me with the description you gave, I'd probably be looking for a problem in that area first.... I might end up elsewhere, of course.... stuff happens.

Hello Jerrold,
Amp guy replaced the C2 Cap, and still have the same issues. The amp guy stated it seemed to get better, but still cuts out. any other ideas as to what we could check next? I really appreciate your help BTW, my only other option is to shipped the amp out... in which I have been avoiding having to do. Thanks.

Jerrold Tiers · #19 07-13-2011, 07:08 PM

Well, I didn't say "replace C2", I suggested that one idea is possible DC bias on the compression control..... C2 is one place it can come from. And that does happen sometimes, a cap goes leaky at some temperature, often making a loud noise.

Did he look for DC? if so and there wasn't any, then C2 is not the issue......

You have to realize that troubleshooting by remote control is not really a happening thing.... And that if I, or a tech elsewhere who was familiar with that unit were actually there with it, we'd have the main possibilities checked out in 15 min..... It's a simple matter of elimination, based on symptoms presented. "I see this happens, that could be here.... nope, or there...... nope, or maybe over in this other area...... AhHA ... there is it...."

or maybe it's a series of "nope" answers, and it goes "under the bench" for a couple days while money gets made and rent paid, and then the $#@! thing comes out for another session.

For "successful" techs, it comes down to " what could do that?" followed by "check those things out", followed by "fix what you find wrong", followed by "re-check to be sure it's really fixed".

Someone on the spot needs to do that.

IIRC, we have a case where after a while there is a loud noise, followed by low/no sound, except that the compression knob in preamp affects the sound, from nothing to really low volume, AND the peak LED flashes when you turn that knob. And it does not always do it (of course).

The compressor is an LED/photoresistor type, op-amp driven, with the op-amp input adjusted by the "compression" control. The connection is completely DC, no caps, to the top of the gain control. C2 is what feeds signal to there, and a leak would turn on the compression. The peak is fed from there too, so DC could affect it too.

Ok, C2 is OK, But we don't know about the opamp, or the opamp power supply. Either could be intermittent, and the PS could surely cause a big noise if it drops the + or -. Opamp problems are less likely to, since it is isolated by the LED/photoresistor. pot problems would probably just make the compression not work.

The photoresistor could really knock the signal down if turned-on hard by a DCed op-amp, possibly due to a bad 15V supply, OR a defect in opamp that makes it go DC when hot, OR a bad solder joint that loses contact when hot, at the opamp, or at the +-15V supply...... etc, etc, etc.

I am focusing on those, because your comments about the compression control suggest it is part of the issue, AND I really don't believe in "coincidences" unless I am forced to.

Tyme · #20 07-14-2011, 01:59 PM

Yes, I asked him to take a look at C2 cap, and he said he replaced it because although it tested fine on his tester, that sometimes the tester doesn't produce enough voltage to recreate the problem, and testing in the circuit may give false readings.. so he just replaced it. I wish I had better options with regards to area certified Ampeg technicians. but it's looking more and more like I'm going to have to bite the bullet, and ship it out. I appreciate your help.