What's wrong with my amp?!?

[webmiztris]

I have a Marshall Valvestate AVT 150 watt combo amp. One day I unplugged the speaker and hooked it up to a Mesa Boogie 100 watt head. It sounded great. When I was done, I put everything back to normal. Suddenly I had a MAJOR loss of volume. I had to crank the master volume to 8 just to make it sound as loud as it did prior set to 2. And there was also a constant low crackle. These problems only happened on the Overdrive channels though, not on the Clean channel. That channel sounded like it always did. Also, there was no speaker crackle.

Sooooo, my question is: Does anyone have idea what the problem could be? I've had this amp at the shop for about two weeks now and he still can't figure out what the problem is. He said he changed the power IC's (????) and that didn't do the trick...and that it's not dirty pots. My invoice is already at $125 and I'm kinda freakin' out. LOL HELP!

[lonewolf]

If your clean channel works OK, then its probably something wrong in the OD preamp circuit--not the power amp. I doubt if there are any power chips in the preamp circuit, except for maybe the output buffer stage to the power amp. That sounds a little like you might be paying for OJT.

As I recall, that is a tube preamp--it might just be a bad tube. Or, it could be a bad transistor or op amp IC in the preamp if the crackle sounds like bacon frying.

You can isolate this problem by using the FX send jack. Connect the FX send to another power amp and use that amp to drive speakers. Switch between the clean and OD channels and if it still doesn't work right, the problem is in the OD preamp circuit. If it works OK with the master V on 2 like before, then you know the problem is in the power amp.

[webmiztris]

Could it still be a bad tube even if the clean channel sounds the same as it always did? I wish I had the amp here to try your suggestion. He said once he tries the next part, my bill will be at $150, and even then there's no guarantee it will work. I'd think if he can't fix my amp, I shouldn't have to pay for a thing! Maybe I should call him and tell him to put the old parts back in and give it back to me because I'm starting to think I'm getting boned.

[lonewolf]

Could it still be a bad tube even if the clean channel sounds the same as it always did? I wish I had the amp here to try your suggestion. He said once he tries the next part, my bill will be at $150, and even then there's no guarantee it will work. I'd think if he can't fix my amp, I shouldn't have to pay for a thing! Maybe I should call him and tell him to put the old parts back in and give it back to me because I'm starting to think I'm getting boned.

Yes. Its possible that the clean channel is solid state and the OD channel uses the tube(s). Its also possible that if there is more than 1 tube that the clean channel uses one tube that is still working and the OD channel uses one that is bad. I can't really tell without looking at the schematic.

[lonewolf]

I haven't found an AVT schematic, but the VS100 schematic shows the FX loop exiting before it gets to the tube stage. This is very important because...

If this is the case with your amp, you can test to see if the problem is in the solid state preamp section by using that method I mentioned with the power amp and speakers in the previous post. If it still crackles, its probably a $1 OP AMP.

Both the clean and OD channels use the tube, so its probably not a bad tube, but it won't hurt to check. The tube runs much hotter in the OD mode and might run OK when in clean mode and start cracking up under more stress in OD mode. If the tech hasn't already tried a different tube, he should.

[onegunguitar]

Just get it to Lonewolf-problem solved.

[webmiztris]

I wish I could do that! I'm really frustrated that I haven't had my amp in two weeks now and that the guy still has no idea what the problem is and is planning to charge me $150. So I already owe $150 and it's not even fixed yet. That doesn't seem right to me.

[onegunguitar]

I wish I could do that! I'm really frustrated that I haven't had my amp in two weeks now and that the guy still has no idea what the problem is and is planning to charge me $150. So I already owe $150 and it's not even fixed yet. That doesn't seem right to me.

Yea,that does suck.Maybe you should tell him to call Lonewolf,haha.

[webmiztris]

I wish I could do that! I'm really frustrated that I haven't had my amp in two weeks now and that the guy still has no idea what the problem is and is planning to charge me $150. So I already owe $150 and it's not even fixed yet. That doesn't seem right to me.

Yea,that does suck.Maybe you should tell him to call Lonewolf,haha.

I'm seriously considering it Would it be wrong to advise the guy to refer to Lonewolf's responses here on Rockpage before ordering any add'l new parts?

From talking to the place today, it seems a lot of parts are damaged but I don't understand it. I worked one day and then didn't work the next. I didn't drop it or crank to 11 or anything like that (lol) so I really don't understand what could have happened to it. It just sits here in my living room!!

[lonewolf]

I managed to find an AVT150 schematic. It is much different than the VS100. If you want a copy for the shop, PM your email and I'll send it to you.

If I was changing out parts on this amp, I would start with all the NJM072BL op amps in the OD preamp circuit, namely the ones designated: IC1, IC6, IC7, IC8 & IC10

You can tell the shop that if you want.

They are available at Mouser for $0.50 each:

http://www.mouser.com/ProductDetail/NJR ... b1H9Ubs%3d

[JackANSI]

I managed to find an AVT150 schematic. It is much different than the VS100. If you want a copy for the shop, PM your email and I'll send it to you.

If I was changing out parts on this amp, I would start with all the NJM072BL op amps in the OD preamp circuit, namely the ones designated: IC1, IC6, IC7, IC8 & IC10

You can tell the shop that if you want.

They are available at Mouser for $0.50 each:

http://www.mouser.com/ProductDetail/NJR ... b1H9Ubs%3d

ESD might have fried the opamps. good call.

[VENTGtr]

Would it be wrong to advise the guy to refer to Lonewolf's responses here on Rockpage before ordering any add'l new parts?

Not at all. It's your money and they don't seem to be getting anywhere on their own.

From talking to the place today, it seems a lot of parts are damaged but I don't understand it.

This would concern me. Not because of the amp as much as who is looking at it. Any trust quotient would drop substantially.

[todd17063]

take it to Sarge over at Rainbow Music in State College.. He will tell you whats wrong with it and he would call you and tell you what it needed before he fixed it to see if you want it done. I had my Fender deluxe do the same thing as yours and he fixed it for right around a hundred bucks.

[Banned]

Have you tried testing the speaker it self? If you hooked up a speaker at different ohms you could have damaged the coil in the speaker itself. If it was working fine before hand I would diagnose it as something the Mesa did.

[Banned]

Also how would any part of the amp be damaged if the amp wasn't recieving or sending power?

Also I keep thinking of things but...... This happened to a friends Mesa Tripple rec. The tube came out of the socket just enough to function and clean worked fine but same symptoms actually.

[JackANSI]

That's why I assume ESD on the opamps might be it (and generally agree with lonewolf that the opamps are the best chance for a fix). There is a lot more to it, but you could say that the opamps are the tubes in your buddy's case.

You wouldn't have to do anything but touch the right connection with a big enough static charge to cause breakdown in an opamp. And that charge wouldn't really need to be high enough you'd feel it.

If it was the speaker, anything coming out of it would sound funny/crappy/etc and she said the clean was fine.

[witchhunt]

Also how would any part of the amp be damaged if the amp wasn't recieving or sending power?

That's what I was thinking. The way she described what happened shouldn't have affected her amp unless the speaker wires from her amp came in contact with the speaker wires from the Mesa. I fried a car stereo once by letting the speaker wires touch. Just a guess.

[lonewolf]

When repairing electronics, what the owner says happened before the equipment failure may or may not have a bearing on fixing it. Unless it is inherent to the system, the cause of the problem may not matter.

Look at the symptoms, not the reputed cause:

Clean channel works OK with power amp section.
-clean channel probably good
-power amp probably good

OD channel works but with lower output level & crackle noise.
-Transistors amplify signal level.
-Bad transistors make crackle noise.
-Op Amps are transistor amplifier circuits on a chip.

Conclusion:

Bad transistor or op amp in the OD circuit.

There may also be other bad components in that part of the circuit that caused the problem.

[JackANSI]

She could have applied "power" to the system when the static electricity jumped from her finger, to the speakers connector, then through the circuit (power transistor might have been able to take the ESD no problem) to the opamp (would have a very hard time handling an ESD). Tada.

http://www.charleswater.co.uk/PPT/ESD%20Basics.ppt

Its not like the only way to damage a circuit is through connecting power or even connecting something wrong...

[webmiztris]

Wish you had my amp, Lonewolf. lol It's been 3 weeks and no phone call since I posted this topic last week. I have no idea if he's figured out the problem. I really wish I had taken it to Rainbow now.

[onegunguitar]

Wish you had my amp, Lonewolf. lol It's been 3 weeks and no phone call since I posted this topic last week. I have no idea if he's figured out the problem. I really wish I had taken it to Rainbow now.

I'd just call them and ask them how much you owe for the work they've done and go pick it up and take it elsewhere.It sucks they didn't fix it and they want to charge you an outrageous amount but at least you know not to have them fix anything again.

[Ron]

She could have applied "power" to the system when the static electricity jumped from her finger, to the speakers connector, then through the circuit (power transistor might have been able to take the ESD no problem) to the opamp (would have a very hard time handling an ESD). Tada.

http://www.charleswater.co.uk/PPT/ESD%20Basics.ppt

Its not like the only way to damage a circuit is through connecting power or even connecting something wrong...

My main question is "Why did he replace the power IC's when the clean channel works?".

I also highly doubt if it was ESD. The charge would have had to somehow jump past ground to the tip connector inside the jack. I don't see how it could. The op-amp circuit would also have to be more robust than that. Consider how often the tip of a guitar cable is touched while it is being plugged into a guitar, while the other end is plugged into the amp. On the input side it is practically going straight to the op-amps.

For a static charge to fry a bipolar analog circuit like those op-amps, the charge would have to be really hefty. I've purposely tried to fry op-amps before with ESD... not with simple static charges, but a cattle prod that could spark across a 2 inch gap and make you scream like a little girl. After hours and hours of abuse, we could not get any of the op-amps to fail, and they were cheapo 1458s. A JFET op-amp is a different story.

But, like Jeff said, the actual cause isn't really the point. With the described symptoms, I would suspect the power supply to those op-amps in the OD preamp section. They may be running with true +/- voltage rails, and one of the rails is down or weak. If they are using a voltage divider to supply the op-amps, it could be a 1 cent resistor causing the whole issue.

If it is a transistor breaking down in the op-amp, you will likely hear the crackling sound all of the time in the OD channel, even with no guitar plugged in. When this happens in a preamp circuit... look out, since the noise can get worse quickly over time and easily push the amp to full output... really, really loud and potentially very harmful to those freshly replaced output chips.

It sounds like the tech needs a schematic, signal generator, o'scope and a troubleshooting primer.

[mjb]

i wouldn't pay him anything if he didn't fix it. couple bucks maybe. three weeks...... he doesn't know wtf is going on. go get it, give him 20bucks and take it somewhere else. if you have to give him 150, give it to him i guess and then go back at night and throw a brick through his window. about all you can do.

[webmiztris]

If it is a transistor breaking down in the op-amp, you will likely hear the crackling sound all of the time in the OD channel, even with no guitar plugged in.

I forgot to mention this symptom. If I had the amp plugged in, but no guitar plugged into the input, it crackled constantly.

[webmiztris]

LOL, I don't know....I took it to Music Emporium. Is anyone familiar with who does the amp repairs there? I don't know the guy's name. I figured they probably have a lot of experience fixing amps but at this point I wish I had tried somewhere else instead.