Noobie wants to fix his own amp... help!

[diegs]

Hi,

I feel like this might be a long post, so apologies in advance. I have an early 90s Mesa Studio .22+. Consensus is that this amp is somewhat special, and a keeper. Most importantly, I really like the way it sounds, and the size.

Unfortunately it has a few problems that keep me from using it to play out (I'm getting a jam group together):

1. It makes a nasty, loud static noise when you turn the master volume up past 3. Even with nothing plugged into it. I haven't had an opportunity to test it out in a live setting, but I'm thinking it's gonna be unacceptable.

2. These amps are known to self-combust from over-heating. The tube sockets are mounted on the PCB. My PCB looks ok for now but I don't want to push it. I have read that adding a fan would be a good way to help this.

I tried to get some professional help, but failed. To make a long story short, I had bad experience with the first tech I lugged it to, and I don't have a car to take it from tech to tech trying to get it fixed properly. (N.B. Carrying an amp on the subway and around tens of city blocks sucks.)

I want to try fixing it myself before I give up, because I don't think I can afford a better amp to replace it. I have no experience with a soldering iron but I am good with electronical things (have previously built computers for a living) and I have a latent interest in learning this stuff. So...

What would I have to check to fix it? Is there a good resource on how to get started? I have heard that I might have to "test resistors"? If I went through with a multimeter checking all the components on the circuitboard and resoldering broken ones, would I have a good chance of fixing it?

And how would I put a fan in it? Where would I hook it up to for power?

Am I asking for trouble? Thanks in advance for your thoughts/opinions.

[Blue Strat]

Start with tubes. They're the most user serviceable item and the first place a tech would start. Replace all the preamp tubes with known good tubes (from another amp you or a friend owns).

[diegs]

Thanks for the reply Mike--I think you helped me in my last thread too. I should have mentioned that I did a tube tapdance and it seems like the problem is in the preamp (pulling V5 or V2 was the only way to stop the problem).

I'll try and borrow some "known good" tubes from someone but I'm not sure how soon I'll be able to do that (I could buy some new ones but I don't know if they'd be good).

[Blue Strat]

(I could buy some new ones but I don't know if they'd be good).

You don't know if they'd be good? Who are you buying tubes from? ;)

You should always have tube backups for times like this or to assess the condition of the tubes you've been using for a year or more.

[boobtoob]

Maybe your volume pot is going/gone bad???

[John Phillips]

I could be wrong, but from the fault description, this could be a nasty problem involving slight board damage. It sounds like a loose joint, where the component has heated up enough to slowly desolder itself over time. I've come across this several times on these amps, usually around the power tube sockets but also on a couple of high-power resistors elsewhere. At a certain volume setting the vibration from the speaker becomes enough to rattle the joint and start the noise - even if there's no signal, the slightest touch on the amp or vibration in the room will set it off and then it becomes self-sustaining.

In fact, this is exactly why I got mine - my first Mesa amp :) - cheap.

Check the tubes first, but if that doesn't cure it you could start looking for something like this. The good news is that you can see the joint side of the board easily; the bad news is that it may be difficult to identify the exact problem component from this side. You also really need to know how to solder well before trying to work on a PCB amp, unfortunately - you can do a lot of difficult-to-repair damage to the board otherwise. And remember that tube amps contain VERY dangerous voltages... you will probably need to work on it powered up to find the fault too, unless the problem is very obvious to inspection.

So you may still need a tech... :(

[LesPaulMan]

I could be wrong, but from the fault description, this could be a nasty problem involving slight board damage. It sounds like a loose joint, where the component has heated up enough to slowly desolder itself over time. I've come across this several times on these amps, usually around the power tube sockets but also on a couple of high-power resistors elsewhere. At a certain volume setting the vibration from the speaker becomes enough to rattle the joint and start the noise - even if there's no signal, the slightest touch on the amp or vibration in the room will set it off and then it becomes self-sustaining.



This is exactly what I experienced on mine. I had a Mesa Studio 22 (not the plus version, which just adds EQ). After about a solid hour of very high volume playing, the amp just stopped making any sound. Long story short, when I picked up the amp to check for a blown fuse, I could hear a part rolling around in the chassis. Sure enough, a component actually came off the PC board. Can't remember if it was a cap or resistor (was fairly small, but not that small). It was in the area of the power tubes, and this amp did get very hot (which I am still a little surprised, especially with it only having two EL84's). Funny thing is that the two solder pad/holes didn't even have to be cleaned up. The part could easily be inserted. I resoldered it and the amp was fine (didn't even blow a fuse when the part fell out). The Studio 22's were neat little amps, but the build quality wasn't the greatest.

Not sure if this is the case with your amp, but John Phillips (as always) gives great info on what might be some potential problems.

[diegs]

You don't know if they'd be good? Who are you buying tubes from? ;)

You should always have tube backups for times like this or to assess the condition of the tubes you've been using for a year or more.

I would hope a new set would be good, but how can you be 100% sure? Probably would have to try them in another amp. The ones I have in there are brand new JJs, and I have the previous set of Mesas and Fenders (!!!) which are mostly microphonic.

I think John Phillips nailed it on the head (as usual). I should probably get some experience by building some BYOCs and maybe a p-to-p amp or two before I muck up my PCB or kill myself, and try to get this fixed professionally.

Are any of you guys in the NY area and willing to fix this or know someone who could (for appropriate monetary compensation of course)? For some reason I trust people who give me selfless, generous help on the internet more than repair people who will take my money whether they fix the problem or not, and I would be happy to rent a Zip Car and drive it out to you sometime.

And as noted, the heat problem is a concern, so adding a cooling fan may be worthwhile to prevent this from recurring. Or I can try and cut my losses, sell it and buy a cheap Crate Palomino or something, but I feel like I would be disappointed.

[John Phillips]

Yes, fitting a cooling fan is well worth it no matter what the cause of the problem. These amps aren't the best-designed things Mesa have ever put their name on, to be honest. Mine was my first experience of Mesa in general, and it really put me off them - I knew Boogies were 'PCB amps' from reading an interview or article by Randall Smith, so I assumed that meant all Boogies were done the same - PCB-mounted power tubes, with no extra support (worse, the tube retainer pushes on them quite hard), upside down under the board with no means of diverting the heat elsewhere. It was only a couple of years later when I had to fix a MkIII that I found out they weren't all like that.

They do run very hot - the EL84s are run close to, or at, their maximum dissipation at idle, and you really need to use good ones. I assume this is why the amp is labeled 'Class A' - it most certainly isn't, and nor is it even cathode-biased which is often assumed to be the same thing. (In fact, it's the only amp I can think of with a fixed-bias preamp stage too - the PI.) At the very worst the board around the power tubes chars and cracks under the strain, which involves some tricky work to fix, and can never really be done perfectly.

They are great-sounding little things though, and incredibly loud for their size. The clean sound is especially good... the Lead mode hasn't quite stood the test of time as well IMO. Even with the known PCB problems I would definitely keep it... I sometimes wish I still had mine.

[acorkos]

replace the tubes and spray clean the master volume pot, and all the pots for that matter, before you start looking for heat damage to pcb and solder joints