My MarkIII died, can anyone help?

[newtomesa]

Haven't played the amp in about a month because I was moving.
I take it out today for the first time for a little band practice.

Here's what it did:
[1]
I set all my settings, power on into standby.
A minute later, I flip the switch into on.
The first thing I notice is that the amp is unnaturally quiet.
I have the master at 3 and it is a comfortable bedroom level.

I turn the master knob, to no avail, it felt like I'm using an attenuator.
I could hear more saturation, but absolutely no volume increase.

I flip the amp back into standby, check all the tubes, they are all glowing just fine.

[2]
Next I plug my guitar into the return on my effects loop. Again I hit a wall for the volume, and its very quiet.

[3]
I check the power tubes, swap them around utilizing my 1/2 power switch.

[4]
I check my V1, swap it with another, to no avail (although at this point, it seemed logical that I had a problem w/ power/output and not the preamp)

[5]
I play around with the amp, and about a minute later it shuts off. I can't power the amp on anymore.

Help me please!
Thanks!

 

[user 2126]

Maybe the fuse.

Boogiebabies help needed here :wink:

 

[Blueracer]

Hi,

In terms of diagnosing problems of this nature I would start with the easy stuff first. Are your cables good. Check them with another amp if possible. Same for the speaker. As far as item 5 goes it sounds like your fuse went. I am assuming that there are no lights, no glowing tubes no anything. After this it starts to get harder given that the speakers and cables are ok. It doesn't seem like a short or else the fuse would go immediately. That you detect increased saturation with the master but low volume would point to something wrong with the output tubes with saturation occuring in the phase inverter. Kind of tough without going in and snooping. So at this point I would stay with the easy stuff. We all fall pry to not plugging things in completely or using a broken cord or such at least once in a while. Once you confirmed that all the external stuff is working then you could proceed from there but it will get kind of technical. Hope this helps.

Gary

 

[newtomesa]

Thanks for the reply guys, I did check all of my cables, tried 3 different speaker cables, nada. I'm going to take a look inside of the chasis tomorrow, to see if there is anything visible that could be wrong.

The fuse does not appear to have broken circuit. There looks to be a tiny smudge on the inside of the glass near the middle, but i don't see a break in the wire.

But yes you were correct, the lights do not switch on.

I'll try to post tomorrow with my findings, thanks for the help so far!

 

[newtomesa]

Popped open the III, I don't see any burn marks, all the resistors and capacitors look like new. There is nothing superficially wrong with any of the components, nothing obvious at least.

I know this doesn't help much, just fishing for ideas.
Does mesa operate on sundays?
And how does shipping go, do I have to pay for both shipping and return fare?

Thanks

 

[Paul Secondino]

Mesa will likely charge you for shipping and include it in your bill. I have a bad feeling that your output tranny is the culprit. Not 100% though but whe nthe volume drops way way down and everything sounds distorted, that is a classic sign of a output tranny going bad.

 

[jamme61]

did you check the speaker? Try another cab or speaker if you can.

 

[Boogiebabies]

Take the fuse out and check it with a multi meter for continuity. That smudge may be where it poofed enough to blow. Before I send it to the cash cow, I would try a new fuse and maybe some power tubes.

 

[newtomesa]

I'll try swapping the tubes out with some spares I have, I figured I could figure out if it were the tubes using my 1/2 switch and rotating the tubes, but it could be i busted a few of them.

And I checked my fuse with a multimeter, what do you know, it is broken. <- can a faulty tube cause a fuse to go out? Sorry i don't know too much about the electronics.

 

[Paul Secondino]

A faulty tube can cause alot of problems. SOme very minor like a blown fuse and some actually damage other parts of the amp.

But I'm glad you found it was only a fuse. Occasionally, a fuse will appear ok and do crazy things liek it did in your case.

ON one other amp , a Mesa Recto, I found that a seemingly good fuse was causing all kinds of wierd phasing sounds in all channels . Almost like a faser or flanger were added to the signal. Replaced the fuse and all was well.

 

[Boogiebabies]

I'll try swapping the tubes out with some spares I have, I figured I could figure out if it were the tubes using my 1/2 switch and rotating the tubes, but it could be i busted a few of them.

And I checked my fuse with a multimeter, what do you know, it is broken. <- can a faulty tube cause a fuse to go out? Sorry i don't know too much about the electronics.

Yes.

Get a box of fuses and replace the bad one. If it pops the fuse again, replace the power tubes. One of, or two may have an internal short.
The worst case scenario is a blown cap if the fuse and tubes do not fix it, but sometimes and amp will run on a bad cap and hum pretty loud.

 

[newtomesa]

Figured it out.

It is my cab. I ran my 3/4 back cab and the amp works fine with no tube replacements or anything.

I didn't realize having a faulty cab would pop my fuse so quickly.
I checked the impedance of my cab, and it is exactly where it should be. Sound comes out of both speakers as well.

Thanks for all the help guys! All your responses helped me figure out what was wrong.

James