Too good to be true: my new/old Mark IV

[ikkyu2]

So I bought a Mark IV long head yesterday. Total impulse buy, guy's like "I lost the speaker cabinet, can't play it for you, but it sounds excellent," and the price was, well, too good to be true.

I got it home and retubed it with known good brand new tubes (Mesa brand; I had a few lying around). The switching and preamp are all in good shape; the direct line out (DI) sounds exactly like what a Mark IV should sound like on all 3 channels.

However, turn up output level on any of the channels and what happens is.. nothing. I slid the chassis out of the wood and I'm pretty sure I've isolated the fault to the small transformer, the one that sits right behind the last two power tubes. That's the output transformer, right?

I'd like to try replacing it myself. If anyone knows where to get the part or has any tips on doing this, I'd be interested to hear your perspective.

What I don't need to hear: I am smart enough not to fry myself on the filter caps, I think; I have a decent working knowledge of electrical safety and I'm good with a soldering iron. I do have a good option in my local Boogie repair facility (I live pretty close to Petaluma). So I realize that's an option; I'm not soliciting opinions such as "Take it to your local Boogie repair shop" unless you think you have something to add that I haven't thought of.

 

[Boogiebabies]

Hey, I am sorry but if you cannot spot an output transformer from a mile away, I would not even think of replacing it myself. Is the Pull Silent on the Master knob pulled? It could be a bad switch or Master pot. Even if it is the output transformer, only mesa makes the Simul-Class OT for your amp. Call Mesa and talk to tech support.

 

[t0aj15]

If you don't have the knowledge to properly diagnose the problem, you certainly shouldn't attempt to repair it. There's a big difference in knowing how to solder and being qualified at amp repair. Take it to Petaluma and let a pro fix it.

 

[Boogiebabies]

Oh, how are the fuses?

 

[ikkyu2]

Yes, I'm sure the pull silent isn't off, and yes, I'm sure it's the output transformer that I'm looking at and that is causing the problem - you can hear the darn thing arcing when it ought to be outputting instead. It actually does output a miniscule amount of current to the speaker, but it's not enough to drive the cone, just to make a couple of hissing and popping noises.

Fuse is fine - I'm aware of one fuse, unless there's a hidden fuse somewhere I don't know about. As I say, the amp powers up and the preamp stage works flawlessly at the DI output.

 

[Boogiebabies]

Is it arching with without the power tubes or with them?


It is a Mesa only part.

 

[ikkyu2]

It only arcs when the power tubes are installed. I've tried two separate sets of power tubes; one of these sets is known good at this time (they're brand-new Mesa tubes, the 6L6's work in my IIC+ and the EL-34s work in my little ADA amp.)

The arcing noise is definitely coming from the OT, not the tubes. I moved my ear all around to make sure of that.

 

[Boogiebabies]

Is there any type of resistance reading from the OT? Are the bias supply caps and resistors OK. A bad set of these will take out every tube you put in. It may have taken the OT out with it.

 

[Boogiebabies]

Where did you go?


Hey look, I'm a MK IV....

 

[fdesalvo]

Dude, I don't have the MKIV layout, but I think that small transformer is actually the choke for the power supply!