Think I blew the OT in my Mojave Peacemaker.

Telephant

Telephant

Active member
:cry:

A friend of mine has gone over it and we cannot get it to stop blowing the mains fuse everytime you flip the standby off to play. My buddy sent Mojave an email with the details and I'll post it below (any advice is welcome!). I hope I can get a replacement from them. After gigging with it relentlessly since I got it, going to back to my Superbass is actually hard. It sounds... thinner. Not as much muscle. The PM has a cool midrange that makes it more unique sounding to me. Admittedly, I didn't notice it as much when I first got it, now I can't live without it. :cry: :LOL: :LOL:

Heres the email:

Mojave Peacemaker blowing mains fuse immediately after flipping standby switch. 

Started with old tubes. Fuse blows. 

Replaced with new matched quad of Mullard reissues. Fuse blows. 

All resistor values on preamp board, filter capacitor board, and tube sockets are good and correct. 

b+ fuse is good. 

Rectifier diodes test good. 

Bias voltage is ~ -40v on both sides of 220k bias splitter resistors.

130VAC on each B+ tap of power transformer,

Steps I've went through so far, and I have a sinking feeling it's the OT, but was hoping to see if you could verify. 

- I disconnected the B+ connection from rectifier to mains caps. power on and all the way up, voltage fine, no blown fuse. 
 
- Reconnected B+ to caps but disconnected OT B+ Lead (red wire) and choke from mains caps. power on all the way up, no blow. 

- Reconnected choke but left OT B+ Lead disconnected, power up, no blow.

- When OT B+ Lead is connected is when it blows.

- Also according to http://www.geofex.com/ampdbug/outtrans.htm
I followed a couple of their simple test and think i have a faulty OT while still connected in the amp. 

When testing the B+ lead to the Power Tube plate leads, it seems normal ~17ohm from red to brown, and ~19ohms from red to blue. 

Then when I tried testing each of the primary leads to ground, as suggested on the geofex page, this is what started to bother me. Off the geofex page: "While you're at it, measure the resistance of every lead to the chassis. This reading should be very high, hundreds of K or preferably megohms. A low reading here indicates a short to the transformer core - again, dead transformer."

the red = 16ohms, brown = 2.5ohms, and blue = 34ohms, which certainly would make one believe this tranny is dead according to these troubleshooting tips. 

I tried to be as thorough as possible, because I would really just like a verification of what i'm dealing with so I don't have to deal with more downtime. 

If, in fact, it IS a blown output transformer, what is your expert suggestion for what I should do? Can these special Mercury Transformers be bought? Can Mercury Repair them? Can any 100w Marshall type OT be used in it's place if I have to wait to have either of the above done? 
Click to expand...
 
Bummer.....hope you get it worked out Shea. I bet you sound good through a Peacemaker. :rock: :rock: :rock:
 
Good luck on the fix.

Having to go back to the SB isn't that bad of a problem to have.

I loved the clips you posted of that bad boy!


btw....are they still around anywhere?
 
Greazygeo":39b7smz2 said:
Does it blow fuses with no tubes in the amp?

Check filter caps?
Click to expand...
Good point George. I am no tech but do dabble. If it blows the fuse with no tubes he probably has a shorted filter cap. A recap may do the job.
 
Telephant":1aqic83r said:
Greazygeo":1aqic83r said:
Does it blow fuses with no tubes in the amp?

Check filter caps?
Click to expand...
It does blow fuses without tubes. :aww:
Click to expand...
You have a shorted filter cap. Replace one cap at a time or just do all of them.
 
Those aren't an off the shelf transformer are they? I bet Mojave wants a pretty penny for one.

BTW, was there an incident that led up to the blown OT in the Mojave?
 
glip22":2l0pytga said:
Telephant":2l0pytga said:
Greazygeo":2l0pytga said:
Does it blow fuses with no tubes in the amp?

Check filter caps?
Click to expand...
It does blow fuses without tubes. :aww:
Click to expand...
You have a shorted filter cap. Replace one cap at a time or just do all of them.
Click to expand...

"Reconnected B+ to caps but disconnected OT B+ Lead (red wire) and choke from mains caps. power on all the way up, no blow.

- Reconnected choke but left OT B+ Lead disconnected, power up, no blow.

- When OT B+ Lead is connected is when it blows."

I think it's the OT. Contact MM right away and ask Paul ( paul@mercurymagnetics.com ) what your options are.
 
Maybe a stupid question and not to doubt your friends qualities but did he measure the values on the OT with the wires disconnected from the circuit ?

Giga
 
It sure sounds like the OT is done unfortunately. Only thing I could add is to have your friend double check the power tube sockets again to make sure there isn't a small carbon track shorting between pins 2 and 3 just for the sake of being optimistic. If Mojave wants an arm and a leg for a replacement OT I'd give Brian at Marstran a call for one of his kick ass Marshall style OT's.
 
If the fuse blows with the power tubes removed, I doubt it's a bad OT.

Are the primary resistance readings of the OT with it removed from the circuit? At least remove the primary CT before measuring from any of the primary leads to ground.

Does it still blow after pulling all the preamp and power tubes?
 
glip22":1jul6ee2 said:
I still think its a shorted filter cap.
Click to expand...

It might be, if there are filter caps after the standby switch. Usually there are for the preamp circuits, but they tend to have series current limiting resistors. If one of those caps were shorted I wouldn't expect it to pull enough current to pop a fuse. Granted, I have no clue what the circuit looks like in this particular amp...

It's very odd too that the B+ fuse is fine and the mains fuse is consistently blowing. Could it be something as simple as the mains fuse being under-rated? Or, perhaps the B+ fuse is over-rated? I would expect it to blow when the standby switch is flipped, not the mains fuse.

Do you have a light bulb limiter? That's the best and simplest tool to use in situations like these, and it saves you from wasting a lot of money on fuses...
 
protoplasma":2tovfim4 said:
glip22":2tovfim4 said:
I still think its a shorted filter cap.
Click to expand...

It might be, if there are filter caps after the standby switch. Usually there are for the preamp circuits, but they tend to have series current limiting resistors. If one of those caps were shorted I wouldn't expect it to pull enough current to pop a fuse. Granted, I have no clue what the circuit looks like in this particular amp...

It's very odd too that the B+ fuse is fine and the mains fuse is consistently blowing. Could it be something as simple as the mains fuse being under-rated? Or, perhaps the B+ fuse is over-rated? I would expect it to blow when the standby switch is flipped, not the mains fuse.

Do you have a light bulb limiter? That's the best and simplest tool to use in situations like these, and it saves you from wasting a lot of money on fuses...
Click to expand...
I would up the mains fuse a little to see if it solves it. It could be as simple as that. If it was the HT fuse blowing that may point at the OT. Mains fuse points more toward the PT.
 
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