1993 Maverick 410 Keeps Blowing Fuses--Not a "Normal" Issue

[TrentJ]

I'm having a problem with my Mesa Boogie Maverick Dual Rectifier 4x10 combo amp. The fuse keeps blowing and after every "normal" check/fix (replaced all tubes, checked everything out for obvious damage, etc.), this thing will not work. With all tubes removed, no rectifiers firing--NOTHIN'--it still blows (if the power transformer is plugged into the PCB).

With the power supply header/connector (lines go from power transformer to PCB, like a plug) disconnected from the PCB, the fuse won't blow, but when connected to the PCB it will immediately blow (with no tubes--rectifier, power, preamp). I thought perhaps it was the power transformer, but all power tranny voltage readings are within 7-8% (all a bit hot) of the listed values on the schematics I found here: http://www.freeinfosociety.com/electronics/schemview.php?id=288. Is this normal or considered acceptable?

This is the most relevant comment I have found, regarding a similar issue (with a Mark 3) and I have essentially eliminated the whole list:

If it blows the fuse while on Standby [which mine is doing], there are only a few things that it could be.

Power Transformer
Wiring in the Power Supply
Filter Caps
Rectifier circuit (4 diodes)
Filament wiring

You might want to take it to a tech to get a check up and change the Filter Supply Caps. They are getting old and may be the cause of the intermittent fuse blowing... I would have the guy clean all pots, jacks, sliders, etc. as well as tighten and clean the tube sockets. You will be pleasantly surprised by the tone when it comes back. Replacing the filter caps will bring back a lot of frequencies that you never realize you are missing until they return.

Source: http://forum.grailtone.com/viewtopic.php?f=3&t=36667 (Thanks for this post Monsta-Tone, by-the-way)

I've posted a page with some high fidelity photos of the "guts" of my amp here: http://trentstuff.com/1993-Mesa-Boo...e-Problems-Inside-Board-Photos/Mesa-Pics.html.

Can anyone see anything jumping out, or is there anything else I can check?

Some history:
I bought this off a friend about 6-7 years ago and have no idea what the history is before that, but I believe my friend was the second owner. I used it for a few years on and off and it was excellent. Aside from swapping a couple preamp tubes that were bagging out I replaced the "rhythm/lead" switch at one point (because a guitar accidentally fell across it, snapping the toggle off and rendering it difficult to operate), all with no ill effect. Then, at some point about 4 years ago, this amp began failing, blowing fuses just about immediately after turning it on, in "standby" mode. I switched a few preamp tubes and it didn't help and then for various reasons I left it in limbo for the past (about) 4 years.

Now I want to get it up and running again and recently I replaced ALL the tubes, fired it up and enjoyed about 6 hours of some amazing rocking,' thinking maybe the whole problem was just some faulty power tubes (though none of them ever stood out that much). Then it happened again…The fuse blew out while I was conversing with my pals in between jams. I replaced the fuse right then and immediately it failed (exactly the same way it occurred 4 years ago--strange I got 6 hrs of playing in between though). I had a knowledgeable friend by to help me diagnose the issue since I was coming up empty handed and we are both at a loss…

All input is GREATLY APPRECIATED!

-Trent
me[at]trentstuff[dot]com

 

[94Tremoverb]

It will be the solid-state rectifier diodes, almost certainly. They're in the circuit even with the rectifier tube removed. My Blue Angel did the same thing.

The diodes are between the main filter caps and the large white ceramic resistor, near the right-hand end of the PCB and visible in your third pic.

If it's not the diodes for the high-voltage circuit, it could possibly be the ones for the low-voltage channel switching supply.

It's definitely not the power transformer, since if it was it would blow the fuse without the board connector being on.

 

[TrentJ]

Thanks, 94Tremoverb!

That is about the only thing we didn't get to last night. If that's all it is I will be relieved, and upset I let it go so long without getting it right. I'll definitely keep you posted as soon as I get a chance to replace.

 

[TrentJ]

It worked!!! Very happy now. Thank you.

Just to understand the situation a bit more, do you know what causes this/why this happens to the rectifier? Just age?

Also, would you please point me to the low-voltage channel switching supply area, just so I know exactly where it is?

Thanks so much for your help!

 

[94Tremoverb]

Usually a power supply voltage spike I think. It seems random and not particularly an age thing. If they're correctly spec'ed (which they are in Mesa amps) there's really nothing in the amp that would cause them to blow normally. The one that blew in my Blue Angel was in the preamp supply, and under very little stress... just one of those things.

I don't have a full schematic for the Maverick, but I think the low-voltage diode(s - there may only be one) are near the vertical black caps in your pics.

 

[Monsta-Tone]

Just out of curiosity, which diode or diodes were causing the problem?
I've only seen a few go bad. I have a couple of vintage Fenders with original diodes. Still working very well!

 

[TrentJ]

These were 1N4007 diodes. I replaced all four. I think I'm having the same issue (fuses blowing immediately, while on standby) with an old Vintage Club 50 Crate amp. Same diodes, same vintage, basically, but there are 12 of them. I just loaded up on these little 1N4007s and am going to replace all in the Crate today and hope for the best.

 

[94Tremoverb]

You may need a power conditioner or at minimum a surge suppressor. Something is blowing those diodes if you have two amps with them gone - failure is fairly rare normally, and I would say it's too much of a coincidence that you have two amps with the same problem at the same time. The voltage rating of the 1N4007 is significantly above what it should be seeing in the amps and should be plenty unless there's something else going on.

 

[TrentJ]

Well, I replaced 8 (closest to power supply) and after 5 seconds on in standby, poof, another fuse. Both these amps blew originally at my rehearsal space (both around the same time, 4 years ago or so--hmmm), which is in an industrial building with fairly recent electric. Several other amps were there at that time and are always in and out with no problems. Now these two are at my house, electric up-to-date. No issues so far with the Boogie since replacing the diodes, but it does appear this is a separate issue with the Crate. I Just posted a few pics and a link to the schematics here: http://trentstuff.com/Crate/crate.html. I know this topic has strayed from the original subject so I understand if this needs to end here, but I'd rather not make a whole new post and any more advice here is much appreciated.

As long as my Boogie is in order, I'm happy. The Crate would be great to leave back at the rehearsal space to take more beatings though (the only decent sounding Crate I've ever heard really)...But at this point I am kind of at a loss. I suppose I'll be calling my friend to come by with his multi-meter and go through things one by one.