Mesa Boogie Lonestar Power Failure Troubleshooting Help Needed

[godbones]

I have a Mesa Boogie Lonestar Classic and am experiencing total power failure. It is blowing fuses immediately powering up WITH all power tubes removed (pre-amp still in place) and in standby mode. This came out of nowhere. I was fine tuning some tones last night, all was well, put it in Tweed mode for a minute then put it back, as I was comparing overall tone. I then shut it down. About a half hour later I turned it back on and poof, I blew a fuse. Tried another and poof. Then removed all power tubes to verify it wasn't a bad tube. Same thing. I'm reading that it's almost certainly the power transformer, although various folks say they've never seen a MB with a bad transformer. I had recently replaced all four screen resistors, as one blew out from a bad tube. The amp has been working fine since, but could this be related? The big question of course is if it is the power transformer why did it go out? I wasn't prepared for something like this to happen, total bummer. I have no repair facility nearby, so this is likely to involve shipping if I can't fix it myself, which is I prefer to do. Thanks for help or suggestions to move forward on this.

[AZJimC]

I had a situation like that with an old Ampeg. After a couple of fuses, I did some checking and found the output transformer had shorted the primary to the secondary side, and the B+ was shunting to ground via the speaker ground. Try taking loose the B+ to OT and see if it powers up. Other likely suspects are diodes, and then maybe power trans.

I haven't looked at your schematic, but some of this should apply. Since it happens in standby, I'd lean toward diodes though.

EDIT: After looking at schematic, I'm assuming you had the rectifier tube pulled too? Still diodes if that's the case. Also, the first power supply caps could be shorting. They look to be "pre stby" as well.

[shooter]

If I'm looking at the right schematic it shows an MOV on the primary side of the PT, probably a red 2 lead thing about the size of a nickel or quarter.  disconnect at least one side and ohm it, don't know about the new ones but they used to be very problematic in powersupplies n they either shorted or sounded like a shotgun going off when they failed.  Otherwise what AZjimC said.  The schematic also shows a LVPS, try disconnecting the 2 brown wires going to the bridge, pull the 5U4, select tube rectifier, leave the MOV out and see if your fuse blows.

[godbones]

I had all the power tubes pulled, so no relation to rectifier tube.  I'll check diodes.  I'm not a technician, done lots of basic repairs, so I'm a little lost on the MOV and B+ references.   It could be I just need to get this off to a competent tech to sort out.  I should be able to easily ohm out he diodes, just need to know where to look.  Definitely not real clear on the other recommendations.   Thanks!   

[shooter]

If you are not TOTALY comfortable with deadly voltage, component identification, understanding how the parts work in general, reading schematics, Then you are better off having someone qualified workin on it, the last thing you want is to make matters worse, read dead or injured to you or your amp.  The moderators here are some of the best but they need someone on the other end that can understand what they're suggesting and get accurate feedback in order to reach a safe and happy conclusion.

[godbones]

Oh yes, I totally got that a long time ago.   As this involves the power supply and voltage that is probably the smartest thing.  I just wanted to make sure I wasn't missing something simple, which is likely not the case.  Thanks for caring! 

[godbones]

Thanks.  I'll definitely check it out.  I would not be surprised if there were a power surge.  I guess I need to identify it from the schematic.

[godbones]

Hey Shooter- Bingo!  It's the MOV.  It's open-no ohm reading.  It's blue not red. I de-soldered one lead and measured-zip ohms  From the schematic it's showing 130V.   The question is why it went out I guess.  Given where it's located I assume it had to be a surge from the wall and nothing within the amp.   One thing I suspect is that switching from Tweed to Regular on the power switch may have done it.  That switch reduces the power to the amp and I had switched it just before this happened.  Supposedly you can do this on the fly, but I'm guessing that perhaps it does stress the MOV doing it.  Just a guess.  I'll buy a couple of the MOVs and hope for the best.  Any other thoughts? 

[shooter]

No ohms or infinite ohms?  Guessing the amp works, indicating no ohms/short.  I believe they ohm very hi/infinite resistance when good, low ohms/short bad.  That is what an MOV does, handle surges, it's on the primary side, so it takes care of power company surges etc.  The cheap part probably sacrificed it's life to save your power tranny.  Glad you're back up playing!  As to ideas, find the ohms law "pie chart", understand the math, read lots of sources on working with DC voltage, VERY high DC voltage, buy a good meter, study the schematic, look at the layout, then when you are ready start "benchmarking" your amp, B+, voltage on the tubes etc, then when it fails you have a good idea of normal, without understanding normal, broke will make no sense.  Probably a 101electronics at your community college and your ready to get started.

good luck
 

[godbones]

No fixed amp.  My mistake in expecting the varistor to have a ohm reading. It was zip-no reading, both the old one and the new one.  So I obviously misunderstood what I was looking for.  So I'm bailing on fixing it.  I've got a buddy who's a certified Fender repair tech (actually playing keyboards on a gig for us and owns a sound company) who will take over and figure it out.  Yes, an electronics 101 would be nice-maybe years ago.  Presently I can repair a LOT of things, all my guitars, all of my basic gear (as well as fixing a shit load of other things here and there) but when it comes to figuring out a power supply failure like this, I'm obviously not very prepared.   Thanks you guys that offered up the help, appreciate it. 

[Willabe]

IMO, a PCB is a lot harder to try and start learning how to fix an amp. So much harder to see what's hooked/wired up to what.


                    Brad