Boogie Nomad 100 +15v supply

[lowell]

Amp came in with no sound. Opened up and found that the low v supply resistor was heating up so much as to melt the solder joint completely... disconnecting the supply and causing a "no sound" fault.

I've replaced the dropper resistor with a 350 10w and it is still overheating, and the +15v supply is at +14v and slowly decreases after turn-on. -15v supply is -16v.

I've replaced all 4 low v supply caps and the +15v zener. Still same symptoms. I've scanned the schematic for the most likely culprit and nothing jumps out.

The amount of Boogies I've worked on this year that I've spent 10-20 hrs on and had to give up is already about 3. This is getting off to a bad start already. The former 3 repairs I had to diagnose as "conductive pcb."

Anyone dealing with this with Mesa Boogie? To add to it, it's not easy or fast simply getting to the solder side of the pcb. So after each "repair" it all has to be rewired again.

Any suggestions on this specific repair appreciated. I'm "this" close to not going any further wasting my time.

[The Dude]

http://www.thetubestore.com/lib/thet...-Schematic.pdf

There probably isn't much time, but can you see if something using the 15V supply is getting hot- an IC, a bypass cap, etc.? Is there some part of the amp that isn't working that might give you a clue as to where to look?

[nosaj]

http://www.thetubestore.com/lib/thet...-Schematic.pdf

There probably isn't much time, but can you see if something using the 15V supply is getting hot- an IC, a bypass cap, etc.? Is there some part of the amp that isn't working that might give you a clue as to where to look?

How about this?

Take a variable DC supply disconnect the 15 volt supply sub yours in slowly dial up voltage checking components heat with your finger. it may give you enough time that way. Of course I realize that this may be inpractical considering it's a mesa.

Just ignore me if I'm off base but it looks like the 15 v rail has entry points at the FX loop , the Graphic eq, and the Solo Footswitch, the record circuit, switching matrix,mute circuit.

One at a time disconnect the 15 v rail at each entry and see when that one resistor stops heating up. Should allow you to focus in on the issue at hand.




nosaj

[Leo_Gnardo]

Take a variable DC supply disconnect the 15 volt supply sub yours in slowly dial up voltage checking components heat with your finger. it may give you enough time that way. Of course I realize that this may be inpractical considering it's a mesa.

Indeed! The way Mesa boards have lo voltage and hi voltage so close, side by each, testing heat by fingertip could lead to an unpleasant shock. A cheap and safe method - "laser" infrared thermometer. I got mine from Parts Express under $30 and it's saved me many a singed fingertip plus a couple shocks I'm sure. It's also handy for finding which tube in an output set is running hot or cold. A cheap enough tool, every workshop should have one. Plus they're easy on batteries as long as you don't use yours to entertain your cat. Here's one for $25, not the same model I have but I'll bet it works just fine.

https://www.parts-express.com/triple...meter--391-074

I'm not much looking forward to a return visit from a Mesa Mk IV I repaired - or thought I had repaired last year. Similar symptom, roasted 39 ohm resistors in the LDR control circuit. It's possible there's an intermittent arc from the hi voltage department. Really dread the hours of poking around to no avail. Owner says he's had enough of this nonsense, simple amps for him from now on, good choice! Anyone like to buy an "almost good" Mk IV?????

[The Dude]

How about this?

Take a variable DC supply disconnect the 15 volt supply sub yours in slowly dial up voltage checking components heat with your finger. it may give you enough time that way.

Or, just wait and see what explodes! (not being sarcastic- I've actually done that as a last resort)

[lowell]

All good suggestions thank you. The Dude thanks for posting the obvious...the schematic.

My initial gut reaction is this is gonna be like the others...but hey I think, thanks to the suggestions, I realize I have to learn something here and take a different approach. That being use a sub bench supply so I dont have solder/desolder the psu every damn time I make a change IE lift a transistor or opamp.

Thanks y'all I'll report back on my defeat or god willing...success.

[Randall]

I second the laser thermometer. And I also dread working on Mesas. Sometimes I decline. I can't be all things to all people after all. Perhaps Enzo will offer a lesson this to me, but this is my reality, such as it is.

[Jazz P Bass]

I'll second that.
Mesa amps suck to work on.

[Enzo]

Mesa amps suck to work on.

I doubt I could have stated it any clearer than Jazz did.

[lowell]

Ok well good to know I'm not alone here

And duh I missed the 5532 opamp in the schematic. I DID touch it, and the other ICs to see about heat but nothing jumped out. I have a therm-gun too, maybe need to leave the thing on a bit longer to monitor. But really, the 5532 seems most likely culprit here. Most other components have one or more decent sized resistors in series so I'd doubt, even with a shorted transistor, that THAT could be it. Nothing in series with the 5532 supply rails.

Troubleshooting a 3 channel Mesa is like troubleshooting 6 preamps simultaneously. 3-channels plus 3 modes per channel. Not to mention other "add-ons." Just a lot of, or dare I say too many, options.

[The Dude]

No argument here. Mesas do suck to work on. Thankfully, they're horribly overpriced. It gives us a bit of license to charge more for the extra hassle. Keep in mind the replacement cost when you're working on one.

[lowell]

Mos def

[lowell]

Ok so I realize I left out a very important detail. The +/-15v rails are actually +14/-16. It seems skewed or offset. Does this narrow the possibilities? I'm gonna check that the ground points have continuity.

[lowell]

Sooo... I'm an idiot. I had my probe grounded at the chassis earth, but not true circuit ground. Reason being I only had SOME of the PSU wires soldered to the board while testing my repair. So.. low v supplies look good. +/-15.3v.

Now, the dropping resistor is still VERY hot. I'm measuring 110degC after amp being on for about 5 minutes. I have a 350ohm 10w cement resistor in there. It's dropping 35Vrms. That's 3.5w So, seems like it's normal that this resistor heats up quite a bit.

Thoughts? Am I wrong?

Thanks!

[nosaj]

Sooo... I'm an idiot. I had my probe grounded at the chassis earth, but not true circuit ground. Reason being I only had SOME of the PSU wires soldered to the board while testing my repair. So.. low v supplies look good. +/-15.3v.

Now, the dropping resistor is still VERY hot. I'm measuring 110degC after amp being on for about 5 minutes. I have a 350ohm 10w cement resistor in there. It's dropping 35Vrms. That's 3.5w So, seems like it's normal that this resistor heats up quite a bit.

Thoughts? Am I wrong?

Thanks!

So the input is 50VAC to the 350ohm?
Is the low voltage stable now? Is it still melting the solder?
I got 50VAC from schematic.
nosaj
Is this correct? see pic