Mesa Boogie Nomad 55 Combo

[cjm808]

Have a Mesa Nomad 55 combo in with low output and poor tone. Plugging signal into the fx return produces decent clean output, slave out, headphone out/recording out and fx out has no signal present (scoped at these points-nada). Opened amp and started checking voltages-tubes seem correct-found one of the opamps (5532) and checked supply voltages-pin 8 shows +15v dc but pin 4 shows -2.6v dc. Traced negative supply back to a 68R resistor and followed it back to the rectifier diodes (these apparently were replaced once in the amp's lifetime). Low voltage out from transformer looks okay and as mentioned am seeing 15v positive on pin 8. Lifted the end of the 68R (there are two in series-lifted the end furthest from the rectifier diodes and closest to the opamp) and got -30v dc-pin 8 voltage dropped to about +6volts dc or so-assume something is drawing the negative supply down-what should I look for next? I am relatively new at the 'game' here so if I am overlooking something very obvious to check next please go easy The layout of the board is a real PITA to get to the underside and since I have no schematic or board layout and the PC is double-sided I am totally lost. Any assistance (or a prod in the butt) to the next step would be greatly appreciated.
TIA-Craig

[cjm808]

Finally got chance t put amp back on bench and start more troubleshooting. I pulled all the pots and switches from the front panel to access the circuit board from under to try tracing. Clipped the 5532 op amp and dropped another one in-now reading both positive and negative voltages on the IC. Plugged amp back in-no change-still low output. Got on the phone and got a tech at Mesa Boogie-gave him symptoms and he led me to a J175 fet which was missing voltage-it was in the mute circuit or something. Thanked him and ordered some parts from Mouser-cut to the chase-put the new fet in and all is well! For master-t who was having issues with the Nomad 45 try contacting Mesa support-maybe I just lucked out and got a good (great?) guy to talk to-reaffirms the normal good vibe I've always got from MB!
Goodnight for now-must sleep.
Craig

[master-t]

hey thanks for the update Craig...I stumbled across your post. That FET has been under suspicion. I think I'll swap that out and if no luck I'll call Mesa and hope for the same luck!

T

[cjm808]

FYI to master-t:
The tech I talked to at Mesa was George. Mesa phone 707-778-6565.
He apparently had a board available to him and he was able to look at it while talking to me and following the traces on both sides-real cool.
Good luck!
Craig

[master-t]

OK, called George today...What we found out was that although the reverb tank worked fine with a different amp, it was actually NOT the right reverb tank for the mesa nomad. The nomad uses a tank that shares ground between input and output where the one I had in my nomad (which some jackass must have put in and just left it there) had isolated grounds. Soldered a wire between the two and BAM! Reverb!

Mighta taken me years to figure that out without ol' George! Good guy, great service. My money (albeit expensive) is on Mesa for now on!

Thanks!

Todd