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Mesa Nomad had funny acting treble

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9/16/2004 11:33 PM

Mesa Nomad had funny acting treble

I have a Mesa Nomad on my bench right now. It's the 55 watt version. 3 channels. On each channel the treble (and to a lesser degree the middle) controls are acting really wierd.

As you turn up the treble from 0 all is well until it gets to about 6. Then there is a pronounced step and then as you continue to turn it up, it almost sounds like a wah pedal or something. As the treble is maxed out, the volume fades and treble is actually rolled off.

The middle control is wierd in kind of the same way but not quite as strong.

My guess is that the tone pots on each channel control the same tone stack through switching relays and that there is something wrong with the stack, the relays or ? Anyway, it must be something they all have in common.

Anyone have a schematic for this thing that they wouldn'd mind sharing. Also,any tips on tracking down the problem will be appreciated.

I haven't tried swapping any tubes, but I suppose that is worth trying.

9/17/2004 6:31 PM
Dai Hirokawa	IIRC, I think I've read something where that was normal operation for the control. Not sure if that's true, but here is something I found re: apparently super interactive controls in the nomad. http://groups.google.com/groups?q=mesa+nomad+treble+control&hl=en&lr=&ie=UTF-8&client=firefox-a&selm=dlamkins-4F8365.22143123052000%40news.teleport.com&rnum=2

9/17/2004 10:44 PM
Jack Collins	Thanks for the info. There is definitely something wrong with it though. I can't believe that Mesa would purposely design the treble control to do what I hear the ones on this amp doing.

9/18/2004 12:44 AM
Enzo	Call Mesa and ask them about it.

9/18/2004 7:47 AM
Don Symes	Do the controls of different channels interact at all? That would point to a problem in either the LDRs or the switching logic. Any reason to believe the pots may be mechanically bad - wipers losing contact, leg(s) not quite soldered to the PCB? Are the pots on a sub-board? Are those connections stable? HTH!

9/28/2004 5:23 PM
Brad	I don't have the solution but I do have one of those amps that I use often right here. It is NOT supposed to act like that! I don't think I have the schematic (thankfully I have not needed it) but I'll take a look and drop you a line if I find it. Brad

10/1/2004 11:24 AM

Problem Solved...With Much Help

The problem was...bad pots.

Apparently this is a common problem with these amps
as some of them were built with a run of bad controls.

More testing revealved that every treble and midrange pot was bad.

Called Mesa Boogie and they ask me to read the pot
codes. The ones in the amp match the ones known
to fail. Bought new pots from Mesa at reasonalbe
cost.

Thanks ampage, especially to the one that helped me
with this via email.

I would have never guessed that 6 pots would simultaneously fail.

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