Mesa Boogie Nomad 45 tube amp repair question

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Does anyone here have experience with repair of a Nomad 45.

Channel 2 not working properly and reverb not working.

I have fixed all the usual issues on this amp...power supplies rebuilt,replaced all electrolytic caps, relays replaced, op-amps replaced. LED IC replaced, new tubes.


I have all the tools needed...scope, function gen...ect

Need some help troubleshooting.


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Mesa Boggie Nomad 45 repair help needed


Does anyone here have experience with repair of a Nomad 45.

Channel 2 not working properly and reverb not working.

I have fixed all the usual issues on this amp...power supplies rebuilt,replaced all electrolytic caps, relays replaced, op-amps replaced. LED IC replaced, new tubes.


I have all the tools needed...scope, function gen...ect

Need some help troubleshooting.

I have replaced spring reverb unit before, the input driver coil is opened. You can tap lightly on spring reverb unit to find out if still have reverb action, if so receive coil is good. So you can measure the DC resistance of both coils to confirm.
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Please be specific, what does "not working properly" mean? What does it do wrong, or what does it not do right?

Reverb not working? As in no reverb at all, or as in reverb sounds bad, or is very weak, or ???
1. Check the reverb pan, is the transducer open at either end?

2. Isolate the problem. Is the issue the drive or the recovery side? Pull the plugs from the reverb pan, and with the amp running, reverb up midway, touch the tip of each plug with your finger. The return cable should make hum out the speaker. That one plugs into the OUT jack on the pan. If it does not hum, then there is a problem with the recovery stage. The drive side is the output of an op amp. Look on the drive cable for a good signal with a scope. Or plug the drive cable into some other amplifier and see if the signal is coming out of it. If there is an electrical problem, it is most likely the op amp itself.
Hi Enzo,

Sorry I was away for a bit. Had to do some stuff for the

What's working

The amp will play on all three channels. There was a bad 1M gain pot on channel one which I just changed and improved the sound on that channel.

Just so you know, when I got this amp not much was working and I don't have a good idea what it sounds like when everything is working normally. This amp is not easy to work on because the switching matix is a bit complex and there are traces on both sides of the board.

Both the channel switch on the amp and the foot switch will change the channels but channel three LED stays lit all the time which I assume is not normal.

The channel 2 tone stack (treble, mid, bass) is not working and the master gain is not working. The gain control is working. Gain control on channel three affects channel 2 which I assume is normal as they are in series. I need to look at the schematic to be sure that is true. The channel 2 normal/modern switch does not change the sound of anything.

I would like to address the reverb issue a bit later after getting the basic amp working if that seems like a good idea. I do have a good signal leaving the board based on a injected 1K signal at 100mv rms. Actually I am not sure just what that signal should look like. It looks like the signal I injected but with greater amplitude.

Because I found a pot issue I am now going to closely check the other pots. I have done a lot of work on the amp and think I have all the power supply issues solved. There is some indication of incorrect LM3914 pin 5 voltages. Not off by a lot but not dead on. I can recheck and give you the readings if need be.

I don't know if you are up this late and if not perhaps tomorrow.


I believe that is one of the amps with the pots mounted on the panel with three wires each feeding back to the board. In my experience, many of those pots can get sprung. If something hits the knob and pushes on teh shaft, it can push the back cover off the pot, or loosen it. And when that happens the wiper inside the pot loses contact. SO go down the rows of pots, and check if the rear metal cover is loose on any of them, and tighten the tabs that hold any if so. Usually the shaft rocks back an forth more than normal when like this.

Channel 2 and 3 gain controls are not in series. Note the ground connection between them. They are just drawn in an unconventional manner.

ANy tone stack that stops working, I first check to see if the bottom end has lost its ground connection. Same with an inop volume control.

I have to admit the arcane channel switching arrangement baffles me too. I understand the electronics, but in practice, trying to set up the selector switch the assign the red and whatnot is way too complex. I like on/off.

I wouldn't expect all the channel lights to remain on. The FS switching circuit is pretty basic TTL. They drew the ICs as boxes with pin numbers. I think it would be easier to follow if they used the logic symbols instead. I'd wager that 74564 might be confused. If the ch3 LED on the amp chassis stays lit, that probably means the relay #4 is also staying on. And that might be the reason the signal path gets broken.

I suspect if you clear up the not working tone stack and volume control issue, the modern switch will probably wind up working again. Simple enough to see if the switch works. All it does is short across the 220k resistor, so use an ohm meter, the switch opens and closes or it doesn't.

Sure, I'd leave the reverb for later too, those are not hard to diagnose, and shouldn;t be involved in the other issues. It was the only thing I could give specific advice for from your initial post.
Hi Enzo,

I changed two of the pots. No real change.

I think that most if not all the issues with the amp are related to the channel switching.
I disconnected to foot switch and am using only the rotary switch to make things less complex.

I am currently trying to understand how the switching works.

I ASSUME that based on the block diagram the NC and NO mean normally closed normally open. I also ASSUME that switched to channel one no signal should be on channel 2 or 3 center tap or on the grids of V4a and V3a and b. I am going to check that now.

This amp was given to me and I am starting to wish I had never seen

Be back after a bit...need


Um... I'll have to look.

The block diagram is about signal flow conceptually, but it may differ from the schematic. But I tend to agree with you, at least for now. I reserve the right to be difficult later.

From all the various things you have described, I'd be thinking the witching was bad myself. I am going to have to think aout it some. I cannot see how the transistor for RY4 ever can be turned on at its base. Same with RY2. Oh... unless they are wired to the collector of the next one over. Sorry, I see they do hop-over bumps in the wire lines, so a straight crossing line must join. SO that means RY4 and RY3 have an inverter between, so when one is on the other is off.

I think I might just go through the switching circuit wholesale. Get a voltmeter or scope on pins 1 and 17 of the 3914 and see if they toggle as you turn the selector switch. If so, then you ought to see it at the bases of transistor 1 and 3. And so the collectors of Q1, Q3 (Q for transistor, I am lazy) ought to also toggle from maybe +3v to what, maybe -2.5v? And if that happens, the collectors of Q2, Q4 ought to do the same. Just at opposite from what Q1,3 are doing.

Q1,2,3,4 control relays RY1,2,3,4. And as each transistor turns on, so should the relay. You can verify that by touching it or by reading resistance across the contacts to see if they transfer.
While checking things I found that the Darlington NPN transistor marked 6426 4 connected to the channel 3 LED is Shorted based on in circuit testing. I also ASSUME that RY4 coil is always on as a result. How else could it get to ground. I ordered some 6426s and they will arrive the end of the week.

Note: I disconnected the foot switch and I am only using the rotary switch for the moment to make thing less complex...well..I

The input voltages to pin 5 from rotary switch are less that stated on the schematic.
CH 1 stated -2.6v actual -1.9
CH 2 stated 0v actual 0v
Ch 3 stated -1.4 actual -0.9
Meaningful ???

Pin 1
CH 1 -2.75
CH 2 -1.42
CH 3 -1.45

Pin 17
CH 1 -1.4
CH 2 -1.4
CH 3 -2.77

Base for the Darlington when switching reads -1.4v to -2.0v

Pin 8 -2.8
Pin 7 -1.5 at the pin before the resistor
Pin 6 2.08
Pin 4 2.8
Pin 3 2.1
Pin 2 -2.8
Pin 1 -1.4 after I replaced the 4.7v zener in the power 2.1 before

I have no skills at designing a three channel amp but I don't think I would have done it this way!!!lol:hypno2:


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I don't know a 6426, but this circuit works on about 6v top to bottom, and the relays themselves don't draw much current. The drawing doesn't show them as darlington, but I believe you that they are. (They should either draw both parts inside the circle or other indication such as two little arrowheads in a row on the emitter line, or a D in there, etc)

Most any little darlington should work, I have a ton of MPSA13 for example, that would I am sure work. If you have any little darlingtons, you might try them.
Thanks Enzo,

I ASSUME that as the transistors are marked on the transistor as Motorola 2N6426 they are Darlingtons. All the data sheets I see describe a 2N6426 as a NPN bipolar Darlington transistor. I don't have any ATM.

I have little experience with solid state electronics so I have my share of issues trying to figure what many solid state parts are. That lack of understanding prevents any logical guessing on my part as to possible subs. I learn something new every day with the kind help of people like you.

MY advice...start learning electronics before one gets to be

To make matters worse Motorola stopped making discrete transistors some time ago.

Two or three of the transistors in the amp are marked F930 MPS A20 which Mouser or Digi-Key could not find a replacement. I found some NOS on ebay and ordered them just in case.

Obsolescence in solid state parts seems to be more prevalent than I assumed it would be.


Motorola became On Semi. They still make the same line, just a new name. They make transistors by the millions still.

Mouser has 354 MPSA20 in stock at 42 cents each.
MPSA20 Central Semiconductor | Mouser

For that matter, Mouser has 9800 and some 2N6426 in stock at 40 cents per.

What is getting harder to find is TO92 transistors, they make more and more of them in surface mount these days. But just as in a tube amp, if you don;t find 6BQ5 on the shelf, then EL84 will do, or put a 6L6 in place of a 5881, you likewise may not find a particular transistor, but the same needs for such a part still exist, they just fill them with a newer type.

On Semi has selector charts on their web site. I have the old book format from Moto from back when. It groups the parts by basic application, so small signal or power transistor or switching transistor or high freq transistor, or, yes, darlingtons. Within a category, you can select by voltage and current needed, or gain, or dissipation, or what have you.

In the audio signal path we might exercise a little more caution, say select a lower noise part, but this is just acting as a switch to turn a relay on, so it hardly matters how noisy it might be. The stock part is a 40v 500ma darlington. On Semi shows MPSA13 as "last shipments". meaning they won't be making more after they run out. But Central Semiconductor makes lots of older types, and on their web page, looking at through-hole transistors, under darlingtons, in the TO92 package, they show not only your exact part, but also quite a few alternatives, like my MPSA series ones and others.

And just so you know, I will be 70 myself in a few months, and I started learning at age 7. So I have to agree, it helps to start earlier than 70. If it helps any, I learned digital logic (TTL/CMOS) before I ever really learned anything about transistors.
Thanks Enzo,

I was able to order everything I needed from Mouser except the MPS930 which I got NOS off ebay. The Mouser order will be here tomorrow. No rush,I am to old to get in a hurry anymore. lol

I try to study something every day. Some of it comes easy and other stuff not so much. I don't like TV and don't drink so messing around with electronics is a fun thing to keep me out of trouble!!

Thanks for the information above. I had not run across On Semi before.


Motorola broke the company into different divisions and spun off the semiconductor division, which became On Semi. The name Motorola continues in the communication industry.

So a On Semi MJ15003 is identical to a Motorola MJ15003 except for the name on it.

I installed a new transistor which turned the channel 3 LED off. All the LEDs are working as expected.

Something is really messed up because when I switch to channel 2 with everything else turned down there is only loud hum. If I turn up the channel 3 master I get very loud sound while still selected to channel 2.

Perplexing stuff.

I have most all the issues fixed at this point. This has been a PITB amp to repair!!

The only real outstanding issue is the reverb. The reverb is working now but is producing high freq. oscillation (feedback sounding) when turned up past a certain point on all three channels.

Any ideas?


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