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I've got a Mesa Nomad 55 combo amp that's making the volume swell up and down as if it had a vibrato effect but it doesn't. I've isolated down to the preamp section. Any ideas where I could look to pinpoint the faulty component?
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No, but I do always check the pots on the panel to see if any are coming apart. The rear covers can get pushes out, leaving it loose, and the shaft wobbly. This results in intermittant contact in the pot.Education is what you're left with after you have forgotten what you have learned. -
And please do not overlook the power supply.
If a preamp decoupling cap is bad, the signal may ride on the power supply.Comment
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It's caused by the capacitors in the power supply. It's a design mistake.
The 1-2 capacitors are not stiff enough, one of them drains too far...when a note or cord is played.
In other words, too much sag in the preamp power supply...
This causes a temporary low frequency oscillation in the power supply. Similar to "motorboating" oscillation.
But the motor boating is not continuous. It only happens when a note or cord is played, then it stops, afterwords.
When this temporary oscillation occurs, for just a brief moment, the grid of V4A is driven too far positive.
This causes a "cutoff" distortion, producing the vibrato effect.
Also, if you listen very closely, with the preamp cranked up, and the guitar volume knob turned down to a lower level...
you will actually hear a (short and temporary) buzzing noise, as the guitar note decays.
This buzzing is actually temporary saw-toothed ripple on the plate of V4A. This ripple occurs (following) when the grid of V4A goes briefly positive. (as above...)
However there is a simple way to cure the design error. This will stop the preamp power supply from oscillating.
It will also stop the buzzing noise, that so many guitar players have complained about.
If you are looking for this problem with a scope and sine wave generator...you are out of luck Charlie.
It only occurs when a GUITAR is driving the amp. Your test equipment is futile.
Unless, your guitar is an important piece of test equipment...which it SHOULD be.
The guitar produces ADSR - attack decay sustain release, which causes the problem to occur,
The sine wave is continuous, and ignores the existence of the problem.
That being said, we can beguine correcting and testing...
There are two proven methods, one which is very short and simple, the second one is more "technically" pretty.
I like the second method, because I am a belt and suspenders kind of guy. I like backup on my backups.
but you know, the #1 is going to solve it most of the time, if just minimal
#1 Solder a 50uF cap (47uF) across power supply filter in the preamp, at point C or at Point D
I am stiffening the power supply for the preamp.
There is 10uF already, and I am going to solder a 47 across it.
I am going to observe the + - polarity marked on the cap, it's got to be installed the correct way.
I am using a 450 volt or 500 volt cap...
And this is going to stop most of the problem in the simplest way.
#2 I am taking circuit board out, which is rather a pain in my PITA bread.
The caps that are changed are in the power supply, not the FX loop.
The FX loop mod is cute, but it really won't cure the underlying problem.
Specifically, The cap at point "C" in the power supply should be changed from 10uF to 47uF
The cap at point "D" in the power supply should be changed from 10 uF to 47 uF
The caps at point "B" in the power supply should be changed from 2X 10uF in parallel, TO 2X 22 uF in parallel.
At point A in the power supply, following the standby switch, you have 6 resistors in series.
220 ohm,365 ohm, 365 ohm, 365, 365, 365...
then you come to a 10uF filter cap. Change that cap to 22uF.
That's a wrap for stiffening the power supply.
Then at V1 input, pin 7, you should install a 33K resistor in series with the ferrite bead.
That cleans up the performance quite a lot. It's just about perfect now.
Sounds like a whole new amp. The sag and mud are gone, the highs are way clearer.
You can crank the gain much higher, without any oscillations (motor boating) occurring.
However, you should have a professional do it for you.
I do not recommend that inexperienced person attempt this update.
The circuit board must be removed to do this update. And, it must be done very carefully.
The tracks on the circuit board are plated thru holes, and easily damaged by an inexperienced person.
Then, there are issues:
as long as you have pulled the circuit board, carefully test ALL the mechanical 6 volt relays.
A worn out relay will cause problems. Make sure your relays are all good, before you put it back together.
I prefer to install sockets for all the relays and ICs.
Then I can change them anytime, without pulling the circuit board.
This saves a huge amount of time in all future maintenance hours, yes.
Was the power supply cap "bad?"
No not really, just not stiff enough to start with.
Not for a high gain design such as this. If the design was lower gain and not so extreme... then the 10uF might have worked...
I mean sag is cool, but too much sag...to the point where it causes oscillation...Last edited by soundguruman; 04-09-2014, 03:34 PM.Comment
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If the amp didn't do this when new out of the box, then it's not a design flaw, it's just a bad component or connection.
You can mod the living daylights out of the power supply, or you can locate the defect and repair it.
Sounds to me like a bad cap.
You've isolated it to the preamp, so it's probably a bad filter cap for the preamp, or possibly a bad cathode bypass cap.Originally posted by Enzo
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They all do it to one degree or another, no matter how old or new.Originally posted by g-one View PostComment
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