What is wrong with my Mark III

[monstermesa]

Good evening fellow mesa boogers.

I was playing my Mesa boogie Mark III Simul Class 100W blue stripe today when I realized it wasn't playing at full volume.

(God I love this amplifier)

As I turn up the Bass knob from 0-10 it starts to get loud, but then completely goes silent when I reach 10. '
This same thing also happens on the far left Volume knob, but is more apparent in the bass nob.

The amplifier is loudest ONLY when the Bass knob is at around 5. At that point the amp is incredibly loud. Once you spin the bass knob to 9 the amplifier goes almost completely silent.

All of the tubes are lit, and none make noise when I tap on them.

The volume nob and the Bass nob are the only 2 nobs that crackle when I tap on them from the front. They make a lot of crackling noises ESPECIALLY when I spin the Bass nob in and out of the VOLUME SWELL zone.


Any advice you can give me to save myself a 500 dollar diagnostic and repair would be greatly appreciated.

Help me get rockin at full volume again! Whats wrong with my Potentiometers?

Thank you !!

 

[monstermesa]

boogie?...


mesa..... boogie?

 

[nor]

$500? Part of my pcb blew up and the repair + full recap cost me $250. You are rolling an old amp. If you, like me, do not have the skills to diagnose the problem - you probably don't have the skills to resolve it. Get it to a pro, bro.

 

[McBarry]

Totally agree with Nor..

this doesn't sound like a simple or easily found fault for non-techs.. sorry to say...

I'll bet the pots anomalies are only the tip of the iceberg...

A tech should be able to diagnose and make a plan with you, the owner, quite easily..

Prolly not what U wanted to hear, but so be it...
Cheers, DB

 

[nor]

There is a single ground wire that loops through all the pots. Maybe some cold solder joints?

 

[Third Age Amps]

I'm having the same issue with mine EXACTLY! Did you get yours fixed?

 

[Third Age Amps]

Copy from our previous post ...

Thanks Restless, I just figured that out myself.

vigo1999, change your tone stack caps! Since it's open, just change all four. Two ceramic and two film caps at the left bottom of the board right behind the Volume and Bass pots. It's kind of a ***** because of the grounding wire and the other wires, but just cut the tie-wraps and push them out of the way. May as well check your PS caps while you're there too. They probably need to be changed.

I changed my tone stack caps to 250p, 500p, .02u, and .02u as I wanted more mid and mid/high punch. You can use the original values, of course, but it always seemed a bit too scooped for my taste.

Good luck! Now to do the Rhythm 2 pot mod, and build a proper 3 button footswitch!

 

[darkbluemurder]

vigo1999, change your tone stack caps! Since it's open, just change all four. Two ceramic and two film caps at the left bottom of the board right behind the Volume and Bass pots. It's kind of a ***** because of the grounding wire and the other wires, but just cut the tie-wraps and push them out of the way. May as well check your PS caps while you're there too. They probably need to be changed.

I fully support this suggestion. Probably the bass tone cap is leaking DC - moreso when the pot is turned up since then the resistance to the next grid is lowest.

 

[monstermesa]

Thank you for all of the responses!

I brought my amp into my local music store and 4 weeks later they were able to replace a volume pot and a cap.

They did not fix my problem.

They did add an extra nut to the pot on the inside of the shell, so the rod that protrudes outwards it a sixteenth of an inch shorter than all the others causing the volume knob to rub against the back plate.

Putting that behind me now, I would like to get my amp fixed.

Can one of you tell me what I am looking for as I go through to test my different caps?


PS. To the two fellows that had absolutely no constructive thoughts to help fix my problem at all, I came here for advice and guidance, not to be told I can bring it somewhere and pay to have it fixed. Good tip

 

[McBarry]

MM,
pls don't get offended when U don't like what you hear..
Consider that from what you've said, and what you've NOT said, a probably difficult diagnosis is likey beyond you.
But let's suppose I'm wrong, firstly I apologise if I did indeed offend you.

Second, please supply whatever voltage measuements you have taken thus far, and we can begin helping you in earnest.
DC measurements are a good start, and a sig-gen traced through is also great.

Third, I'm not quite sure why you're jumping to checking caps, and indeed which caps are you suspecting?
FWIW, Do you understand the tip that Darkbluemurder gave you?
It's good, BUT NEEEDS TO BE PART OF A SEQUENTIAL DIAGNOSTIC PROCESS.

Fourth, have you checked the vol and bass pot function out of circuit? (Assuming the new pot is fine, and whatever fault exists has not simply damaged the new pot also - unlikely, but must be considered). I find it helpful to manually plot a graph on paper - I can see in an instant if they have any flat spots, or any irregularity in their function.

Also, some assumptions: you have the correct schematic for your amp, you can read it, you have competence to work safely inside a live chassis, your amp is not hiding any peculiar mods (but you would have noticed that when the schematic was different from what is physically in front of you), you have a work area with appropriate tools...

Diagnostics MUST be sequential, and prove each step/point before moving on to the next.. Make NO assumptions.
Anything less is random, possibly guess-work and potentially dangerous. To you AND your amplifier.

Does this sound like something you'd be able to do??
If so, fabuluous, let's begin !!
Dave

 

[Third Age Amps]

monstermesa Two things ...

1. You can just change the two orange drop caps right behind the Volume pot with the same value, new caps and that will most likely solve your problem. .047uf/400v and .1uf/400v.

2. You can t-shoot the issue, but the t-shooting tips I'm about to tell you have to be done with the power on, so if you haven't worked with high voltage DC before, give it to a qualified tech. You can get killed working with these voltages.

With a guitar plugged in and the volume down, turn on the amp. Set your DMM to DCV and put it across pins 2-3 of the Bass pot. Now turn the Bass knob up and watch the DMM. There should be no DC voltage showing. If it does show a voltage, that means the cap associated with that pot is bad. You can do the same thing for all the pots if you want.

A good cap should block all the DC from hitting the pot.

 

[monstermesa]

Well it was a Mylar capacitor. All that trouble and it was one of the easiest components to replace. (Music shop clearly has no diagnostic experience having told me they replaced the caps)

I took it to a repair shop and paid 150 dollars and was told it was a mylar capacitor. As I turned up the volume on the bass it was shorting more and more of the signal to ground.

I then went home and purchased myself a multimeter for 24 dollars :roll:


ROCK ON GUYS and thanks for everyone who gave me advice other than, you should take it to a shop!

 

[Third Age Amps]

Well it was a Mylar capacitor. All that trouble and it was one of the easiest components to replace. (Music shop clearly has no diagnostic experience having told me they replaced the caps)

I took it to a repair shop and paid 150 dollars and was told it was a mylar capacitor. As I turned up the volume on the bass it was shorting more and more of the signal to ground.

I then went home and purchased myself a multimeter for 24 dollars :roll:


ROCK ON GUYS and thanks for everyone who gave me advice other than, you should take it to a shop!

Hopefully they replaced it with an OEM cap (Orange Drop 715P), otherwise you might be changing it again in a few years!