Mark III blue stripe issues, Please Help!!!!

[Wolfchild]

Hi folks, new to the board and need some help:

I have recently acquired a Mesa/Boogie MKIII (blue stripe) that has a couple of problems...
Firstly, there is a strange oscillation occurring on the lead channel even at low volumes, the pitch of which can be tuned with the eq controls (both the graphic and the pots). I have replaced the preamp valves but this has not cured the problem.
Secondly the reverb effect has a low level distorted signal coming through in addition to the main amplified signal. Apparently this is a common problem with these amps (and easily fixed?), but I wouldn't know where to start. The reverb is an authentic boogie unit but has been added as an upgrade (ie it was not factory installed).

Thanks in advance,

W.

 

[MrMarkIII]

For general tube problems, I'd recommend downloading a manual from the Mesa website.
Here:
http://www.mesaboogie.com/manuals/user_manuals.htm
Pull the reverb tank and look for the numerical code stamped on it -- it should be 9AB2A1B. Other tanks won't work and/or sound right.
You can buy reverb tanks from Accutronics.
Here:
http://www.accutronicsreverb.com/
Google is your friend.

 

[Wolfchild]

Thanks for the tips, the reverb tank checks out ok. I think I read somewhere that the reverb glitch has something today with an incorrect resistor value? Can anyone shed some light on this please?
Regarding the oscillation, I have the manual from Boogie and have followed their troubleshooting guide re. preamp valves but to no avail. Essentially any medium-high gain setting (including Mesa's own recommended settings), results in feedback/oscillation unless the lead master is so low that the channels cannot be balanced.
Any more suggestions guys?

Thanks once again for your time,

W.

 

[Wolfchild]

Just uploaded a clip of the oscillation problem on youtube:

http://www.youtube.com/watch?v=oR2qdDTpxuw

W.

 

[Monsta-Tone]

I haven't seen the oscillation problem or the Reverb problem yet, sorry.
The Oscillation might be a bad ground, solder connection, or pot, but I can't say for sure.
Without seeing the amp in person, I couldn't tell you what it was.

I bet if you do a search here though, some one has had these problems.

 

[Wolfchild]

I have searched the entire forum and come up with some possibilities for the reverb issue which I will investigate. Still no luck with the weird oscillation on the lead channel though.

Any more suggestions greatly appreciated....

 

[Monsta-Tone]

I'll think about it and see if I can come up with something.

It's pretty hard to diagnose something I haven't seen yet, sorry.

 

[silentrage]

Can you post any progress on this? I have pretty much the same oscillation going on with mine.
Mine is triggered by 1. R2 pulled, 2. Lead Master pulled and 3. Lead Drive pulled and higher than 9.
That's the exact condition required for it to oscillate like that.

 

[Wolfchild]

The amp has been sent to a tech who is confident that he can fix both problems. I will post his findings when I get the amp back.

 

[Wolfchild]

UPDATE:

Got the amp back today and she's as good as new

Here's what the tech had to say:

Reverb problem was the grid reference resistors which had been snipped (possibly to increase the reverb level). Once reconnected the reverb was not distorted but still very quiet. Original resistors rated at 20k(?) a piece subsequently replaced with 1meg(?) value et voila: Lush 'verb minus the distortion.

The strange oscillation on the lead channel was due to faulty input earths which have now been fixed rendering the amp capable of all manner of hi-gain mayhem (if that's your bag).

Hard to find a competent tech here in blighty so thanks to Steve Rowse of Godalming in Surrey for a great job. He can be contacted at

http://www.stevesamps.co.uk

...check him out if you need work doing and your in the south of England.

 

[bartchamdoh]

Amen for the guitar tech most guys i,ve went to just don,t have a clue! I had a 2c(many moons ago) and eventually traded it for a mark 3 cause i couldn,t get it fixed. The tech i sent it to did a complete re valve(to the tune of 180 pounds at the time)and still it came back with the same probs.....needless to say we had a little heated debate on the matter....but that,s another story!

 

[Monsta-Tone]

That is good news!

To be honest with you, the reason I started fixing amps was because there was nobody in my area that was worth a ****. I went through several great amps because I was always told it needed an output transformer or some other B.S.

I've only seen 2 or 3 bad O.T.'s in thousands of amps that I've repaired.


Glad there is a good tech near you!

 

[silentrage]

@gts
Sorry to make a blanket statement like that, but this is where I'm getting it from.

http://homepage.mac.com/mesaboogie/dot.html

#1 - No mark or a little dot. Only a few hundred then some balck marks or "+"'s .

- Lean and powerful amp with more output power than a IIC+

#2 - Purple: reshaping of R2

- R2 was shaped to be more "rounded" and less gain, with improved level

#3 - Red: R2 is like current Mark III

- R2 further developed and very hot. Lead mode is also tweaked to close in on the IIC+ sound

#4 - Blue: Reshaping of R1

- More aggressive preamp gain - reshaping of R1, Power section made akin to IIC+

#5 - Green/Simul-Class: Final R1 and Lead Channel reshaping

From your "possibly" sarcastic remark, I'm guessing I'm totally wrong here?

 

[CoG]

way too much gets made of the stripes. I have a "late" green and an "early" purple and if they have the same tubes in them you can't honestly tell R1 and Lead apart. If you use exactly the same settings they sound different but any sound you can dial in on one you can dial in on the other. The R2s are different but one's not better or worse.