Mark III help needed!!!!!

[Monsta-Tone]

Well...
Usually, I'm trying to help someone else out with their amp problems! Now it's my turn!

I have a Mark III (can't remember the stripe) in my shop.
It works, but has an interesting problem that I simply cannot track down. Maybe I'm looking too hard or over analyzing things. I'm hoping that one of you guys here has had this same problem and knows what the hell is up!
I called the Mothership yesterday. Mike B is on a much needed vacation and won't be back till next week. The guy just bought the amp and lives in Guam, so he's dying to play it. I'm hoping I can get an answer for him sooner than next Monday or Tuesday!

Here is what the amp does......
It works perfectly with these settings:
Gain 6
Treble 6
Bass 6
Mids 6
Master 3
Lead Drive 8
Lead Master 5
R2 Master 4

If I turn the Gain (Volume) up to 8 or higher:
The Mid pot only works on 4. Any other setting and the sound is muffled and garbled!
If the Mid pot is not on 4, then the amp will not go into R2!

I was guessing LDR's, but it was late on Sunday and Monday was a holiday, so I just stopped where I was and thought I would call Mesa yesterday.

Any help would be greatly appreciated!

 

[Blueracer]

Recently I did have a weird problem concerning the volume/tone stack. Tracked it down to a leaky cap in the tone stack. Not a bad thing to check for. Look for any dc on the tone control pots and the volume control with no input. Make readings relative to ground.

An interaction with getting into R2 from the mid control is interesting. Look at what is happening dc voltage wise in the 6 volt dc supply as you vary the mid control. I would meter across the 3300 uf 10 volt cap. If you aren't getting into R2 it could mean low voltage here. Notice also that the heater for V1 is on this voltage also which could account for the distortion.

 

[Blueracer]

So what ever happened with this amp. Did you locate the problem. Just curious.

 

[Monsta-Tone]

:lol: 20 minutes ago! :lol:
I talked to Mike B today. He said the same thing about the tone stack caps.
Sure enough, the mid and bass cap (.1 & .047) were leaking just a little bit of DC!

The way they were leaking is strange though. I would read 0 volts until the Volume was above 8 (no matter where the mid pot was)! Then, the mid pot would vary the voltage from .5 to 14.65 volts.
The amp would not go into R2 if the volume was above 8.
It would go into R2 if the volume was below 8, but there was a huge popping noise!

I changed all 4 (mid boost cap too) caps in the tone stack and tested the resistor. Now everything is beautiful, and REALLY LLOOUUDD!!!!!

Thanks for the help!
I guess I didn't look hard enough at those caps when I tested them! Should have messed with the damn knobs a bit more!

 

[Blueracer]

Good to hear that you located the problem. Measuring dc from leaking caps can be tricky. It is a good place to use a VTVM (vacuum tube volt meter) which has a very high input impedance. More conventional meters can have an input impedance that will sufficiently load the circuit which will lower the measured voltage. Tone Lizard has a good article on this. Here is the link.

http://tone-lizard.com/VTVM.htm

Hope this is helpful and loud is good!!!

 

[Blueracer]

One other thing to note is that the volume control goes straight into the grid of V1B. If the grid goes positive relative to the cathod the grid will pull current. This could account for the "weird" voltage readings you encountered when you varied the volume control. Removing the wire between the top of the volume pot and the wiper of the treble control would isolate the grid circuit of V1B and the tone stack making DC voltage readings up and down the tone stack more straight forward. Just a thought for future trouble shooting exercises.

 

[gummx97]

Well, I was the customer that owned that amp that Andy was working on. Finally received the amp today. All I can say is that now the amp sound phenomenal! Lead channel has that liquid sustain gain Mesa is famous for, R2 mod works as advertised with great crunches, clean sound is as Mesa as it can get. All potentiometers now work great and has the correct taper.

Plus, seeing the old parts being sent back to me shows Andy's professionalism regarding his work.

When he got the amp fixed, the wait was killing me!

At the end of the day, money well spent with Andy's work!! Thought I'd let everyone know since I heard of Andy ( aka Monsta-Tone) through this forum.

Just another satisfied customer with Andy. Great Work!