MP-1 Classic tube channels problem

[Davide]

Hi everyone,
this is my first post here :)
My MP-1 Classic has a strange problem: the S.S. channel works like a charm, the tube ones (clean, brown and dist) not so much.
The dist at least has got some good level of volume: but I've to put the OD1, OD2 and Master at high level and the sound is not so distorted, it seems to get only a crunchy level.
The brown channel is the one that virtually doesn't work: I've to put the OD1, OD2 and Master at very high level to hear a bad and confused sound, with a lot of noise.
The clean channel has got a very low level of sound if I set the master under 9 (and over) and if I don't set OD1 and OD2 to very high values (around 9-10) I can't hear anything than a hissy and weak sound.
I already replaced the R151 resistor (as suggested by MarshallJMP  ) but I didn't notice any improvement in the sound: yesterday I noticed a small burning on the tube board below the vactrol U22 and I was wondering if it can be the reason of the problem... I hope you will be able to suggest me a solution, you are my very last hope  8) Thank you guys. Davide

[Dante]

I'm interested to hear how this goes...my Classic has an issue with the Brown sound being lighter than it should be...I'm assuming it's on it's way to being where yours is now. 

Either way, I'm keeping an eye on this post, until I have time to dive into my own Classic to see what's up

[MarshallJMP]

I mailed you back, did you got a chance to try it out?

[Davide]

Hey P, yes  ;D

[Dante]

and...?!?!? what happened?

[Davide]

Hi Dante, I checked the wrong Resistor , then I took a look at the right (R115) O0. At the moment, still waiting for MarshallJMP reply.
It seems that the Vactrol U22 could be the problem (you should check if the R115 Voltage value drops when you increase the overdrive setting; in my case, if I only increase OD1 level - keeping OD2 on 0 - there isn't any voltage drop). Don't know if it is correct: If I got it right (comparing Classic with MP1), Tube 1 should work for OD1 setting and Tube2 for OD2 setting). When MarshallJMP will tell me something about it, I'll be back with the answer 

[MarshallJMP]

The classic works different compared to the mp-1. OD1 is the input volume, OD2 is the amplification of the first section of the first tube. I'll try to get my classic out tonight and do some measurements so I can compare it to yours. Send me an email if you don't hear from me in the next few days. I'll try not to forget when I get home. ::)

[Davide]

Hey P, thank you for your precious help Waiting for your reply, hoping to solve this annoying problem  ::) ::)

[Davide]

Hi P, I sent you an e-mail just now. Hope you could have good news for me  :D :wave:

[Davide]

Hello guys, great news here!  :waving-banana-smiley-emoticon :headbanger: :banana-skipping-rope-smiley-e
We finally fixed my Classic. Here the answer:
- we replaced the first OPAMP (on the left) near U23 Vactrol;
- we get rid of two (NEW!!) Electro Harmonix 12AX7EH tubes, probably damaged or not suitable for this preamp. Now I have on board two TAD ECC 83 and my Classic shines  :banana-rasta: :banana-trip:

[MarshallJMP]

The opamp in question was U21, comparator that drives the vactrol.