MarkIIC converted in IIC+ Lead channel issue (loss of gain)

[djemass]

Hello everyone,

I own a MarkIIC converted by JVK in IIC+ some years ago. This amp was also modded by him (C++ mod, and so on...). if I remember well, it was a RP-10 board

I was playing in the lead channel without any problem and then suddenly, the full organic distortion of the lead channel became a "thin very few crunchy" tone. No loss of volume, but 95% of the distortion was gone. I thought it was due to a faulty LDR (I had this issue 2 years ago with similar symptoms).

I sent the amp to a guitar tech. He checked all the tubes, they are all OK. LDRs are also OK. For the moment, he hasn't found the cause.

Perhaps a bad resistance ? The problem is that with time the values have disappeared on some of them... he has to draw me a circuit scheme and send it to me with the "missing" values. Other problem is that the scheme of the IIC of Boogie is far to be 100% complete, and with all the mods JVK made, he may be confused now

FYI, the clean channel works perfectly, and the 2 master volumes of the lead channel seem to work ok.

Any suggestion ? Thank you very much for your help

 

[Elpelotero]

Any chance you can send it to Mesa Boogie to have a look at it? Estimate around $200 for the work, plus shipping.

 

[djemass]

Hi,

Thank you for your answer. Unfortunately, it would be impossible for me to send it to Petaluma.

 

[loylo69]

I would have suggested a Vactrol failure also. I had this issue on my mark III and the symptoms were exactly the same, " lead channel becoming a thin very few crunchy tone".
Have the clean channel also been altered?

It is hard to make a diagnosis remotly.
The schematic your tech is drawning may give you (us) a hint.

 

[djemass]

Hi Lyolo69,

Do you know if the schematics from Sloclone forum (schematic made by The Haunted V1.20 of 09/11/2009) is accurate ?

I would send it to the the amp tech to help him

 

[loylo69]

Yes, it is accurate. But as you may already know, two IIC+ may differ from one to another because of a production discrepency depending on which components Mesa had in stock.
However, the schematic should be enough to check the range of the resistors' values in case of such a failure.

So if your tech is good enough, he should find out the problem thanks to the schematic.
The mods JVK performed (C++, vol1.1...) shouldn't alter the comprehension of the amp's layout because they should be located in very specific parts of the circuit.

 

[djemass]

Thank you for your answer

Cheers