Black/No Stripe or Black Dot - Essentially the original schematic based off the incorrect IIC+ schematic with R130 and R126 for R2 channel. C214 added in later for R2 channel on transition models between late 85 and early 86. There was a PCB board change to do this.
Purple - A late black stripe with basically a IIC+ lead circuit minus a few slight component value changes. But then C26 removed to make it less like a IIC+ in the EQ section... VERY underlooked amp. Easiest to + mod in my opinion. Great tones from it too and gain (this is NOT a low gain amp despite the rumours).
Red - A purple stripe but some weird preamp values going on. Lead circuit now almost identical to IIC+. R2 channel changed to remove R126. In its place sticks in an automatic treble shift to be enabled when in R2 mode via an additional LDR. This supposedly causes humming issues and you need to turn treble shift off to get it to sound more like a IIC+ in lead mode.
Blue - 2.2M for R106. Crazy gain lead channel that was completely overhauled. Same R2 channel as Red. Power section finally has 0.0047uF presence cap and 1.5K presence control resistor (same as IIC+)
Green - same as a blue but has 3.3M for R106. Outer simul power valves wired in pentode.
There was a Reverb mod/fix done at the factory. If this was done there will be a big R written on the back. This essentially adds a 120K 2W resistor from R116 to ground and also a 1K or 3.3K resistor on the SEND/RETURN black lead. I'm not 100% sure on the reason for this but it's supposed to sort an issue with the reverb distorting or causing interference. Personally I have also found you can get a popping noise on the speaker sometimes causing interference without this 120K resistor in place. The fix hasn't got anything to do with increasing reverb strength and, in my experience, it does change the overall tone of the amp and I wasn't a fan so removed it. The 1K/3.3K is worth keeping though for definite. This was a Reverb mod done on the IIC+.
Some people say you can sort this reverb issue by moving the reverb tank around 180 degrees so the reverb tank plugs are no longer under the power transformer. I had some success with this.
I ended up removing this 120K resistor and made the voltage rail going to R116 from C to B on the schematics. I did this with running wire off the B rail 30uF cap to R116 with one leg of R116 lifted off the board going to this wire and this got rid of the popping or distorting issue and for all and made the reverb all so sweet (make sure you secure it properly – glue gun etc.). This B rail is how it's done in the C+. I'd recommend upping R116 to a 2W resistor though in this scenario definitely.
The Mike B III+ mod is something I have documented but would rather not reveal. Like the IIC to IIC+ upgrade steps I don’t want idiots going and ruining their amps. The whole point of me doing this is so competent people might be able to fix other people’s screw ups! Basically, always send stuff to Mike B at the Mesa factory if you’re in the USA! If you’re unable like me living too far away all I can say is that it mods a III to how you would think it would be done – lead section and EQ section have major changes, especially the EQ section. Power section remains basically the same with minor changes. Again, the Preamp section only has minor changes.
Regarding the cathode for the preamp in general - if your amp has 22K and the schematic says 15K relax or 22uF vs 20uF vs 15uF – relax. Those values seem to be used interchangeable. Plus if you’re expecting one schematic for a IIC+ to be exact with no random component values every so often think again!
NO STRIPE/BLACK STRIPE/DOT CP1D LAYOUT AND SCHEMATIC:
SCHEMATIC: https://drive.google.com/open?id=1yG8gK ... Cvo5cu0CK2
BOARD VIEW: https://drive.google.com/open?id=1boo0U ... 1mIyigNuog
TINYCAD FILE: https://drive.google.com/open?id=1ATlQr ... eAVviBnAU4
NO STRIPE/BLACK STRIPE/DOT CP1E LAYOUT AND SCHEMATIC:
SCHEMATIC: https://drive.google.com/open?id=1fAjkr ... rhnay8ZXjJ
BOARD VIEW: https://drive.google.com/open?id=1vurMa ... kTfTuyGAuH
TINYCAD FILE: https://drive.google.com/open?id=1WQjZ_ ... bWj3TG7F9r
PURPLE STRIPE CP1F LAYOUT AND SCHEMATIC:
SCHEMATIC: https://drive.google.com/open?id=1QRuyC ... MqApbrxWXv
BOARD VIEW: https://drive.google.com/open?id=14wHka ... Jz0HFRDXON
TINYCAD FILE: https://drive.google.com/open?id=1-0lSs ... v34OuzKMMo
RED STRIPE CP1G LAYOUT AND SCHEMATIC:
SCHEMATIC: https://drive.google.com/open?id=1o965Z ... sDycfzlBJX
BOARD VIEW: https://drive.google.com/open?id=1kf7xB ... C1fO-vkEAO
TINYCAD FILE: https://drive.google.com/open?id=1hVEeG ... 4A3AkPH6l0
BLUE STRIPE CP1G LAYOUT AND SCHEMATIC:
SCHEMATIC: https://drive.google.com/open?id=1xnKpo ... rckxwbt8oJ
BOARD VIEW: https://drive.google.com/open?id=1Mv0qN ... dVgBiBVUhw
TINYCAD FILE: https://drive.google.com/open?id=1y8kds ... WfUlITuv0t
GREEN STRIPE CP1G LAYOUT AND SCHEMATIC:
SCHEMATIC: https://drive.google.com/open?id=146K06 ... _5CU9_hXlV
BOARD VIEW: https://drive.google.com/open?id=1MmBK- ... idtRplSIIl
TINYCAD FILE: https://drive.google.com/open?id=12VF0_ ... TKhDVBG1VT
Hello! So a lot of stuff has been posted on here about the Mark IIC+ and there is the excellent guide for dummies, however there is not too much info on the Mark III.
I really want to get a guide up but I'm missing some info on component values used in the different stripe values. I have most of the info I need, schematics ready to go and board layouts for these components but ideally some board photos or gut shots would be great of any of the following:
The 100% concrete info I have is from my Early 86 where there was a transition period between the Black and Purples using the CP1E board. I'd like to do one for all the Mark IIIs and get a full guide up and running along with general maintenance info. Even better if you don't mind going through your amp using my board view and schematic as a guide and checking the component values that'd be amazing!
I should also be putting up a III+/++ mod guide for each stripe if I can get the info needed for each. HOWEVER, please note the IIIs are great amps and shouldn't be overlooked as being massively inferior to the IIC+. Originally they were designed to be a IIC+ with switchable ++ (the R2 channel) but they went beyond that into new territory with them. Whether this was good or bad is for debate but it happened - deal with it. The other big change is the IIIs use a simplified single PCB board instead of the many individual PCBs used in the IIC+. In developing this amp into a single PCB, Mesa missed a few components though compared to the IIC+. The early IIIs are identical to the official III schematic which was based off the official IIC+ schematic, which is incorrect in a lot of places.
Any info you could contribute would be great! Please don't post full serial numbers but thing like 178XX are cool along with the stripe and amp dates (the bit in sharpie on the chassis) would be amazing!
Many thanks in advance.