Mark III to Mark II mod

Mark I's, II's, III's and the almight IV

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xdg999
Donating Member
Posts: 168
Joined: Tue May 05, 2015 9:07 am

Re: Mark III to Mark II mod

Post by xdg999 » Mon Nov 05, 2018 9:14 pm

Got the SRG lll+ back from Mesa. The amp sounds and feels fantastic. It’s not as bass heavy as the DRG’s. I had Mike B. install the R2 knob on the underside of the chassis so I could keep the Direct Out.
With an MC-90 speaker it’s just light enough to be a grab and combo.
c+ x 14, III+ x 5, D-180, Studio Preamp, 50/50, Mark V 25,1994 Rev G Dual Rec x 2, Mini Rec and lots of Mesa cabs.

How many guitars and amps do you really need!

lions
Mark II
Posts: 75
Joined: Thu Feb 22, 2018 11:37 pm

Re: Mark III to Mark II mod

Post by lions » Tue Nov 06, 2018 6:31 pm

xdg999 wrote:Got the SRG lll+ back from Mesa. The amp sounds and feels fantastic. It’s not as bass heavy as the DRG’s. I had Mike B. install the R2 knob on the underside of the chassis so I could keep the Direct Out.
With an MC-90 speaker it’s just light enough to be a grab and combo.
How pronounced is the difference in low end? Is the tightness comparable?

xdg999
Donating Member
Posts: 168
Joined: Tue May 05, 2015 9:07 am

Re: Mark III to Mark II mod

Post by xdg999 » Tue Nov 06, 2018 7:42 pm

I really like the SRG with a Telecaster. Definitely spankier than the DRG’s which is why I decided to send it in for the lll+ mod. It’s similar to an HR c+ switched to 60 watts. It’s tighter than the DRG’s but the new caps need to get broken in so it should loosen up a little bit. I have it in a combo with Mesa Russian 12ax7’s, 440’s and an MS-12 speaker. I didn’t get to spend a lot of time with it since I also just picked up another c+ and a D-180. And I’m usually out of town for work. The D-180 came with 4 Tungsram’s and 6 of the 415’s. When I get home Saturday I’m going to try the Tungsram’s and 415’s in the SRG.
c+ x 14, III+ x 5, D-180, Studio Preamp, 50/50, Mark V 25,1994 Rev G Dual Rec x 2, Mini Rec and lots of Mesa cabs.

How many guitars and amps do you really need!

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III to Mark II mod

Post by jrb32 » Mon Dec 17, 2018 1:01 pm

Regarding the III to IIC+ mod, after buying a IIC+ to compare and double check my schematics, I've just finished off the III+ mod for lions on a Green Stripe. A few things I'd like to revisit after getting a IIC+ finally and comparing the two and tracing that properly as there are some mistakes in the IIC+ schematic I drew up before which I have now corrected (links to google drive are the same):

1. The 47K and 0.047uF on the V2A anode for the III can be swapped around (R113/C505). This is how it's done in the RP10A IIC+ boards. I think this changed for the RP11A board though. It shouldn't make much difference in sound though.

2. Adding in the extra voltage rail with an extra 30uF filter cap will work but there is a 4V drop in the IIC+ between D and D+. For all intense and purposes replacing R140 with a 5.6K resistor and making the power rail for the Preamp/Lead/Reverb 12AX7s as D will do exactly the same job. It's better that way and more stable than running a load of wires and putting in an extra cap. You're talking about 4V plate voltage difference by doing all that effort. NOT worth it IMO. The extra cap a hangover from the IIC where there was a significant voltage drop between each of the rails. Then they stuck a 1K resistor in instead of a 6.8K for the IIC+ and it does basically nothing. In the later IIC+s the voltage was increased even more by replacing the 5.6K resistors with 1K, meaning there was virtually no voltage drop which is the reason they ended up using 3x30uF caps in the III.

3. R205 should be put between R239 and R225/R122/C516 and where R205 was before make this go to ground with bus wire. This makes a BIG difference to the sound. Be careful in doing this as you'll need to lift components off the ground and form a triangle pyramid of components, similar to when you add the extra 100K and 10uF/63V in for the EQ section.

4. R238 resistor that goes to the presence pot is 1.5K on the IIC+. NOT 3.3K. I have never seen this as 3.3K in any pictures I can see of a IIC+.

5. Do not try to wire the III+ pots or grounding exactly like the IIC+. Leave everything as is. If you try to change it you'll get weird grounding and hum issues. I have this now on my III+ which I went overboard on and now need to change back. It'll all look a lot neater too. It's wired that way for a reason because of the PCB style. If you want exact wiring of a IIC+ you need to build a clone board of everything and basically build a DIY IIC+ clone in a III chassis. That's way overboard for a 'mod'.

6. The extra treble shift LDR in the Red Stripes onwards and the extra diode/resistor can be removed without any impact (well perhaps - see No. 7)

7. I can't seem to engage R2 when the Lead is engaged on the Green Stripe. I can on the Black Stripe. I'll look into why this is done (guessing Mesa changed the LDR layout slightly to prevent this) but without changing will prevent the ++ being switchable on the Lead channel which kind of defeats the point. I will check the LDR rails and fix this though. Is this how the Blues and Greens are supposed to behave regarding R2? I can't tell since I've already done the mod... :(

Side by side the III+ Green Stripe and IIC+ sound very similar. Each has a slightly different characteristic. However, they sound and feel the same and I can play each for hours. There are also some component difference values which would count towards this but no more than how two C+s will sound different side to side. The EQ sliders have a wider range on my IIC+ (measured with a multimeter) so if you set the controls the same they sound different, but you can make them sound the same by trial and error. Same with the pots due to manufacturing at the time.

I'd still pick my IIC+ over the III but just because of the more high quality hand built feel and separate PCB boards/twisted wires everywhere rather than the all in one PCB. Overall there is less hum in the IIC+ because of this and it's quieter overall.

I'll give you an update once resolved the R2 LDR for lead mode issue.

Cheers,

Jon
Last edited by jrb32 on Wed Jan 02, 2019 4:58 am, edited 1 time in total.

Agustín Collia
Mark II
Posts: 128
Joined: Sat Oct 12, 2013 1:36 am

Re: Mark III to Mark II mod

Post by Agustín Collia » Fri Dec 21, 2018 9:07 pm

jrb32 wrote:Is this how the Blues and Greens are supposed to behave regarding R2? I can't tell since I've already done the mod... :(
I can engage R2 when I'm on the Lead channel with my Blue Stripe. AFAIK, you can engage R2 and Lead at the same time on all Mark IIIs.

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III to Mark II mod

Post by jrb32 » Wed Jan 02, 2019 5:03 am

I can engage R2 when I'm on the Lead channel with my Blue Stripe. AFAIK, you can engage R2 and Lead at the same time on all Mark IIIs.
That's what I thought! Supposedly the R2 volume boost is minor on the lead channel on the green stripes from what I've heard. Either way in the end I bypassed the R2 switching completely and wired the ++ mod (the 15uF cap on the cathode with a 100K between the treble/volume pot) as a simple ON/OFF switch for when the Rhythm pot is pulled. Works really great and is foot switchable too! Can be engaged on both Lead and Rhythm mode.

Just finishing off. I've noticed some voltage differences in some of the 715P power caps (they're 200V in the IIC+ and 400V in the III) so changing these just to see what difference it will make (none in theory right but you never know...)

Thanks,

Jon

xdg999
Donating Member
Posts: 168
Joined: Tue May 05, 2015 9:07 am

Re: Mark III to Mark II mod

Post by xdg999 » Sun Jan 06, 2019 6:45 pm

I picked up a mint Mark lll HRG no stripe combo yesterday. The amp came with the original tubes. A quad of STR-415’s, a Mesa labeled Chinese Military in v1 and Tungsrams in v2-v5. The amps sounds really good stock but I like my 60 watt lll+ better. The cleans are better and the lead channel is more touch responsive on the 60 watt. Look’s like the HRG is gonna make a trip to Mesa so Mike B. can work his magic.
c+ x 14, III+ x 5, D-180, Studio Preamp, 50/50, Mark V 25,1994 Rev G Dual Rec x 2, Mini Rec and lots of Mesa cabs.

How many guitars and amps do you really need!

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III to Mark II mod

Post by jrb32 » Thu Jan 10, 2019 11:11 am

I picked up a mint Mark lll HRG no stripe combo yesterday. The amp came with the original tubes. A quad of STR-415’s, a Mesa labeled Chinese Military in v1 and Tungsrams in v2-v5. The amps sounds really good stock but I like my 60 watt lll+ better. The cleans are better and the lead channel is more touch responsive on the 60 watt. Look’s like the HRG is gonna make a trip to Mesa so Mike B. can work his magic.
Living the dream! I've spotted a really cheap IIB near me that I'm really tempted by... going for the whole set one day probably :lol:

xdg999
Donating Member
Posts: 168
Joined: Tue May 05, 2015 9:07 am

Re: Mark III to Mark II mod

Post by xdg999 » Thu Jan 10, 2019 4:14 pm

I have a hard time resisting buying an amp if the price is right. The Mark lll HRG I just bought was $950 with a road case, EV speaker and the 105 transformer. I had to drive almost 500 miles round trip to pick it up. Never thought I would have this much stuff. All I wanted was another c+ HR combo.
c+ x 14, III+ x 5, D-180, Studio Preamp, 50/50, Mark V 25,1994 Rev G Dual Rec x 2, Mini Rec and lots of Mesa cabs.

How many guitars and amps do you really need!

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III to Mark II mod

Post by jrb32 » Tue Jan 15, 2019 4:53 am

Same here! Similar amount of driving to get my IIC+ and III but the price was right so I couldn't say no...

Back to the III+ mod! On the green I swapped over the 715P power caps with 200V versions as in the IIC+ and made the B voltage rail so it goes to the reverb 12ax7 (lifted R116 and ran a wire off the B rail filter cap neatly around the PCB board where the other wires go, again same as in the IIC+). It really does sound identical to my IIC+ and made a huge difference. Really quiet and reverb works great as it should! Take off the weird 2W 120K reverb fix resistor going to ground coming off R116. Instead add the new B voltage rail along with, ideally, a 2W 10K resistor in place of R116.

Weird how much the reverb circuit can affect the rest of the amp but it definitely does!

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III to Mark II mod

Post by jrb32 » Tue Jan 29, 2019 9:50 am

So for anyone looking to do this for the III, here are the schematics for the IIIs and a IIC+ schematic to compare to. I'd suggest changing the orange drops in the power and EQ section to the same voltage rating as the IIC+ as well.

BLACK STRIPE/DOT CP1D LAYOUT AND SCHEMATIC. ORIGINAL 1985 BLACK:

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

BLACK STRIPE/DOT CP1E LAYOUT AND SCHEMATIC LATE 1985/EARLY 1986:

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: 1986

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: 1987

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: 1988

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: 1989-1997

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

IIC+ SCHEMATIC (mix of RP10A and RP11A C+s with ++ info)

SCHEMATIC: https://drive.google.com/open?id=1ieTnM ... RIDwbLAcUQ
TINYCAD FILE: https://drive.google.com/open?id=1_GA53 ... 9ozJfqaQdZ

Any mistakes you come across feel free to edit and let me know. Easy to compare the two and go from there but the rules are:

A) Send it to Mike B if you're in the USA.
B) Do not attempt unless you have a lot of electronics or soldering experience and cannot send it to Mike B due to geographical location. Lead solder. Iron set to 410 degrees C. The traces can be easily damaged and might need to be repaired. Having a III+ mod not done by Mike B may harm its resale value.
C) The 22K cathode resistors can be left as 22K. Close enough to 15K but you can change them to 15K if you want.
D) Orange drop type (225P vs 715P) resistor type and tantalum cap type vary between IIC+s, especially the orange drops in the power section it seems.
E) For a ++ switchable mod I like to add a 100K resistor off the treble pot with R2 to use C214 only with a 15uF cap instead of a .47uF. You might be able to do something more clever though with the relay.
F) Don't add in the extra D+ voltage rail. It's messy and not worth it.
G) DO add in the B voltage rail for the Reverb like on the C+. It makes a big difference separating this from the other 12AX7 power rails and don't get any interference that the III 120K 2W factory reverb mod tried to address. You have to run a wire though.

Cheers,

Jon
Last edited by jrb32 on Thu Mar 14, 2019 4:25 am, edited 5 times in total.

lions
Mark II
Posts: 75
Joined: Thu Feb 22, 2018 11:37 pm

Re: Mark III to Mark II mod

Post by lions » Wed Jan 30, 2019 12:16 pm

Isn't C26 missing in Blues and Greens (and possibly earlier?)? Great work all around man.

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III to Mark II mod

Post by jrb32 » Wed Jan 30, 2019 1:43 pm

Isn't C26 missing in Blues and Greens (and possibly earlier?)?
Yep already done that check the schematics - C26 is removed for the EQ section from the Purple stripes onwards. It's marked on the boards though just not populated with the cap.

Cheers! I think this is pretty much there. Only main thing I am unsure on is the values for some of the ceramic caps in some of the stripe versions.

lions
Mark II
Posts: 75
Joined: Thu Feb 22, 2018 11:37 pm

Re: Mark III to Mark II mod

Post by lions » Mon Feb 25, 2019 5:17 pm

Might have found a typo/error in the IIC+ schematic, it says delete C30 but that's going by the III's naming. Looks like C30 on a III is C32 on a II?

jrb32
Mark III
Posts: 194
Joined: Wed Jan 31, 2018 8:32 am

Re: Mark III to Mark II mod

Post by jrb32 » Tue Feb 26, 2019 4:40 am

Might have found a typo/error in the IIC+ schematic, it says delete C30 but that's going by the III's naming. Looks like C30 on a III is C32 on a II?
Yeah just checked some pictures of a couple of DRG IIC+s and they have what I'm calling 'C32' removed and 'C30' present on the IIC+. These numbers are not official as there's nothing written on the board and no overall official schematic or board layout. Have updated thanks for letting me know :)

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