Photo request: Mark IIC+ preamp circuit board

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

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woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Photo request: Mark IIC+ preamp circuit board

Post by woodbutcher65 » Wed Aug 01, 2018 12:41 am

Would anyone here who has access to such a thing be so kind as to post as clear, and as large, a photo as possible of the preamp board in a Mark IIC+?


I want it for reference. And because I want to see for myself how different, or similar, that preamp board is to that of a Mark IIB or IIA.

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Wed Aug 01, 2018 10:52 am

It's a lot different! Mainly the LDR switching. It's why only the C board can be upgraded to a C+. Were you thinking from an upgrade point of view or just curious? You can turn a Mark IIB into a C+ (I think also a Mark IIA) but you need a C or C+ circuit board. Mesa ran out of those a while back.

I can send you some pictures though if you want of an RP-11A board? For reference the are loads of different C+ boards:

RP-10A - "Based in the style of the IIB with LDR's for switching instead of the one relay. Early production IIC PCB. It was changed somewhere in the mid range
to the RP-11 in mid to late 1983. The reason the RP-10 can be upgraded is the five LDR's. The rest of the
circuit Mike B. just does his magic on."

RP-11A - Simul-100 and 60 Watt Reverb
SP-11A - Simul-60-100 Watt. No Reverb

woodbutcher65
Mark III
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Joined: Fri May 04, 2018 10:36 pm

Re: Photo request: Mark IIC+ preamp circuit board

Post by woodbutcher65 » Wed Aug 01, 2018 3:01 pm

I wanted to know just how much might be involved if I were to consider reworking a IIB to a IIC+.

If it goes beyond minor changes and goes to full rework, that's not practical.


But with really good clear photos of a IIC+ board, I should be able to dupe the trace pattern well enough to be able to cook up my own board, which would go into my SOB chassis. (While retaining the original SOB boards and parts for full restorability to stock.)

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Thu Aug 02, 2018 6:54 am

I wanted to know just how much might be involved if I were to consider reworking a IIB to a IIC+.
A lot! Main thing though is to replace the preamp board or make one up and get hold of a load of those optocouplers used (LDRs as they're also called). They are double sided PCBs so unless you get pics of one with components removed or both sides you're not going to be able to recreate it identically. I doubt anyone has that info out there! Not only that there's loads of wires going everywhere on the IIC+ and is a real pain to figure out. It's possible though I guess but you'd ideally need access to a IIC+ and take it apart (good luck finding someone who will let you do that!)

Regarding the SOB? You'd also need to get a PWR- 7-C1 or PWR -7D power board for that depending if 100W/Simul or 60W. On top of that I don't even know if the transformer is correct... could be possible though. You'd also need to source all the Push/Pull pots.

I just posted on another thread about finishing off my Mark III to IIC+ conversion. That's your best bet! Trust me. A LOT easier.

Fair play if you can cook up PCB boards. Have you seen the sloclones schematic? The PCB they drew up is completely off but the schematic is basically correct apart from Reverb and Lead share half a 12AX7 each of V3 and V4. Probably the best place to start!

I'll ask permission and see if I can send you over the IIC+ board pics a member on here sent me.

Cheers,

Jon

woodbutcher65
Mark III
Posts: 170
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Re: Photo request: Mark IIC+ preamp circuit board

Post by woodbutcher65 » Thu Aug 02, 2018 2:33 pm

Two people have been kind enough to send me photos so far, and it appears that they're slightly different board variations, which makes it interesting!

I SUSPECTED that the board is double sided but wasn't sure. Well, even that's not a problem because I'm not exactly inexperienced in electronics. I've been repairing everything you can break for the last 30 odd years.

If I have only the component side layout and components, I can figure out what has to be on the back side, with some thought.


The harder part is starting from a photo of a populated board and creating trace artwork from it. With components in the way, and obscuring some important details.

woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Re: Photo request: Mark IIC+ preamp circuit board

Post by woodbutcher65 » Sun Nov 04, 2018 12:05 am

I'm still working on this in what spare time I have available. It has been more of a pain than I might have hoped.


I swear, if I had a real Mark IIC+ here, I'd use extreme care but I'd strip the preamp board to a bare board, photocopy it, have masters made from it, and then rebuild it using all the original components in their original locations, done with the utmost care.

I'd do this without hesistation if I had my own IIC+ to work with. I know my workmanship and attention to detail. Removing a part and reinstalling it just as it is, is the same as if it never happened.

If anything, refreshing the solder connections would only help it to work better.

If somebody who has one were willing to join up in the project with me and allow THAT level of reverse engineering to be done on his amp, well, first, I'd be greatly surprised, but second, he'd get full access to the repro boards I'd be making, OF COURSE.

I'd make exact RP11 and RP12 (either or both depending on what I could copy) replicas, but with just enough changes that you could tell it was not a true original board. Including my initials or a logo and a date.

So what would I do with the boards? Well, I have an SOB which is built in a Mark II era chassis. The SOB has never quite made it for me. So it'd be gutted, its internals carefully preserved, and a Mark IIC+ built in the chassis. Might have issues finding a 105 transformer but I think Mercury Magnetics makes a match for it.

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Wed Nov 07, 2018 5:05 am

woodbutcher65 wrote:I'm still working on this in what spare time I have available. It has been more of a pain than I might have hoped.
Could you PM me what you've got already and I'll send you what I've got too? I was looking at doing a similar thing (should emphasise NOT for profit or resale!) and got fairly far on an RP11 board from pictures and the endless (ongoing) hours I put into getting a decent schematic and board view (about 90% complete at the moment)

Still trying to get my head around some of it but I reckon with some joint effort and some guess work for the back of the board we could come up with something near enough identical.
Last edited by jrb32 on Mon Nov 26, 2018 6:08 am, edited 1 time in total.

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Wed Nov 14, 2018 5:38 am

I've just bought a 60W Mark IIC Combo (SRGX) which I'll be collecting this weekend.

I'm not sure if it's an RP-10A or RP-11 but they're both the same PCB boards which are used for the Mark IIC+. Either as a IIC to + conversion or a IIC+ stock board which was used in production with the + conversion already done to it.

Please do not think there is a simple and nice IIC+ PCB board and layout which just slots in. There was a whole load of fudge work done to do the + conversion to the stock IIC boards plus how it wires to the pots, reverb and the jack connectors. This fudging was basically done on the production line in the factory but there's not a separate IIC+ PCB board or layout or anything like that. It's all a bit of a spaghetti mess.

If you're still interested I'll send you a blank board PCB layout once I trace mine including both sides and the schematics for the IIC and IIC+ but you still need to get your head around these amps fully (I'm not there yet either) if you actually want to replicate this.

You'd need to source the relevant push/pull pots, LDRs and some other vintage components if you're looking to do a IIB to C+ conversion but it is possible with a cloned RP-10A/11A board (or SP-11A for non reverb) blank board if you can get your head around the spaghetti mess. You could probably take most of these components from a IIB board but it's sacrilege!! Plus you need the LDRs. Basically don't do it! Build your own from scratch.

As you said
If it goes beyond minor changes and goes to full rework, that's not practical.


This is definitely on full on rework, even if you have a blank board! So it's not practical, no, but if you're still interested let me know.

woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Re: Photo request: Mark IIC+ preamp circuit board

Post by woodbutcher65 » Wed Nov 14, 2018 11:14 am

Yeah, I'm still interested. I sent you a PM with my email address.

I'm not planning to rework a IIB, I'm planning to take out the preamp board of my SOB head and preserve it intact, and reuse the chassis and build a whole new Mark IIC+ circuit inside it.

I'm fully capable of scratch building an amplifier. When working with the SOB chassis I'll already have the transformers and output stage. Just have to build the power supply and preamp boards, and adjust voltages to be what they need to be by changing power supply dropping resistors as needed. I'll be working with different transformers than a IIC but that's OK.

Every part needed is available, including the LDRs. Don't have to salvage anything for this build.

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Wed Nov 14, 2018 11:31 am

That's a shame in a way because a IIB upgraded to a C+ is something that has been done before by Mike B and supposedly sounds incredible. I don't think anyone has ever done an SOB to Mark IIC+ conversion so I'm a bit skeptical, but if you reckon the transformers are powerful enough fair play!

I'll take out the power board and preamp board and send you pictures of both sides of those along with any other info I can get once I pick up the amp at the weekend.

Don't forget the push/pull pots! ;)

woodbutcher65
Mark III
Posts: 170
Joined: Fri May 04, 2018 10:36 pm

Re: Photo request: Mark IIC+ preamp circuit board

Post by woodbutcher65 » Wed Nov 14, 2018 6:13 pm

Well, I'll have a 60 watt amp when I'm done. The SOB is a 60. So I may end up missing a little bit of the tone due to not having the right transformers but I can live with that.

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Fri Nov 16, 2018 4:17 am

Sounds good! What's the marking on the transformers you have currently?

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Mon Nov 19, 2018 9:13 am

I just got my 60W IIC. RP10 board. All original and has some maintenance to do but sounds great. I'll definitely pull the board for you and take pictures of both sides, removing the larger orange drops so you can see the traces and see the component layout. Guessing a photocopy would be good too?

I'll trace it as well and get you schematics of both with a board layout. From there you'll hopefully be able to work out the C+ conversion.

I'm tempted to leave it stock but I might work out the C+ conversion based on comparing the two, perform it and see which I prefer. I'd send it to Mike B but international shipping from the UK is crazy.

Hope that helps! Sorry it's not an RP11 which is apparently easier to do the + conversion but should get you where you need to go.

annaindat
Bottle Rocket
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Re: Photo request: Mark IIC+ preamp circuit board

Post by annaindat » Tue Nov 20, 2018 2:44 am

I believe Every part needed is available, including the LDRs. Don't have to salvage anything for this build.
Ochedin Valanor

jrb32
Mark III
Posts: 190
Joined: Wed Jan 31, 2018 8:32 am

Re: Photo request: Mark IIC+ preamp circuit board

Post by jrb32 » Fri Nov 23, 2018 6:51 am

I've started on a board layout and schematic for the IIC RP-10A. Should be finished this week or so. I've also been sent some great photos of a IIC+ with RP10-A so will be using those to do the + mod myself as the UK mesa distributor will not offer this or offer shipping to the mesa factory for my amp to have this done by Mike B. Shipping costs would be crazy plus it would be nice to document for myself the steps involved and the variants in the C->C+ amps vs the factory C+ amps (the two are similar but have some very different component values in places)

As mentioned before, the RP-11A and RP-10A boards were both used for the IIC. The C+ mod involves some SERIOUS board modding to either, with wires going off in crazy directions, but once I have the info up it should give you all the info you need to build an RP-10A with + mod.
I believe Every part needed is available, including the LDRs. Don't have to salvage anything for this build.
To get it authentic you will need to go hunting for some vintage components for the cathode caps and lead circuit (kemet yellow bullet, kemet sealed tantalum etc) and get hold of some decent VTL5C1/VTL5C4s. The rest of the components should be easy to source.
Last edited by jrb32 on Mon Nov 26, 2018 6:13 am, edited 2 times in total.

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