Photo request: Mark IIC+ preamp circuit board

[woodbutcher65]

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]

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]

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]

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]

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]

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]

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.

 

[jrb32]

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]

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]

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]

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]

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

 

[jrb32]

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.

 

[jrb32]

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.

 

[woodbutcher65]

Don't even think about putting the tantalum caps in. There is no such thing as a tantalum capacitor that sounds good and when they go bad they become hard shorts.

Put in electrolytics instead, best of all, non-polarized ones.

Mesa uses those awful tants only because they are cheap and they offer a large amount of capacitance in a small package. They're still hot garbage.

 

[jrb32]

All IIC+s have military sealed tantalums on the cathode caps and an important .22uF one in the lead circuit that should be a T110. Are you really saying IIC+s sound bad and you would rip these out of a IIC+ ever came into your workshop and replace them?

I'm going for authenticity here. So would Mesa if they still had RP10/RP11 boards available. Stick with the original parts if you want it authentic. If I bought a IIC+ that had the wrong type of components I would correct it 100% without question regardless of how much of a POS tant caps are. The hermetically sealed ones seem to be fine I haven't seen any fail on me yet.

http://forum.grailtone.com/viewtopic.php?t=18617

All IIC+ cathode caps were tantalum or hermetically sealed tantalum. They never used electrolytic or ceramic disks.

http://forum.grailtone.com/viewtopic.php?f=9&t=49710

Replaced .22 cap on Pull Bright on Lead Master with Kemet T110 Hermetically Sealed

 

[woodbutcher65]

I'm saying that you can always do better than a tantalum capacitor as far as tone goes. I think you will improve the sound of even a IIC+ by replacing those tants with something better sounding.

 

[lovetoboogie]

Don't even think about putting the tantalum caps in. There is no such thing as a tantalum capacitor that sounds good and when they go bad they become hard shorts.

Put in electrolytics instead, best of all, non-polarized ones.

Mesa uses those awful tants only because they are cheap and they offer a large amount of capacitance in a small package. They're still hot garbage.

This statement kind of caught my attention...

As a musician, amp and guitar tech of 30+ years with 100's of scratch builds and a custom amp builder...I would never encourage a vintage Mesa Boogie owner to "rip out" all the tants because they are crap or because Mesa Boogie was "too cheap". Molded tantalums started showing up in the late Mark IIA models and have been a consistent in all Mesa Boogie models. Tantalums are super stable at below their rated voltage, they don't need to be replaced unless they short(which is almost nil in the cathode bypass), they are super compact and help the real estate of a populated Mesa preamp board. I can't speak for Randall Smith or Mike B but the mindset at Mesa has always been durability and using components that don't fail. The pedigree and originality of older Mark Series amps are at stake with bad info on the internet. It might be prudent not to further that cause....

I might add I mean no harm or judgement of your experience. It is admirable to do period-correct work on old Boogies so I try to promote that.

 

[jrb32]

I might add I mean no harm or judgement of your experience. It is admirable to do period-correct work on old Boogies so I try to promote that.

Definitely agree on this. If it was my own personal amp and I wanted to experiment and maintain these I'd be fine changing some caps but if I was ever to repair or maintain one for someone else or sell it on, definitely go for originality all the way!

I've finished a Mark IIC board view and schematic done from my SRGX '83 with RP10A board. Hopefully that will get you where you need to go.

IIC RP10A Schematic https://drive.google.com/open?id=1mpJFsu2gHA2gIiNmg6FGbC81rlggkt6M
IIC RP10A Board view: https://drive.google.com/open?id=1Lx0AofY9eI34mGY3RbgNDSV5IgEUf5cS

I'll do the same for the IIC+ RP10 once I do the upgrade.

 

[Authorized Boogie]

Don't even think about putting the tantalum caps in. There is no such thing as a tantalum capacitor that sounds good and when they go bad they become hard shorts.

Put in electrolytics instead, best of all, non-polarized ones.

Mesa uses those awful tants only because they are cheap and they offer a large amount of capacitance in a small package. They're still hot garbage.

This statement kind of caught my attention...

As a musician, amp and guitar tech of 30+ years with 100's of scratch builds and a custom amp builder...I would never encourage a vintage Mesa Boogie owner to "rip out" all the tants because they are crap or because Mesa Boogie was "too cheap". Molded tantalums started showing up in the late Mark IIA models and have been a consistent in all Mesa Boogie models. Tantalums are super stable at below their rated voltage, they don't need to be replaced unless they short(which is almost nil in the cathode bypass), they are super compact and help the real estate of a populated Mesa preamp board. I can't speak for Randall Smith or Mike B but the mindset at Mesa has always been durability and using components that don't fail. The pedigree and originality of older Mark Series amps are at stake with bad info on the internet. It might be prudent not to further that cause....

I might add I mean no harm or judgement of your experience. It is admirable to do period-correct work on old Boogies so I try to promote that.

+1!! Sadly, there is so much misinformation presented as the number of internet experts multiply!