Mesa Boogie MK2C+ EQ Transistor question?

[msnooks]

Hi there,

I have been looking into the mk2c+ eq among the slight differences between other mesa models that use the 5 band graphic eq
which has left me a tad confused.

I understand that the supply to the EQ is negative and tapped off the bias supply which seems to be anything between -30v to -67v.
The schematic i have been looking at is the sloclone version which has a -67v ac supply. http://milas.spb.ru/~kmg/files/schematics/Mesa Boogie/Mark II-C+/Mk2C_(SLOClone_ver).pdf

I have tested a similar power supply setup at 25v ac and rectified to negative which brings the supply up to -42v let alone use a 67v ac and rectify that.

The schematic says the supply should be -30 to -40v dc to the transistors (op amp config circuit), yet how could it possibly run at -40v when the mpsa20 & mpsa70 are rated at -40v Collector emitter voltage and the mpsa63 is rated at -30v collecter emitter voltage?

is this one of their mistakes in the schematic? surely the mpsa63 start heating up and fail at -40v and the mpsa20 and 70 would maybe scrape through at -40v?

can anyone enlighten me on the subject?

 

[Henz]

Thanks for the link to the schematic. The one I've been using has a few errors. Anyway, I think the EQ isn't taking from the bias supply. It's tapped off the 220ohm on power board. On my C+, there's a white lead going to EQ board which mine reads - 60v ish.

 

[Firefight]

Here's a better print...http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/mk2c2.gif

http://guiguijones.free.fr/Electronique%20&%20Bouine/electronique/Schema%20Amplis%20Effets%20etc/Amplis/Mesa%20Boogie/mk2cpwr.gif

 

[Henz]

Here's another one. But as I mentioned , there are a few errors when I traced my C + with it. Between this one and Slocones, there are some inconsistencies. Slocones PI circuit is pretty accurate. https://drive.google.com/file/d/1ieTnMLeSiNDoXu_9Or0ky2RIDwbLAcUQ/view

 

[jrb32]

Hey Henz sorry I've not been here for a while! Could you let me know the differences between the sloclones and the other schematic so I can make changes as necessary.

Regarding the negative supply I don't have a multimeter to check mine right now but this is from another post:

http://forum.grailtone.com/viewtopic.php?f=33&t=39690#p288351

EQ Supply
----------------
Mark V: +24V
Mark IV: -30V
Mark III: -42V
Mark IIC+: -42V
Mark IIB: -47V
Mark IA: -37V
-----------------

So, what I actually find surprising is that this is the first boogie to use a positive voltage supply instead of a negative voltage supply for the GEQ. This means that they had to swap all of their transistors from PNP to NPN and vice versa. To me, that's really interesting. The GEQ has been one of the most stable portions of the design since the Mark series began.

 

[Henz]

Nice to hear from ya! I may have been a bit hasty when saying a few inconsistencies. I just now looked at the Slocone version and saw in lower right that it is for a C+ DSN. Not sure what s n means. Anyway ,the D is for Simul Class and in my SC, there's a diode on pin 3 of inner sockets going to ground. Slocone's doesn't show it. And the Final Version has them but on pin 4 to cathode. Probably not a big deal. Regarding my C+, Slocone's version is consistent with my amps tracing to where the .1 400 or 600v cap is connected to the 1.5k. The Final Version has it connected to the other side of the 1.5k. This may or may not have anything to do with tone or functionality.let me know if it does. You know much more about this amp than me. And as far as the V2 A and B, some are reversed on some schematics. It's up to the individual to figure that one out lol. As far as my EQ measurement, im not sure if I took the reading in the correct spot. I took it from the lead off the 220 on power board to the EQ board off the 470. It read -60 +/- a few. All of my voltages in my amp are running high for some reason. I hope you are doing well. Get back with me what you find out. Blessings, Jim.

 

[Henz]

I forgot to mention something that kinda bothers me. Why didn't Mike and Doug build all of the C+'s the same? I've seen picts of Mallory's in the tone stack, some Sprague. All throughout amps. I remember reading Boogie Babies saying that no two C+'s sound the same due to this. That bothers me. Let's say hypothetically, I went down the street to hear my buddy's C+ and loved it. So I go and buy one only to be disappointed. I maybe splitting hairs here but I'm a perfectionist with a little OCD mixed in. Just had to vent. lol. Later brother. Jim

 

[Dan__O]

Nice to hear from ya! I may have been a bit hasty when saying a few inconsistencies. I just now looked at the Slocone version and saw in lower right that it is for a C+ DSN. Not sure what s n means.

it's just the file extension! (*.DSN).
There's a new version of my SLOCLONE with much more colour added (and a bunch of fixes).

there's a diode on pin 3 of inner sockets going to ground. Slocone's doesn't show it.

Yeah, that's the flyback diode, doesn't affect tone. It's just for protecting the output transformer (to some extent) if the amp is played without a speaker attached.

Regarding my C+, Slocone's version is consistent with my amps tracing to where the .1 400 or 600v cap is connected to the 1.5k. The Final Version has it connected to the other side of the 1.5k. This may or may not have anything to do with tone or functionality.

This shouldn't make any difference, series impedances can be in any order. I think the 'FINAL' version has some funny values in the Phase inverter...

as far as the V2 A and B, some are reversed on some schematics. It's up to the individual to figure that one out lol.

Yeah, mine was wrong, a bunch of the valves weren't labeled correctly - Should be fixed now! Lemmie know if you have any other issues with it...

http://sites.google.com/site/danoampstuff/mesa/Mesa_MK2C+_SLOCLONE_v2.pdf

Dan.

 

[Henz]

Thanks for the reply and info and link! I'll give it a good once over! Blessings, Jim

 

[Henz]

Thanks for the schematic. A short while ago I had a chat with Mike B. about my C+ DRG. I was asking him why my amp still had C21 , 1000pf, and C10 was a 10 pf while the schematics said otherwise. He told me that those caps are a bright reduction mod in earlier C+'s. Mine still has C25, 500pf, but didn't ask him about that. I see that you have a 100k for Master 1. I sent my amp for repair in the mid 90's. Mike put a 250k in. I've been having signal issues with it few months ago so I contacted Kris at Mesa. He sent me a 1 meg. It was the ones that cut in real sharp but I didn't know it till I installed it. I definitely did not like it and I didn't want to do that mod to fix that (R 17 ). So I removed it and took the pot apart to see why. The resistance plate had no trace going all of the way to lug. Why don't they find better pots? So I just used a couple of the new pot parts and rebuilt my old one. Seems ok. Let me ask you something that I can't get any answers from. On the Final schematic there's a 120pf cap on V2A. It says it was removed. Mine still has it. Do you know what it's for? One more thing, if I may, Caps 2 and 3 on your schematic, I was thinking about trying silver micas there. What's your opinion? Thanks again for the schematic. Hope to hear from you. Jim