Mark IIA (1979) circuit deviations; experience?

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iefes

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Hi guys,

I've been reading quite a bit on the forum which has been a great resource so far but I have not yet been actively contributing. So: Here we go!

I have just bought a used Mesa Mark IIA Combo (1979, Export) with GEQ, Reverb and the EV12L speaker. Got it relatively cheap here in Germany (approx 650$) but it had some issues which I'm resolving one after the other. It already sounds incredibly nice and is freakin' loud.

Well, I'm quite interested in the technical aspects and have looked at the schematics of all the Mark II series amps and traced out the circuit of my particular amp. Doing this I noticed that the amp is essentially the actual IIA circuit but there are some resistor values that are slightly off the values given in the schematic. However, I'm pretty sure these resistors are no later modification but have been used already by the factory as they are all the same type as most of the other ones (those red ones, supposedly half-watt rating) and have the same amount of dirt etc on them.

Now I am wondering if this is something that is common to those old Mesa amps...? Did they just use values that were close to the ones needed when they ran out of supply? Or was it done on purpose? I looked up gutshots of other IIAs and couldn't find many. But in those pictures I found most values are more to spec than in my amp even though still not all of them match the schematic.

Some of the resistors I'm talking about are:
Tonestack slope resistor is 82k instead of 100k
plate R of V1B is 62k instead of 82k (also saw 100k on a picture online)
PI tail resistor 10k instead of 22k
resistor between V1B and Lead Drive pot 220k instead of 56k (transition towards MK IIB specs?)
Cathode R of V2B 220k instead of 150k

Reverb Pot is wired with Signal input on the wiper and signal output on pin 3. I suppose that's been done in order to not change the volume in Rhythm mode upon using the reverb pot (pot resistance to ground forms a voltage divider with that 2M2 resistor). Have seen this on pictures of other Mark IIs as well, but not on all.

Additionally there were some other changes which I can tell have been done later and likely not by Mesa. These included some modern blue metal film resistors and many ceramic caps across resistors (both to increase and attenuate treble content). I removed these already and replaced them with schematic-correct values.

So my question is if somebody can confirm that the MkIIA schematic is actually correct, so that I can convert my model back to stock specs? I would be highly interested in your experience regarding this aspect of the early Marks. If somebody would be able to provide some pictures of the guts of another MkIIA I would highly appreciate it. Would love to see if other amps are all to spec or if they have used different values back then.

I'm also wondering if the Reverb circuitry of the Mark IIB is an improvement compared to the Mark IIA...? I like the sound of the Reverb, but unfortunately the noise floor is increasing with increasing amount of Reverb. In the MkIIB circuit it looks like the Reverb pan is driven harder compared to the IIA, allowing to keep the Reverb pot lower and therefore lowering the noise. Is this true?

Well, I'd be happy to read a lot about your experiences! I've already read some of the famous Mark IIA topics here but couldn't really find information on what I am looking for.

Thanks a lot! :)
 
My 2/79 IIa "SSR" is also different from the schematic. I bought this in '81 from the original owner.
60 watts, reverb. No graphic eq. White tolex.

Both plate resistors on V1 are 62K instead of the listed 82K.

The 1M resistor from the grid of v1a to ground is 200K.

The voltages on the "TCR"s were more like 1.3V instead of 3.5V.

There may be other differences, but I don't have it open right now.

Quite a few years ago I disabled the gain boost and used the switch for a different cap in the lead circuit. Tightens up the bottom nicely.
I also recapped it a few years ago. It developed a problem where it would start to get "blatty" and quieter and quieter, untl no sound at all unless I hit the strings really really, and then would just give a little burp. The hard thing is, it *never* has the problem when opened up on the bench. Recapping it didn't fix it. New tubes same story. Part of the problem is a mouse crawled in and peed all over everything and nibbled on the heater wiring! I've cleaned it, but there may be some "contamination" somewhere that is shorting out or something.

Just this past weekend I replaced the TCR's with 1.5K resistors and pulled out the fetron to see if that made a difference.
Again, won't fail with the chassis out. Did fail once with it in for about a minute, but then it came back.

Next time I open it up I'll check and see if anything else is different.

-Dale
 
So here are some other changes. Serial 3390. PC board is RP-6A.

  • Both v1 plate resistors are 62K instead of 82K
  • v1a grid resistor is 220K instead of 1M
  • The 33K resistor by the 12ax7/fetron switch is 47K
  • The slope resistor seems to be unchanged at 100K
  • The 56K between v1b and the lead drive pot is 220K
  • The 47K between the lead drive pot and the relay B is 220K
  • The lead drive pot has a "250K" (250pF ?) between the wiper and ground. Like a reverse treble bleed?
  • The .02 cap between the "56K" and lead drive pot is .01,
    However, I changed that to go through the gain boost switch with a 470pF in parallel.
    So switch in (closed) is .01||470p, switch out (open) is 470pF.
    Allows me to have the bass control above 1 or 2.
  • The 180p bright cap is "250K" (250pF?)
  • v3a and v3b are swapped.
  • The gain boost jack was not wired up. Pretty sure the .005 cap wasn't there.

The reverb pot seems to be wired like the diagram, but the non-grounded leg is also wired to the reverb foot switch jack,

I didn't examine the reverb driver or recovery amp. Or the power amp.
 
Well I was reading this: https://mesa-boogie.imgix.net/media/User%20Manuals/Mark%20I%20Maint-Repair.pdf
Specifically about the fetron:

We use a special type and we select and install load resistors to fine tune each Fetron.
and
If an oscilloscope is available and the Fetron shows a lopsided wave form or
uneven clipping, it can sometimes be corrected by changing the value of the plate load
resistor. Values should fall in the range of 33K to 100K.

So that explains my 62K plate resistors on v1!
 

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