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 Post subject: Mark III schematic
PostPosted: Mon Feb 12, 2018 4:31 am 
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Mark II

Joined: Wed Jan 31, 2018 8:32 am
Posts: 66
I started tracing my Early 86 Black Stripe over the weekend as I feel it would be good to have a verified III schematic out there if anyone is interested. It'll be as a TinyCAD file and I would love for people to copy it and use it as a basis for getting some verified schematics of the different stripe versions for diagnostics/repair/mods.

I'm half way through and so far the original mesa schematic I can find is has a lot of mistakes (no surprises there) - but I just wanted to check one major thing:

On a lot of the disc capacitors they are labelled, for example, 250K, 750K etc . If you type this into a capacitor calculator it comes out as 25pF, 75pF and 50pF at 10%. However, on the schematic these are listed as 250pF and 750pF, for example in the tone stack.

250pF 10% would be code 251K so I just wanted to check if there wasn't some different naming scheme in the 80s when it came to this, as every schematic I can find always lists these as 250pF, 750pF and 500pF so I'm probably wrong here but just wanted to double check as another disc capacitor for the pull bright is labelled as 181K 1KV which is using the correct code for 180pF which roughly matches the 100pF listed on the original schematic.

Cheers,

Jon


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 Post subject: Re: Mark III schematic
PostPosted: Mon Feb 12, 2018 9:31 am 
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Mark II

Joined: Wed Jan 31, 2018 8:32 am
Posts: 66
So I've learnt not to trust the codes on caps! Especially when I know that Mesa sourced parts from various places. I've got my head around it and there are two main naming conventions on the caps if anyone else decides to go through and trace theirs:

Writing the value of the cap in full plus it's tolerance (e.g) 750K would be 750pF 10%, so for the same cap style 75K would be 75pF 10% (confusing!)
Using the correct cap code e.g 181K would be 180pF 10%, 180K would be 18pF 10% https://www.electronics2000.co.uk/calc/ ... ulator.php

Something to be wary of when tracing these things! Anyway I'll continue on my schematic.

Thanks,

Jon


Last edited by jrb32 on Tue Apr 10, 2018 4:55 am, edited 1 time in total.

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 Post subject: Re: Mark III schematic
PostPosted: Wed Feb 14, 2018 4:14 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
Posts: 104
Location: Europe
Jon,

Many thanks for sharing!

The lead circuit is quite similar to my purple stripe 60 Watt, except that

- the resistor in parallel with the lead drive pot is 470k instead of 150k, which would provide for a bit more bass at higher settings. I think the 150k is actually beneficial as it tames bass before the overdrive. It also cuts the available overdrive a bit but the amp has plenty anyway.
- the 120pf cap in parallel of the 470k resistor (either going to ground or the cathode of V3a) is missing - that would make the amp a lot brighter. 47pf is a good compromise.
- C502 is a 0.022uf in my amp and all other schematics I have seen. 0.047uf would give more bass - not a good thing at this spot IMHO.
- R201 is a 3k3 in my amp and all other schematics I have seen. 1k5 would give more gain - not really necessary.
- C30 was also 0.002uf in my amp. I changed it to 0.001uf as in the Mark II C+, which made the amp a bit brighter and more open.

Cheers Stephan


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 Post subject: Re: Mark III schematic
PostPosted: Fri Feb 16, 2018 5:14 am 
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Mark II

Joined: Wed Jan 31, 2018 8:32 am
Posts: 66
Hi Stephan,

That's fantastic thanks! I've finished most of the schematic now for my black stripe - preamp, lead circuit, reverb and the V5 circuit. I have not verified the bias supply, EQ, LDR switching or output valves but it's a start at least and mainly I wanted to get this done for comparison reasons. Anything with an RXXX or CXXX number has been verified by me and should be correct (although I might have made a typing error still!)

I also have no idea yet how the R2 circuit relay works so that is a bit blank at the moment. I've traced what I can for the moment for the R2 circuit but will do this fully soon.

Cheers,

Jon


Last edited by jrb32 on Tue Apr 10, 2018 5:54 am, edited 8 times in total.

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 Post subject: Re: Mark III schematic
PostPosted: Fri Feb 23, 2018 4:24 am 
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Mark II

Joined: Fri Jul 28, 2006 5:58 pm
Posts: 104
Location: Europe
Hi Jon,

Thanks for your great work. Can you please upload one schematic as standard pdf-file? I have difficulties downloading it.

Thanks,
Stephan


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 Post subject: Re: Mark III schematic
PostPosted: Fri Feb 23, 2018 4:37 am 
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Mark II

Joined: Wed Jan 31, 2018 8:32 am
Posts: 66
It'd be great if you could tell me what the other main differences between mine and your purple stripe are. That way I can get a schematic drawn up for the purple stripe pretty easily :)

Many thanks!

Jon


Last edited by jrb32 on Wed Apr 11, 2018 7:14 am, edited 1 time in total.

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PostPosted: Tue Apr 10, 2018 5:53 am 
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Mark II

Joined: Wed Jan 31, 2018 8:32 am
Posts: 66
UPDATE

Full III Black Stripe schematic for you guys! This has taken ages sorry for the delay.

(well about 95% done technically! Some final things to finish off and double check) Everything with a R, C or D, LDR etc number has been verified and traced by me. This includes LDR supply, EQ etc and should give you an idea about how things work exactly! I also finally understand how the R2 circuit works! Hooray!

I have some questions regarding a few weird things that don't make sense to me (mainly why do people use R130 instead of R109 for the R2 mod and why R133 and C28 exist, but will start a new forum about this! - Link is here: https://forum.grailtone.com/viewtopic.php?f=27&t=75514)

It finally gives a good understanding of how the R2 circuit works.

So, if you're reading this and have measured your III Purple/Red/Blue/Green stripe PLEASE let me know where any values differ! Or send me high res pics of your board and I'll do the rest! That way I can colour code component values onto this for all the different stripe versions

Then we can get a nice single schematic that shows pretty much all the differences between the stripe versions. This is similar to what Lions has been doing on here so full credit to him for this idea.

Image
Image
Image
Image
Image
Image
Image

PDF Link to Board Layout: https://drive.google.com/open?id=1CTJOjS-HTaZDfzMj0W_6t50_XyNOSSJ6
PDF Link to schematic https://drive.google.com/open?id=1HwhczDup8EDBPKyCUK4CV19XKAcg0aeT

Thanks,

Jon


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 Post subject: Re: Mark III schematic
PostPosted: Fri May 18, 2018 6:16 pm 
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Bottle Rocket

Joined: Wed Jun 15, 2016 1:18 am
Posts: 11
Amazing work. Thankyou.

_________________
Blue Stripe MKIII head
MK IVb long head
Vox AC30TB
Mesa cabs

Gibson, Fender, PRS and vintage EHX.


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