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

Joined: Wed Jan 31, 2018 8:32 am
Posts: 9
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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Bottle Rocket

Joined: Wed Jan 31, 2018 8:32 am
Posts: 9
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. Here's the lead circuit for a Black Stripe I've verified with my 100W Non Simulclass 01/86

https://drive.google.com/open?id=1MyTIA ... 9wkysID4aG

I should get the full version up by the end of the week.

Thanks,

Jon


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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: 92
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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Bottle Rocket

Joined: Wed Jan 31, 2018 8:32 am
Posts: 9
Hi Stephen,

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. Please someone better in the know feel free to edit this.

I'm finishing off a IIC+ version from information from the sloclone forums where one was 'verified'. I'm can't be sure on the accuracy of that though. Will upload when done.

Because it's a TinyCad file feel free to download and make changes for the different stripe versions if you have one and can verify it. I'll also upload some stripe revision versions of this schematic from info I have found out there but again I can't be sure of the accuracy of them.

Black Stripe

TinyCad file: https://drive.google.com/open?id=1rEpW8 ... vvr6HTNR8W
PDF: https://drive.google.com/open?id=1CpGCp ... V9yptRwIXh


Cheers,

Jon


Last edited by jrb32 on Sat Feb 17, 2018 2:54 pm, edited 3 times in total.

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 Post subject: Re: Mark III schematic
PostPosted: Fri Feb 16, 2018 5:34 am 
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Bottle Rocket

Joined: Wed Jan 31, 2018 8:32 am
Posts: 9
Mark IIC+ redrawn from sloclones 'verified' version. Reverb not verified and could have made a couple of mistakes copying it. If anyone out there could verify if this is correct that'd be great as I'm suspect of a few things!

TinyCad file: https://drive.google.com/open?id=1_l0uX ... JQJ2oEwvVL
PDF: https://drive.google.com/open?id=1gQhK0 ... TyLlHR93f3


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