madbeanpedals::forum

Projects => Build Reports => Topic started by: marauder on September 10, 2013, 03:31:45 AM

Title: Mesa JCM800 restoration
Post by: marauder on September 10, 2013, 03:31:45 AM
Last time Phil came over from Hoboken to the homeland, he brought the last of his hurricane Sandy flood damaged pedals with him, a Mesa V-Twin.  This one is beyond repair :(  Corroded component legs, and pads, it would need everything pulling, and it's a nightmare to get the oxidised solder to melt,  only a pair of vactrols have been salvaged.  The board is still lying around in the corner somewhere , so if anyone wants it for the cost of shipping let me know , but I wouldn't bother to be honest.

Luckily the case is mostly stainless steel and survived ok, but the tread plate panel has suffered pitting corrosion, so has been painted.

Anyway,  the guts are now a JCM800 preamp circuit, with the second stomp acting as the lo and high input on the real amp.  The slider switch on the top switches a parallel 10k resistor on one of the existing 10k cathode resistors to give a little volume boost.

Phil wasn't bothered about some of the text being rubbed off, and said leave it as is, otherwise Haberdasher would have  been called upon to etch a face plate.

Somehow the Mesa 12AX7s survived with no ill effects, and are working well at the 190V the vero SMPS is supplying,  the best sounding tube pedal built by me or John.

Not good.
(http://marauder.dtdns.net/vtwin/dscf4627.jpg)

(http://marauder.dtdns.net/vtwin/dscf4628.jpg)

Lots of wiring.
(http://marauder.dtdns.net/vtwin/dscf4623.jpg)

(http://marauder.dtdns.net/vtwin/dscf4619.jpg)

JCM800 preamp.
(http://marauder.dtdns.net/vtwin/dscf4621.jpg)

9V in, 190V out.
(http://marauder.dtdns.net/vtwin/dscf4624.jpg)

A little repainting.
(http://marauder.dtdns.net/vtwin/dscf4637.jpg)

Title: Re: Mesa JCM800 restoration
Post by: LaceSensor on September 10, 2013, 04:17:04 AM
fackin bad ass man
Title: Re: Mesa JCM800 restoration
Post by: kothoma on September 10, 2013, 04:19:42 AM
Ouch, it really hurts looking at the first two photos.
It's a wonder the tubes survived.
What a great restoration job!
Title: Re: Mesa JCM800 restoration
Post by: juansolo on September 10, 2013, 04:29:11 AM
It does sound utterly spectacular. It murders the Twin pedal we did before. I think it's a combination of the extra voltage and using 12AX7s (which really don't work well at lower voltages). It really is a Marshall in a box now. Obv, dangerous voltages inside this one so don't undertake it lightly.
Title: Re: Mesa JCM800 restoration
Post by: lincolnic on September 10, 2013, 10:04:19 PM
Is it bad that I see "9V in, 190V out" and I just want to laugh?

Terrific work, guys.
Title: Re: Mesa JCM800 restoration
Post by: Cortexturizer on September 11, 2013, 01:31:26 AM
when I saw 9V in 190V out I almost fell from my chair haha, AWESOME
pedal looks fantastic inside out, I don't have to hear it in action, I know it sounds like a dreamy marshall...
Title: Re: Mesa JCM800 restoration
Post by: Darkknigh_t on September 11, 2013, 05:27:01 AM
It looks amazing great job i love peoole who restore things :)
Title: Re: Mesa JCM800 restoration
Post by: pickdropper on September 11, 2013, 06:04:27 AM
Amazing restoration job, guys.  Very sharp.
Title: Re: Mesa JCM800 restoration
Post by: peterc on September 11, 2013, 12:08:33 PM
Brilliant job. I like the 'wolf in sheep's clothing' idea.

How did you do the 12v - 190v conversion?

Peter
Title: Re: Mesa JCM800 restoration
Post by: culturejam on September 11, 2013, 01:10:58 PM
Whoa. That is great work, man.
Title: Re: Mesa JCM800 restoration
Post by: catfud on September 11, 2013, 02:26:59 PM
Man, that's a great restoration, and I'm sure it sounds great in its new guise, but still, a damn shame about the Mesa V-Twin, that's also a lovely sounding pedal itself. I've seen the schematics for the Mesa V-Twin published somewhere too - and the circuit looked too complex for me to consider, but I wondered whether someone somewhere has done a DIY job using that schematic...
Title: Re: Mesa JCM800 restoration
Post by: jkokura on September 11, 2013, 03:15:48 PM
Now that is rad. I love repurposed stuff.

Jacob
Title: Re: Mesa JCM800 restoration
Post by: billstein on September 11, 2013, 04:39:37 PM
Top notch as usual.
Title: Re: Mesa JCM800 restoration
Post by: juansolo on September 12, 2013, 12:39:40 AM
Quote from: peterc on September 11, 2013, 12:08:33 PM
Brilliant job. I like the 'wolf in sheep's clothing' idea.

How did you do the 12v - 190v conversion?

Peter

Using Rick Holt's 555 SMPS (http://www.diystompboxes.com/smfforum/index.php?topic=74088.120 Page 6-7). Though Cleggy has used a bigger inductor. Tried a Nixie PSU and it created a lot of noise for some reason. This one is not ideal on vero, but seems to work fine. It's been given a good long test.
Title: Re: Mesa JCM800 restoration
Post by: hoodoo on September 12, 2013, 01:46:19 AM
Geez, i wish You 2 were my mates that fixed my broken shit and made it better than new, awesome as always  :)
Title: Re: Mesa JCM800 restoration
Post by: soldersqueeze on September 12, 2013, 02:55:34 AM
Damn, this is epic! Really great work as usual, guys.

Quote from: hoodoo on September 12, 2013, 01:46:19 AM
Geez, i wish You 2 were my mates that fixed my broken shit and made it better than new, awesome as always  :)

+1
Title: Re: Mesa JCM800 restoration
Post by: peterc on September 12, 2013, 11:12:47 AM
QuoteUsing Rick Holt's 555 SMPS (http://www.diystompboxes.com/smfforum/index.php?topic=74088.120 Page 6-7). Though Cleggy has used a bigger inductor. Tried a Nixie PSU and it created a lot of noise for some reason. This one is not ideal on vero, but seems to work fine. It's been given a good long test.
Quote

Thanks for that.
Peter
Title: Re: Mesa JCM800 restoration
Post by: marauder on September 12, 2013, 11:42:14 AM
This is the original article about the PSU
http://www.ledsales.com.au/kits/nixie_supply.pdf

Reading the DIYSB thread it seems that a 150uH inductor is better than the original 100uH, mine is a 120uH, 2.5A rated one, it doesnt get hot, but the mosfet does and needs a heatsink.

The vero layout is slightly wrong, the 2.2k and 100pf are on the wrong side of the UF4004 diode.


Title: Re: Mesa JCM800 restoration
Post by: gtangas on September 12, 2013, 01:42:19 PM
Hey...

I have one vtwin open in my table with a stupid problem
.maybe you can help me

The previous owner send it to repair but on the OD channel the output is to weak.you can ear it but...

From the look of the PCB only the vActrols was changed y new ones. Can I jumper the ldr side of the vactrol just to check if the problem is there?

Thx
Sérgio


Title: Re: Mesa JCM800 restoration
Post by: marauder on September 12, 2013, 02:02:02 PM
Should work jumping the resistor side of the vactrols.  What vactols are they?  This one had VTL5C9s.

Here's a schematic.
http://tubefreak.com/v-twinp.gif
Title: Re: Mesa JCM800 restoration
Post by: gtangas on September 13, 2013, 03:49:16 AM
i will post some pics later
Title: Re: Mesa JCM800 restoration
Post by: gtangas on September 14, 2013, 05:20:37 AM
(http://i775.photobucket.com/albums/yy38/gtangas/20130914_130439_zps169b6915.jpg)

(http://i775.photobucket.com/albums/yy38/gtangas/20130914_130446_zps4bf01a0f.jpg)

(http://i775.photobucket.com/albums/yy38/gtangas/20130914_130512_zps47a5a520.jpg)

I think this one was in time moded...
Title: Re: Mesa JCM800 restoration
Post by: marauder on September 16, 2013, 02:59:59 AM
No idea what's going on there.

The jfet is part of the bypass switching.  Is the cap connected between the jfet and pot?

Title: Re: Mesa JCM800 restoration
Post by: gtangas on September 16, 2013, 09:22:42 AM
Yes it's connected to the pot...
The problem with this units is low gain
The tubes are new and the vactrols ware replaced by a guy

Do you think the problem could be on that cap?
Title: Re: Mesa JCM800 restoration
Post by: Vallhagen on September 17, 2013, 12:04:23 PM
Fantastic and ambitious job! Im impressed!

Title: Re: Mesa JCM800 restoration
Post by: masterlk on September 17, 2013, 12:51:39 PM
Yeah, what he said!


Quote from: LaceSensor on September 10, 2013, 04:17:04 AM
fackin bad ass man