Mark III ' Purple ' - possible blown R2 - UPDATED 24-04-2012

UPDATED AS AT APRIL 24 2012 - SEE POST #4 BELOW

Hi all - my first post here - GREAT forum.

I have a 1986 " Purple " Mark III 60w Rev and Graphic " Short Head "

I did the R2 mod - works perfeclty.

In reassembling the head, I picked up the wrong head-suspension screw [ too long ] - as you face the amp front-on, it is the screw on the front right-hand side.

Anyway, turned it on and that screw [ looks like ] it shorted out the diode direclty under it - white and audible burst of light - this is not good !!

Turned it off immedialty, waited and turned it back on.

- I have power
- Ch 1 seems to work fine

however

- when I pull out the pot on the front to manually go to R2, it makes a loud pop and cuts the signal gain / volume substatnitally

Can anyone please point me in the right direction to get my amp back up and fully running.

Is there a downloadable schematic anywhere that might be of use ?

All advice and help will be hugely and sincerely appreciated.

Gid

Does the lead channel sound messed up? Maybe you damaged one of the optocouplers?

Although not familiar with the specific layout and dimensions of a short head, the area U describe houses PSU components and possibly supply wires going to/from switches..
I'm concerned U shorted something, but then re-applied power before looking inside. But your description of a diode is also perplexing - unless U mean those on the PSU board??
Leave it off, take it to a tech before U risk any other damage to the amp, but more importantly to yourself..
Even though Ch1 seems to be OK, this doesn't mean there's not a potentially lethal fault about to unleash fury..
The power supply needs to be verified intact, functioning and safe. Then and only then can U go chasing signal anomalies..

POST #4 - UPDATED AS AT APRIL 24 2012

Gid here - the OP

Some further detail regarding my Mark III R2 problem.

I have now confirmed that:-

Ch 1 - works fine
Ch 3 - works fine
EQ - works fine
Rev - works fine
Presence - works fine
All Push / Pulls - work fine
All Front and Back Rotary Controls - work fine
R2 "mod" - works fine [ I tested it fully before I shorted the head screw to the diode board ]

The *only* thing not working is the R2 MID PULL channel selector.

When I pull it out - to engage R2 - it makes a loud pop and the volume / gain drops significanlty.

Any help / suggestions will be hugely appreciated.

Thanks again,
Gid

There are some schematics floating around the internet for the Mark III. Doing a search in this forum will find some chatter about those as well as where to get them. However, and others can chime in to correct me if wrong, most are for the earlier black stripe/no stripe. There are some changes based upon the stripe color. I beleive someone has been collecting info on the various types and posting as they can to this forum. So, again, a search might start you in the right direction.

Dennis

I hate to say it, but I'd take it to a good tech. Short-out scenarios are very tough to track down especially since you added the extra variable to the equation...the R2 volume mod. Since you never got to thoroughly test your work after the modification, you don't know if the issue is due to the mod or the short.

The problem with schematics for the MkIII is that the factory docs are for the original no-stripe/black stripe model. To my knowledge, updated prints were not released. But a member here re-drew them for the blue stripe model a while back. Like everything you find on the Internet, use them as a guide, not the reference. I have a green stripe and found numerous inconsistencies with all public schematics. It's a better bet letting a good bench engineer track down the problem.

Another good reference comes from documents published by MB titled "Quick Mods" dating from 1990 and 1991. They are the only solid reference for sanctioned mods to MkIIIs/MkIVs/Studio 22s. Some searching should unearth them. Again, use it as reference material, not a definitive guide.

Good luck! When it's functioning at 100%, a MkIII is one of the best amps on the planet, IMHO.

gidloz said:

POST #4 - UPDATED AS AT APRIL 24 2012

Gid here - the OP

Some further detail regarding my Mark III R2 problem.

I have now confirmed that:-

Ch 1 - works fine
Ch 3 - works fine
EQ - works fine
Rev - works fine
Presence - works fine
All Push / Pulls - work fine
All Front and Back Rotary Controls - work fine
R2 "mod" - works fine [ I tested it fully before I shorted the head screw to the diode board ]

The *only* thing not working is the R2 MID PULL channel selector.

When I pull it out - to engage R2 - it makes a loud pop and the volume / gain drops significanlty.

Any help / suggestions will be hugely appreciated.

Thanks again,
Gid

Click to expand...

Hi kdorsey

I am the original purchaser and owner of the amp from new.

I still have all the papers / tags etc.....

I also still have the original schematic that came with the amp - it is a yellow A4 sheet with the schematic on both sides.

Surely [ hopefully ] as this is the schematic that came with the amp from the factory, it is *the* schematic for my specific purple stripe Mark III (?)

Gid

gidloz said:

Hi kdorsey
Surely [ hopefully ] as this is the schematic that came with the amp from the factory, it is *the* schematic for my specific purple stripe Mark III (?)

Gid

Click to expand...

I think I have the same thing...the hand drawn 'Tri-Mode Programmable Preamplifier' schematics dated 1985. My copy shows the power stage in triode mode and the green stripe is in pentode. That doesn't apply to your purple stripe, but my point is to be cautious.

FYI, I'm the original owner of my MkIII too. There seems to be a large number of MkIII owners that never let go of them. 8)

Hi Gid, you're going to have to trace out the schematic to confirm it's correct for your particular model/variation.
Takes a little while but you'll know the circuit back-to-front when U have finished!

BUT, to reiterate, THERE'S DANGEROUS THINGS IN THERE, and I'm with KDorsey - get it to a tech.

Hi all

Thanks for the comments.

The amp [ and a zoomed-up A3 size schematic ] are both going to the tech tomorrow.

I will report back.

Gid

Good call, and hope it's a quick and easy fix for your tech Gid.

Anyone up for some harmless punting??
I'll start with an opto gone bad. Any other takers??

OK, for a harmless punt: As the short circuit is said to have happened on the screw above the power supply board which as far as I can remember has no components to do with the audio signal or R2 on it, and as the amp was originally removed to do a mod on the R2 circuit, my best guess is that it will turn out to be a problem with the R2 mod itself that didnt show up on first testing or maybe was somehow caused by putting the chassis back. Could be a bad joint, short circuit, wrong connection or a damaged opto. I suspect that the short circuit and flash inside is not connected to the R2 problem or if it is the screw was not the cause, but its hard to say as there was not a lot of info to go on.

However as the amp has now potentially had a short causing a big flash on the power supply it needs a good looking over to see what other damage might have been caused and make sure that its safe, so I agree with the posts saying take it to a tech.

Actually I'm a bit puzzled with the whole thing as I cant see why some screws would be longer than others, why Mesa would make any of the screws long enough to touch components that could be at 500V (they would have to go a long way into the chassis to touch anything which is not necesary), why the fuse didnt blow if there was a big flash inside and how it was decided that the screw was the cause as the post didnt mention opening up the amp again after the blow up. I will be really interested to see what is found and I hope that the amp is up and running soon. I wish I knew about R2 vol mods years ago when I had a Mark 3, I might still have it if so.

My AMP is FIXED - WOOOOOO HOOOOOO !!!!!

Hi all

My tech just rang - amp is fixed and am pickiing it up tomorrow.

From what I could understand over the phone [ I'm not electronic-component-tech-savvy-very-much-at-all ] the short I induced blew a 400v EQ Capacitor which he replaced with a 600v Capacitor - FIXED !!!!

He also checked my R2 mod - no problems there.

He also gave it a once over and the only other thing needed was he had to adjust the bias as my Sovtek replacement valves were running way to hot.

All up, $158.

To say I am extremely fortunate it wasnt anything more serious is the understatement of the year !!!!!!!!!!!

Many thanks to all here.

Take care,
Gid