Well, this is the first time I've seen a 6 ohm spec for any type of music gear.  But apparently peavey had a custom 6ohm speaker for this model?
  What speaker is currently in the amp?  Is the weber 8ohm you spoke of in the amp presently or is that what you wanted to add as an external speaker?
  To put it most simply and practically, with a solid state amp if you run it into less than it's minimum rated load, it will overheat and burnout the power amp section.
  Sometimes you will get away with it if you don't play too loud, or add some additional fan cooling.  But other times you will just blow the amp so you really don't want to take that chance.
  The best thing to do would be to wire up the amp so it puts the additional speaker in series with the internal speaker.  Then you will have a little less overall power, but probably not noticeable.  And you will not be in danger of blowing the amp.

 

  It's non-conductive in it's normal state.  But you don't want it on the tracks anyway.  Just where you have mechanical contact of moving parts (aside from the tracks and their contacts).  The tracks you would clean with a regular contact cleaner that contains a lubricant, whatever you are using for regular pots.

  On the borders of the page are numbers (top and bottom) and letters (sides).  It's just a way of describing areas of the schematic.  Sort of like latitude and longitude of  a map.  So find A on the side and follow across to 4 on the bottom and you will have the general area.

  I'll just put both power amp schematics up anyway.  Might come in handy for someone in the future  :cheesy:

  There are a few power amp versions, what are the output transistor types?
Preamp schematic attached.
Deoxit makes a fader lube, as you say it is expensive.  Other types of cleaners tend to change the feel of the faders.  In Canada you can find the deoxit sold by Hosa accessories at music stores.  Alternately you can take the faders apart, use regular deoxit or equivalent, then lube the moving parts with white lithium grease or similar.

  Sorry I wasn't more clear about pins 7&8, using the word "signal" on the data sheet was a bad choice for them, as Roly mentioned, pins 2 & 3 are the inputs.
  The chip has both standby and mute options.  In the Marshall they utilize the standby (there is a delay circuit which acts to mute the chip during power up at sector A4 of schem.pg2).  Because they use the standby, they tie the chip mute to the +supply so chip mute is never activated.

Quote from: Hawk on February 19, 2015, 09:20:55 AM
Shows both having the speaker outputs at W12,13,14,15.

...In my mind these speaker outs should come off pin 7 and 8 as they are the neg, pos signal.

No on both counts.  We do not know where any W connections are going except where shown. W13&15 are shown going to outputs, the others not.  Suggest ignoring them.
  The output IC shown on page 3 is the power amp.  The other circuit with W12 thru W14 is not.
  Pins 7&8 are +/- supplies, not signal.
  The output IC data sheet may be helpful.

  Don't feel too bad, modern ground schemes can be fairly complicated and often require complete re-assembly to run properly.
  So it's all done & ready for use now? 

No. 
This is with nothing plugged into the input?  Do volumes have any effect?
Plug something into FX return jack, still same problem on turn on?

  The connectors are just push-in.  Try not to pull by the wires.  At each end there are little lip edges that you may be able to get your fingernails under.  Or a flat head screwdriver to pry each end up a little till it comes loose.

In you first post you had your +15V.  Now it's gone.  Either something has gone bad or there is a connection issue.  Check at the output pin of the 7815 regulator, there should be +15V there.  If it is not, either the regulator is bad or something is loading down the +15V line, like a shorted IC.  Or the regulator is not receiving it's input voltage (probably around +20V or more).
  If there is +15V at the regulator output, then it is probably a connector issue or a bad solder joint or cracked trace somewhere.  In this case, check all the IC's for +15V, if some have it and some don't, it may help localize the problem.

  Also, have you tried contacting the designer and asking him about it?  He sounds like a pretty helpful individual and may have some suggestions.

  Nothing in that picture looks non-repairable, so I don't know why he would want to replace the board rather than fix it.  Perhaps he does not really repair things at component level, in which case you will need a different tech.
  Suggest you contact this fellow to get a copy of the schematics:
http://rolandtd9review.com/simmons-da200s-schematics

  Also on some marshall models you release the standoff from the outside of the chassis.  Sometimes by screw, sometimes by 2-piece black plastic push-pin.

  Amen to that!  For me that pretty much describes the state of modern Marshall schematics.  Occasionally someone online will make up a drawing showing which connectors go to what.  Otherwise they are a nightmare.  I often wonder if the actual factory techs have secret maps of the connector routing.  If they still have such a thing as factory techs....

Hawk:  D11 is part of the supply feeding the 7815 regulator.  As you mentioned you are getting +15V to the op amps, it is not causing much problem at idle.  But perhaps when passing signal the +15V would be dropping and making trouble.
 

   Seeing as this is your "test mule", you should be warned about the 7293 output IC's.
  At any time they are disconnected, before re-connection their power supplies must be discharged.  If not, they can be damaged.  So, if you disconnect their board connectors, you must discharge their supply caps before reconnection.