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.

  Also note the frequency response.  The 8MRA is designed as a mid-range driver with freq. response dropping off below 400hz. 
  This may or may not be an issue, depending on how much low end response you need.
  But, keep in mind, A440 is the high E string at the 5th fret, open low E string is 82.41Hz.   

  The approx. 30V on both sides of that resistor is good. 
I outlined earlier how both channels are separate until C32 where they join.  As the problem occurs in both channels, it is then likely the fault is after C32.  You said the fault also showed up at the FX send jack, so it should be between C32 and FX send.  If it's a bad connection, it should respond to physical input like tapping the circuit board with an insulated item such as a plastic pen body or chopstick.
  If it's an actual bad component, the most likely suspects in that area are TR12 and C33.

  Yes it needs to be on. 
But before you even do that, measure it's resistance, meter set to ohms, one probe to each side of resistor.  It should measure 220ohms or less.  If it measures much more resistance than 220ohms it is bad.

  No you can't run both outputs into the dummy load as there are 2 separate power amps (stereo).
But what are you trying to accomplish?  Dummy loads are only required for tube amps.
You can run a solid state amp happily with no load.