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.

  Then if both channels are the same, it must be something common to both channels.
That leaves the power supply to the preamp, or the circuitry between C32 and the preamp out jack.
  What are your DC voltages on either side of R20 ?

schematic attached
Is the problem the same for both channels?  Do all the controls seem to be doing something?

  The amplifier power will be split between whichever speakers you use.  If you plugged it in to a 4x12 cab, the 120 watts would split 4 ways.
  With 2 speakers, they will each get 60 watts.  A bit of a safety margin helps, you might consider two 75watt speakers if you play loud.

But just as important is the speaker impedance.  The amp is designed for a 4 ohm minimum load.  So you would want two 8 ohm speakers wired in parallel to make a 4 ohm load.  That is probably what the stock speakers are.
  Two 4 ohm speakers wired parallel would make a 2 ohm load which would overheat and damage the amp.  They could be wired in series and make an 8 ohm load, which would be safe for the amp, but the amp will put out a little less power into 8 ohms than it will into 4 ohms.

  To clear up a couple more design/operation issues:
input 1 is for ch.A use only
input 2 is for ch.A or B
So that should explain some of your questions about your tests
(http://www.award-session.com/pdfs/Award-Session%20History0026.pdf).

[On a side note, it irks me that they have called the clean channel "B".  This is so non-standard, I have to keep reminding myself B=clean here   ]

Now, 2 issues you mentioned previously.  You said clean channel was quite weak and required adding in from dist. channel to get level.
  This does not seem right and is possibly a defect that has been overlooked.  Using ch2 input, switch set to clean, how far up do you have to bring the volume to get decent level (without adding anything from dist. channel) ?
  The other thing you mentioned was a bit of distortion at the trailing edge of the note as it fades.  Maybe a cross-over distortion issue.

  The center position of the switch is where it must be set when using an external footswitch.
See pg.5 of schematic.  They call it "off", but it looks to me that when set in centre, with no external footswitch, everything should work.

Quote from: Peter Blair on January 24, 2015, 03:08:51 PM
I found these at "Just Radios". According to their product information these look (to the untrained eye) like they will fill the bill. They are available in the values that I need, and are very reasonably priced. Would you please render your opinion, and/or make me aware of other options or even a recommendation.

http://www.justradios.com/DMEcapacitors.html

Just Radios are an excellent company and you won't go wrong with them.

  In case you have a 300W bulb in your limiter because of the infamous video on youtube, please see Roly's discussion of limiter bulbs here:
http://www.ssguitar.com/index.php?topic=3636.msg28236#msg28236

  They have 2 versions on the schematic page:   http://www.award-session.com/award-session_manuals.html
If it is the '81-88 version, the headphone or ext. spkr jacks could cause that kind of problem if dirty/oxidized.
  If yours has those jacks, try to work some cleaner into them, or even just give them some exercise by plugging in and out of them (with power off) several times.

  The only thing immediately jumping out at me right now is that the volume pot wiper is drawn going straight to ground.