Dual bananas are also fairly expensive.
Agree with Roly, XLR are a good option for carrying high current.
There are also 1/4" jacks with 2 tip contacts which helps, but they are not very common.

Early version schem:  http://www.drtube.com/schematics/marshall/5210-iss5.gif
Later version ('88):  http://www.drtube.com/schematics/marshall/5210-iss3.gif

Roly, you omitted at which step he should remove the short from FX return.  I'm guessing at the end of step #1 ?

  Thanks for posting that. 
Early on they had too small of a fuse in the F2 spot for the 5V supply.  It's the soldered in picofuse.  They had a T500mA in there and then they changed to a T1A.  It runs right around 500mA and would sometimes blow from turn on surge or high line voltage.  I fixed a couple using T750mA and never had a problem after.  That was before they updated to T1A, which is probably and easier value to find.
  No idea if that is a problem with yours, just thought I'd mention it here.

  Does it respond it you give it a whack when it is acting up?  Sometimes bad solder connections will only show up with prolonged heat.
  Also, a heatgun or hair dryer can help find heat related problems.
And if it has an effects loop, plug into the return after the problem occurs.  If the sound does not improve, then you have isolated it to the power amp or power supply.
What is the exact model/chassis number ?

  If the preamp has IC's, they will typically run on a split supply of approx. +/-15V.
It will often be derived from the main supplies, but the voltage dropping may be done either at the main supply or on the preamp board.
Seeing as we are completely blind about what this amp even is (not a common brand or model), some pictures would definitely help.
  As you did not have fuses blowing, and there is no DC on the output, you are probably ok now to connect the speaker and check for sound.
Does the amp have an effects loop (send and return jacks) ?

  For starters, the speaker should not be connected until you verify there is no DC at the amp output.
  With the speaker disconnected, check the amps output leads.  The - lead is often grounded so don't worry if you have continuity to chassis on the - wire.
  The + lead should not give continuity to ground.
Before connecting the speaker, when you turn the amp on measure for DC volts between the leads that go to the speaker.  There should not be any substantial DC there, maybe up to 50 millivolts.

Quote from: txflood on December 19, 2014, 11:12:13 AMThanks for this tip. I did get the specs from Ashdown now on the OPT and I'm guessing that it is bad as the drawing shows 8K ohms resistance on the primary windings. I am measuring a little less than 600 ohms.

Wouldn't this low primary resistance explain R76 burning up feeding the CT of the OPT?

The 8K figure is impedance, not resistance.  Impedance is an AC only spec. and we can't measure it with a normal type meter.  We can measure DC resistance but it will give a much lower measurement.  Your 600ohm resistance reading does not seem low at all, some amps OT primaries measure even lower.
R76 does not feed the CT.  Rather it feeds everything after the CT and the fact it is burning up implies the problem could be anything other than the output transformer.

  If that's a red knob "the twin", it's a tube amp.
If it's a solid state twin, then it's from the late sixties and I was not aware that they had red knobs, but could have been replaced.
  If it's "the twin", you will need the special 9pin Fender jacks.  Don't even think about trying to modify to use anything else, too much switching going on in those jacks.

  Someone on ebay selling those 1N3754 has the spec listed as:
100 PIV, 125mA, Vf = 1.0-1.1V
Does that sound right?

 Be nice to the tolex when removing the chassis, they can be a pain, be patient.
If someone previously has over-tightened the side screws the chassis wings will be bent and it may damage the tolex.

Well, the picture to me looked like a regular sized toggle, but now that you mention it, I see it is smaller diameter than the jack.
  The Fender switch I mentioned is not expensive at $7.95, I mentioned it because they are easily available, but no matter as it is the wrong one.
  To get the proper switch, as has been mentioned, you need to measure the diameter of the bushing, and count the number of terminals on the switch.
  Without that information, no one can help you, aside from Yamaha or a Yamaha dealer.

posting schematic

  It looks to be a fairly hefty toggle switch, like Fender style.
I think a Fender DPST part #  0036570000 should be a safe bet, if you don't need the extra pins you can just leave them unused.

Edit: That being said, you don't want to drill out a new amp if the diameter doesn't match, so best to measure and check.  That Fender switch fits 1/2" hole.

  If you can't hit it with your hand, try a rubber mallet or big phone book or even lift one end of the cab and drop it back down on the floor.
The purpose is to see if there is a loose connection inside.