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.

  You may well have found some transistors that sound better for this circuit than others.
  However, you can not jump to the conclusion that the frequency spec. is what is responsible for the difference.
  Roly has shown how this is the case.  There are many other aspects of transistors that could be responsible for the difference you hear.
  It could also just be a coincidence.  You would need to try many samples of each type of transistor to reach any kind of conclusion.

  One of the simplest things to try is a spare cord between the FX send and return jacks (or preamp out & pwr amp in).
  These jacks have internal switches that route the signal through.  They often cause problems like this and are a great place to begin trouble shooting.
  If that doesn't help, try plugging your guitar straight in to the FX return or pwr amp in jack.  If the problem stops, it is likely you have a preamp problem.  If the problem continues, you have isolated it to the power amp or speaker.
  Alternatively, you can run the FX send/preamp out to another amp and see if the problem is still there.

  Those views are mostly bots.
Glad to hear you are paying attention to safety.  For solid state stuff we don't usually worry about discharging, but it is good practice to check voltages to make sure things ARE discharged.  With tube amps it is a big concern due to the high voltages and circuits that often don't self-discharge as easily as solid state stuff.
  Sounds like you have a pretty good idea what to do and what are likely suspects.
Also check generally for other bad solder connections.  If you have a good idea of what bad solder joints look like, a visual inspection will go a long way.  Pots and jacks are also prone to solder problems, as well as "hot spots" like power supply diodes and resistors where you may see browning of the board due to heat.
  Pay special attention to the dis-assembly and re-assembly process.  Take pictures and notes if you can.  Pay attention to wire routing (dress) and connectors as well as the heatsink assembly for the output transistors.  It is very easy to break wire connections when getting the board in and out of these amps, or flipping it over for access.
  Connectors are another item to check for bad solder.

Quote from: pudge211the H1- and H1+ voltages are 20.3 respectively

Please measure with your black probe connected to ground, use red probe to connect to H1-
Be sure your meter is set to DC volts.
Specify whether your reading shows negative (-) or positive (+)

  Thank you JP !!!

  Try it with a patch cord between the FX send and return jacks.

  I tried to upload it here, it won't accept as it says it already exists.  Yet a search of this site can't find it?  :loco
Here's the direct link to the one Matthias mentioned, there are 2 halves and a parts list:
http://music-electronics-forum.com/t20702/#post171291

  Are you stuck on the switchable output power feature?  If so, I think you are stuck with this transformer.
  Otherwise, why not look into other tweed Champ (5F1) kits.  That's basically what this is, and there should be plenty to choose from stateside.
  This is a very simple circuit so they should all be roughly equal ease of building.

Doc, there was a thread over at MEF that went into detail about the ground scheme for this mixer.  JPB made up a pdf of the ground info, but there was more said about it later so I'll just link the thread instead of posting the pdf.
The ground info starts here and carries on further down:
http://music-electronics-forum.com/t26867/#post340734