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

  Certainly a very strange set of symptoms.  As all channels but 3 have the same problem, normally you would suspect something common like low voltage power supply, but ch.3 working throws a wrench into the works.  I would suggest you leave the ch.3 issue till you have the other channels sorted.
Find out if the FCL's are lighting when they should, if so, the signal at that point should be correct and the problem is downstream somewhere.
Levels for FCL threshold are shown on page 129 up by where it says "mix".

  And above all,
DON'T PANIC !!!
(and don't forget your towel)

 I've seen it said there should be 25mV between TP301 and 302.  I've also seen it said 2.5mV, however the early versions were mosfet output devices and I believe the 2.5mV was for the mosfet version.
  Can anyone confirm or opine whether that sounds likely?

  The fact that it responds to you smacking it means it is likely an intermittent connection.
  As Roly mentioned, this rules out defects of the capacitors themselves.  However, their solder connections could cause the problem you have.  Often resoldering the filter caps corrects this type of problem.

  Clipping of the power amp will occur when the amp achieves it's full power output.  Where the volume control is set does not completely control it, it also depends greatly upon the level of the input signal.
  A very weak pickup will allow a higher amp volume setting, but a "hot" pickup will drive the amp to clipping at a lower amp volume setting.
  Many amps will clip at around 4 on the volume control.  Turning down the volume control on the guitar itself will allow higher amp volume settings.

And in case you are unaware, the 7815 and 7915 regulators have their inputs and outputs in different positions.

  That inner grommet could well be the problem.  The pin should be in the center of the grommet.  That will allow the inner pan to "float" whereas right now it is hung up on the pin and can't move.
Fix it and see if it helps the issue.

Quote from: Don on November 06, 2014, 10:53:28 PM
The only temporary solution I noticed, which would only last for a few days or until I move my amp or something is to shimmy the reverb tank away from the speaker, but don't hold me on that one. When I did this a few days ago the hum was completely gone and I was able throw the reverb knob all the way over to twelve without a hum.

If this is the case then it's definitely a physical problem caused by the proximity of the speaker.  And I doubt you were able to shimmy it very far so it is operating right on the border of oscillation.  If it's such a critical location, then it could be that the grommet is making a difference, put a temporary support there, even cardboard, to raise that corner as much as the grommet would and see if it helps.

  The hot glue is there in addition to solder, not instead of it.  It helps keep heavy parts or cables secure so they don't move around and degrade the solder connections.
So you want to keep it in place.
Are the four corner springs in place inside the tank?  They keep the inner pan centered  and help reduce the physical vibration.
Are there any screws going through those grommet inserts?  If not, you don't really need the brass insert and can use any kind of similar rubber grommet for the missing one.
As far as the cardboard, keep it but you could try some thin foam between the cardboard and the bag.

As Phatt mentioned, if pulling the tank away from the speaker makes it quit, then it is a physical feedback issue, caused by the proximity of the tank to the speaker.  It is a fairly common issue.
Any chance you changed the speaker?  Otherwise, could the tank have moved in it's bag?  Sometimes it only takes 1/2 an inch.  Or it's possible the springs have stretched enough to cause the problem, in which case a new tank should solve it.

The part I don't get is that the footswitch will kill the feedback, but the reverb pot will not.  The only thing I can think is that the pot does not go completely to zero.
Does the level of the feedback change as you turn down the pot?

 The point about the eq was to try and narrow down the area.  If the eq can affect it, then it is happening before the eq.

  You posted 2 schematics, look at the other one for masters and reverb.
I guess there is a FX level pot also on the back of unit?

Quote from: Stormlord1736 on October 24, 2014, 05:10:49 PM
Tnx, for the answare. I have a few more questions.

Quote from: g1
The belton tank is the equivalent of an accutronics model 8EA1C1B.

On the accutronics website I only found the 8EA2C1B, it's the same?

The fourth digit denotes the delay length.  The 1 is short delay, 2 is medium.
Otherwise the specs are the same.
The codes are explained on this page:
https://www.amplifiedparts.com/tech_corner/spring_reverb_tanks_explained_and_compared