Did you take out the power tubes yet?

  Is there a switch or a footswitch jack?  Some require the footswitch to work.
So maybe your friend had a way of turning it on.
Otherwise, weak tubes and caps (especially the cathode caps) can make the trem weak.
Maybe he had some work done later that mucked up the grid wiring and forgot?
Anyway, try to find the real schematic.

For 120V use, the jumper from 3 to 4 is removed.  A jumper is placed between the two 0 tabs, and another jumper between the two 120 tabs.  Hot goes to the 120's, neutral goes to the 0's.

The various points of the primary side are reading correct, so it does not seem there are any shorts in there.  Plus it is not blowing the fuse, which would be expected with a primary short.

  Looking again at the schematic, the thermal fuse goes between the 7th tab and one of the 0 taps.  So it can easily be bypassed and the transformer rewired for 120V operation.

  Try with power tubes removed.

   Those readings are all good.  Where is the 7th tab on the transformer?
The schematic shows that it is the one with the thermal fuse.  You need to measure resistance from the 7th tab to any of the other 6 on the primary.

The funny thing none of us have mentioned, this transformer can be wired up for any voltage, so it could have been rewired and you would not have needed the external transformer.

Edit:  I think the 7th tab is where the white is connected up on the secondary side (with the orange and black wires).

  The fact that you measure the same voltage on all taps of the primary is precisely the reason we suspect the primary is open.
  As others have suggested, you must check the resistance of the primary winding.

  I also see 19V marked on that transformer.  Aside from the secondaries I don't see what that could possibly refer to.
  Maybe whoever drew up the schematic with the (24VDC listed where it should show AC), specified that the transformer maker also mark it that way?

  I think that the one piece unit would turn out to be a major disappointment.

  The piece of wire connecting those 3 pins on the jack must be removed.  The way it is wired all jack contacts are grounded and it can not work.
  Also, the pot you replaced (3rd from jack), it looks like one of the legs is bent over and the solder may be touching the ground trace.  You need to make sure it is only touching the trace it connects to.

  Thrift shops can be very cheap sources of consumer electronic items to be used for parts.

Doc, is that the service manual from elektrotanya ?

 I don't think it will be like the Yamaha:

  Just to add a little to what Enzo said, to increase the power supply usually means installing a larger power transfomer, and some amount of re-design.
  So your point about needing a different amp to get (a significant amount of) more power is probably correct.

 :tu: