I understand that there is no voltage on the output from the transformer when measured across the yellow and yellow/blk wires. Is there any measurable voltage from the yellow and black or from the yellow/blk and black wires?
Also, can you measure the DC resistance of the primary side and also all permutations of the secondary side?
My thought is that if only one side of the output is open, then maybe you can actually measure the other side and find out what the output from the transformer is supposed to be.
So the things you need to measure are:
With AC power connected to the transformer (be careful about not letting leads touch each other!)
ACV Secondary Yellow to Yellow/Blk:
ACV Secondary Yellow to Black:
ACV Secondary Yellow/Blk to Black:
With no power connected to the transformer
DCR Secondary Yellow to Yellow/Blk:
DCR Secondary Yellow to Black:
DCR Secondary Yellow/Blk to Black:
I am assuming that the transformer being used has a center tapped output, but the center tap is not being used.
I am thinking that the transformer actually looks like a Radio Shack model that I had used once with a 120V Primary and a 12-0-12 secondary. That would give you 24V on the output side, assuming you are using the two 12V leads and not the center tap. But, it could be anything, so you really need to check and see if there is any useful information that could be retrieved from the bad transformer.
Also, It sort of appears, from what others have mentioned, that it may be possible to operate this circuit from a lower voltage, so it might be interesting to see if hooking up 12V from the secondary side would allow it to work? It may be a temporary fix while you get in a replacement transformer.