Now I have all the output transformers wired up it was time to wire them the the XLRs, plug the Molex KK connectors into the backplane PCBs and test each channel's direct out. A couple of hours soldering and it was time for testing. Channel 1 was fine, as were channels 2 and 4, but channel 3 had a very low output. A quick check of the secondary dc resistance revealed it was very different from the other channels. So I removed the transformer panels to check if I had made a wiring error, but channel 3 was wired exactly the same as the others. Further dc resistance checks on the primary and secondary windings revealed that the VTB2291 transformer was wired backwards. I had wired to the correct numbered tags so it looked like the tag panel had been put on back to front by the manufacturer. Fortunately I had a spare transformer so I removed the faulty one and replaced it with the spare one. Channel 3 now functioned as expected.
I removed the links from the faulty one and again checked the winding dc resistances which confirmed the tag panel had indeed been fitted the wrong way round. I contacted Colin at Audio Maintenance who quickly supplied a replacement transformer.