I managed to bring the scope home today and test some voltages, I don't have another scope so I measured VAC with a Fluke 75 (without true RMS) so hopefully multiplying by sqrt(2) gave the peak-peak ripple values, please let me know if this is wrong.
+180V = 184.3V (90mV pk-pk ripple)
-12V = -12.00V (200mV pk-pk ripple)
+12V = 15.6V (325mV pk-pk ripple)
+140V = 105.8V (110mV pk-pk ripple)
These results seem to confirm previous suspicions, the manual says 140V ripple is reasonable (although the DC is strangely low) but +-12V is way out (should be 6mV pk-pk) also the +12VDC is oddly high? So something is definitely wrong with the 12V rails.
I measured the drop across all directions of the bridges and seemed to be 0.49-0.56V. Is there any way to identify which bridge (if any) is faulty? Can the open circuit condition only occur whilst under load?
Since they are only low voltage secondaries on the 12V regulator side I should be able to cobble something together with some 1N4001 to replace the faulty one at some point.
Attached are some pictures of the internal construction