Elpico AC88 bad OT

[Timbo]

I have a customers Elpico AC88 on the bench and I’ve found that the OT primary is open circuit somewhere between the ultralinear tap and one end. Amazingly (to me, because I’ve never seen it before), the amp is still working, albeit with slightly distorted and low power output. This amp has a T100mA fuse fitted into the HT circuit, and that didn’t blow. Can I speculate from that that the OT just failed through old age, rather than because of e.g a short in one of the output valves? Obviously I don’t want to fit a new OT and then find there’s an intermittent short in one of the valves. Or, is there a way that I can test them conclusively?

[Helmholtz]

When the plate connection opens, the screen becomes the anode and screen current/dissipation strongly increases.
This is likely to damage the tube.

[Jon Snell]

Check if the winding has broken at the tag on the transformer and resolder it.
Circuit attached

[Timbo]

Thanks John, unfortunately this is a shrouded tx and there aren’t any tags as such. The connections are buried in the windings. The solder joint at the plate wire on the bad end does actually look dry, but I do measure continuity between the solder and the end of the wire. Either way, I don’t see a way of getting in there to fix it. Customer has purchased a new Hammond 1650e to replace it, so that still seems the most reliable way to go.