Old Boston subwoofer SMPS blowing up MOSFETs

[jjne]

This one has been annoying me for a few weeks now lol.

Aging Boston sub, uses an IR21531s-based dual output SMPS for power. Recently blew the fuse. I get the impression this may be a common issue looking at them on ebay.

Both FETs blown, so I bought some new ones and a replacement 21531.

With these parts replaced it's still blowing FETs. I noticed that the 21531 only seems to be oscillating with reference to VS on the LO output - I initially thought this was the fault as it seems to be the FET linked to HO that is blowing, but the replacement chip is doing the same thing. If you ground VS then both HO and LO are oscillating at approx 50kHz, but as configured I'm seeing no oscillation at HO, just a constant high signal. This would definitely blow the FET, but I don't think it's as simple as that as the chip does seem to be functioning and I admit I don't really understand how that VS line from the IC (it's a floating supply referenced to HO according to the datasheet but this doesn't seem to be doing anything).

I've attached a scribbled diagram of the circuit - this is the same as that on the service data I found online with the not installed bits removed.

It has occurred to me that the insulation on the primary winding of the transformer may have broken down but this still doesn't explain the lack of oscillation on one "phase" - would I be right in thinking that I should be seeing a 50KHz square wave on both HO and LO relative to VS, essentially inverted so that only one FET is on at any one time?

One other thing, looking at the right of the diagram I can see that this is just a pair of unregulated outputs, I believe one for the preamp one for the power amp, and looking at the caps I'd estimate that the pre is +-15v and the power likely +-24v. These are going to be noisy and unstable due to the lack of regulation that I can see so assuming these are regulated downstream (not checked yet).

However, what is the purpose of pin 1 on the lower connector? It appears to be just one leg of the other winding, unrectified and unfiltered. I had thought of replacing this SMPS with a suitable alternative but that line is weird and the service data is less than helpful.

Sorry if this is just a stream of nonsense but there's just so much of this confusing me just now!

All passives appear to test good BTW.