[morzh]

Terry

Yes open choke could have blown 8uF #77 up (this was the reason for me to ask if it was open) but for a different reason.

1. #78 would not blown if its WV is equal or better than the rectifier output. (then again it might be lower though some good tolerances exist for surge and overvoltage, so then you'd be right about it). The cap would only charge to the peak amplitude value whatever it is, usually on the orxer of 500V (this is why I have them rated 500V).
Then again it is not the #78 that blew, but 77.
2. The reason an open choke (notice I am not saying "disconnected") could blow up the #77 is simple: it has cam #80 in parallel. That in turn would result a polarised #77 cap subjected to the AC voltage which is known to make them blow. Of course this would depend on the ripple amplitude and the latter quickly cones to almost zero as the 78 has charged so after all it might not be the case.

In this case we have good 80 tube, good (believed to be) choke and good caps. Possible disconnecting of the 10uF #77 is almost immaterial. So....it is a mistery.