I have also had found R3 can drift high and basically shift the DC bias through out the preceding hybrid making it appear dead, ie signal at input but nothing on the CRT. Having played a bit with these vintage HP CRO's I have found the Hybrids to be reliable and less likely to be the cause of trouble compared to the more generic parts around them etc.
Edit: I mentioned this quite some time ago in the restoration of a 1741 thread by Tekfan.
Quite a deal of info there if you wish to search