[Collins] 75A-4 - burnt resistor question

[Thomas Beltran]

My 75A-4 had some sensitivity problems, so I opened it up to take a peek.
R85 [47k], one end of which connects to the terminal "D" on T4, (the plate
of the AVC amp, 6BA6, connects to terminal D) and the other to a .01
capacitor to ground, is completely burnt.  R87 [3300], which connects to
terminal "C" of that same T4 - the grid of the AVC detector, 6AL5 goes
through C109, a 510 microf. capcitor then R87 to ground, is just about
burnt.

Has anyone had a similar problem?  All the other parts are visually fine.
Thanks in advance.  Tom W6EIJ