The bending A.K.A. "flag waving" or "top hook" is usually coming from the source. On the 9-90 check the 330 ohm at the corner of board. You can also
try the RF AGC control on the 9-87.
Never had an off board vert problem except the fuse. IIRC the outputs
runs off a 50 V supply, look there next.
73 Zeno
LFOD !