Kevin Wood G7BCS
Well, I found the problem. U1909 had what appeared to be an internal short
from the +7.5V supply rail to some of the output pins.
Everything was fine until the range changes started to switch in the AC
compensation networks. Then the 7.5v rail got dragged down and, if the
input level was high enough, the device would latch-up and short both
New 4053 substituted and it's working fine.
Many thanks to Jim for his ideas in troubleshooting this.