Jim Weir: RST-442 intercom query

Jim,
I have assembled and bench tested the RST-442 intercom and the audio quality
is excellent but I have a problem with the squelch.
Prior to reaching the squelch point when setting the squelch control all is
well with excellent audio but as soon as the squelch point is reached it
takes a loud voice to break the squelch though I have the mike close to my
lips.
I've checked the voltages mentioned on the instructions and looked at
waveforms with my oscilloscope but can't isolate the fault. There is a 1
volt P-P signal at pin 1 of U101A and 0.5 volts P-P at X116 and the squelch
circuit voltages appear to be as described in the Theory of Operation.
As things stand I can only use the intercom as a hot mike unit. Can you
suggest where the fault may be which is preventing the squelch being broken
with a normal voice or give me some troubleshooting tips to find it?
Thanks for your help,
Leo