[Milsurplus] GRC 106 loses receive

[W Private]

Hello,
I have a RT 834 GRC 106 that I just got up and running. Then it blew a switching transistor in the DC/DC converter. I fixed that.
Found a replacement for the switching transistors.Good 125 and neg 30 volts out.
Also adjusted 20V DC, receive works better than before.
Now it seems to be behaving quite well----
Except that:
Apparently only at the seven MHz setting, when I go up (not down) with the 10 KHz knob (not the other ones) at some point about at the 40 or 50 KHz level, 
The receive almost shuts off, but not quite. The measured AGC, both IF and RF, go to Zero when this occurs. The signal strength meter also goes to Zero.
If I switch to another MHz and then back, receive comes back to high level. Then if I increase the 10 KHz knob it can die down again.
Sometimes if I key the transmit, receive comes back, sometimes not. It always comes back when I switch to another MHz and then back to 7 MHz.
Does not happen at any other MHz band. I think I tried them all.
Happens if I use antenna or freq generator.
When I was on the net on 40 Meters, I was told I was off by maybe 200KC,
When I use the rig on other bands, (20 and 18) I am told I am right on freq.
Again the problem is only on 40 M.

Sounds like a PLL unlock to me.
Do I need to make adjustments on the turret?? Never done this.
Could it be the front end protection?
Any suggestions??
Thanks in advance,
Wally
KC9INK