[Boatanchors] SB-401 LMO B+ voltage cutting out

[Boatanchor at alliedinformation.com]

I have an SB-401 problem I have somewhat narrowed down. Just need some 
advice from someone who knows or has an SB-401.

The rig's LMO is getting the B+ voltage, but only when the rig's mode 
switch is in USB or LSB mode. When I switch it to CW or TUNE, the 150 volts 
drops out, presumably before the regulating tube. The 3rd wafer on the mode 
switch deals with this voltage line somewhat, but its very confusing to me. 
Why would the mode switch need to turn the B+ voltage on or off at all 
dealing with the LMO?  Clearly, it does for some reason, but why would the 
mode switch need to deal with it rather than just leaving it to the 
Frequency Lock/Unlock switch? As far as I can see, cutting out the LMO's B+ 
voltage should only occur when the Frequency Lock is selected to accept the 
LMO input from another receiver. The wiring looks correct for the mode 
switch wafer from the manual diagrams, so I can't figure out the problem. 
Can anyone tell me why the mode switch should shut off the LMO's voltage at 
all? If so, under what conditions?


Thanks.

MC