Read this thru, because of we've can't determine the Mode switch is bad, then let's return back to what you found when the PLL was hooked up with - because BOTH conversations are in here....
Option 1 - One Line At A Time...And It Put You In A Mode...
(apologies to Johnny Cash)
Well, as a suggestion - because of what you're up against, I'd disconnect all the route lines going to the board from the switch. Not the power ones from the board - leave those connected to the rotary switch..
I'm serious! I've had guts of that radio dragged out onto the bench so I can test jig the modes. to find two bad diodes from a character that got goaded by his late night drunks that told him he was off and wound up screwing up the radio.
Reconnect only the USB - test, disconnect, go to LSB - test, disconnect then AM - test - leave (AM) in. Then connect USB - test - does problem return - then the two sections are related. THEN YOU KNOW - you'll have to search out and look for the short causing this.
OPTION 2 - Then All Else Fails - Pull The Pin and RUN...
ELSE IF THE SHIFT still is in there. then that mode is traceable back to the PLL - it's the only thing we haven't changed.
Your "extra pin" left powered - has me concerned that the PLL will lock but the IF (frequency) it generates as part of a loop is not locked or has lost programming - for the N-shift. IT should not do this, but if the PLL is being sent something - or a line from it is being held high (Pin 6 ok? - goes to TR35, and TR33 for AM mode - it indirectly affects TR47 and TR53) the system will not shift because it thinks it's in a mode where it needs to lock in on a frequency based upon the status of the channel changer line (yes, jiggle it- the channel knob- if the detent can force the shift to return in the PLL we may have something) TR35 helps to tell the MB3756 how to toggle, while TR47 and TR53 handed the mute function when the PLL is not locked per Pin 6 line (S/B High have a voltage about 4V or so - low is down to about 0.4.~ 0.2V) AND STATUS OF TR33 (In AM MODE)
If work has been done to the PLL and the Modes seem to be fine - then the PLL and it's associated LOCK line from Pin 6 - has to be checked...Means all the Bases of two transistors are connected to it. If any one of hem blow shorted - there's another headache - power flows into the line from the bad one you don't need. Telling the other Pin 6 is ok...
:+> Andy <+:
