MOTM

About ICs - am I right in thinking U3A would be the side of the IC with the
little circular "dimple" and U3B the other side?
 

The general-case answer is not necessarily. It depends on what the IC is and
how the person (or software) who drew the schematic and laid out the PCB
assigned the pin numbers and part designations. The only way you can tell
for sure is to look at the schematic and see which pin numbers are used for
sections A and B. Pin 1 is near the dimple and the numbers run along the
edges. If it's an 8-pin DIP then pin 8 is near pin 1, on the opposite side.
 
However, in the specific case of U3 in the 420, it appears from the
schematic that A is on the dimple side, since U3A uses pins 1-3.
 
If it's an intermittent problem--that is, it changed when you did something
mechanical (inserting the cable)--then odds are good it's a bad solder joint
somewhere. I'd probably start by going over the entire PCB solder side with
a magnifying glass. Yes, that means you'll have to mechanically disassemble
the module, but that's why I go over the entire PCB with a magnifying glass
before I assemble it further. :-)
 
Good luck!
 
--Adam