Problem solved!
Got back from a trip to the US today. And brought with me some “Goo Gone” (which is apparently very good for removing mylars – according to several articles on the internet) an imperial sized “stumpy” ratchet set (for getting to those hard-to-get places in the pinball machine), a cutter bit for my “Dremel” equivalent and some Fairchild Semiconductor QVE11233 Optos.
I couldn’t wait to test my theory on the opto switch board and so changed the Thing-Up opto. This wasn’t as easy as it could have been, as two of the pins on the opto are soldered on the backside to a very small solder pad. These pads connect through the board to the upper side of the board where connections are made to other parts on the board. When de-soldering pins from such a small pad great care needs to be taken not to overheat the pad itself otherwise it’s easy to “burn off” the pad. I did this on one of the pads which then left me with just a small section of pad to solder the new opto to. I managed it though.
Having soldered the opto in place I connected up the board and tested it……
No more switching of the Thing Up opto when the left outlane switch was activated! A success!!
Time to sit back and decide how to proceed further with my renovation.