7B92A time-base plug-in odd behaviour

Colin Herbert
 

I have a 7B92A which is puzzling me. When turned on, it will only show the "ALT" time-base, which isn't altered by the time-base Time/Div knob, but after warming-up for probably only a minute or so, everything seems to work correctly. I have a feeling that this is associated with one or other of the switches on the "Pull for Alt / Push for Dly'd" switch. I am a bit puzzled about these two switches and the Service Manual isn't enlightening me too well. There is a micro-switch close to the front-panel, which I _think_ is S490 (the mechanical parts list refers to it as "SWITCH, SENS:(SEE S490 REPL)). This only operates when the "Time/Div or Dly Time" knob is pulled out with the two time-bases locked. It then stays in that position until the delaying and delayed time-bases are at the same Time/Div again and the knob pushed in. It seems to be connecting something to chassis ground when the knob is pushed in, but I can't locate it anywhere in the manual. The other switch that is operated by pulling/pushing the time-base knob looks to be a slider-switch on the Readout board, but it is on the inner face of this board and difficult to see without removing the board. I can't find out what it does, as I can't locate it in the manual.
Can anyone enlighten me as to the identity of these two switches and their function? Also, some idea as to what might be causing the odd behaviour before the time-base plug-in warms up?
TIA, Colin.

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