Re: 7B92A time-base plug-in odd behaviour

Albert Otten

Hi Colin,

S490 is shown at diagram <4> top-right (in my manual Rev. D, April 1976). Switching contact goes to GND, one choice is open, the other(LOCKED) connects it to P911 at/to the A6 Interface Board. From there also to R334 at <3>. Somewhat vague, as if R334 is in the wire , then to P936 and P204. I didn't look at the function (yet).


On Wed, May 22, 2019 at 02:10 PM, Colin Herbert wrote:

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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