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

Colin Herbert

Thanks for the info, guys. I have now located where these switches go, but I am unclear as to what their functions are. Perhaps I don't really need to know, after all.

I am still perplexed by the odd behaviour of the plug-in in terms of the behaviour before warm-up. Once the scope has warmed-up, the time-base is still showing some instability which can be induced/stopped by wiggling the time-base "Time/Div" knob. I am now thinking that it might be caused by poor contact on one or more of the cam-switches. This looks like it will involve taking the cam-switch assembly out and doing the "paper-soaked-in-IPA" procedure. As there are about 50 of these, has anyone any idea which might be the dodgy one(s)?

-----Original Message-----
From: [] On Behalf Of Roger Evans via Groups.Io
Sent: 22 May 2019 16:44
Subject: Re: [TekScopes] 7B92A time-base plug-in odd behaviour
On my 7B92A, the microswitch near the front panel is connected by a single wire to P911 on the interface board. P911 is to be found on schematic <4>, 'Delaying Sweep', near the top right >corner. It is marked as going to R334 on schematic <3> and the switch is marked as S490. S800 on 'Horizontal Logic', schematic <7> is marked with 'pull for alt, push for delay' and is surely the >second switch on that shaft, it connects through to the Readout board.
I noticed while investigating my 7B92A that the whte plastic 'cotton reel' that activates the microswitch didn't seem to be an entirely tight fit on the shaft but settled down happily after a few >iterations. I hadn't noticed any odd behaviour when it is operating in the mainframe.

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