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Old 30th Jul 2020, 11:25 am   #14
brian_mk
Tetrode
 
Join Date: Mar 2015
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 69
Default Re: Marconi TF2431 Counter Service Manual / Schematic

From my earlier investigation, I think the schmitt trigger is working as it should.
If I've understood things correctly, the hysteresis is set by the diode forward voltage drop and is about 0.7V.

I'm pretty sure the problem is the interfering signal that I can't get rid of.
The counter makes a pretty good wideband receiver.
If it was FM broadcast, I would have expected to see a whole bunch of frequencies and general noise rather than a fairly clean looking signal that the scope can sync to.
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