Re: 465B won't trigger

Fabio Trevisan

Ah, in time... about your writing:

On Wed, Apr 25, 2018 at 12:28 pm, blackholenulled wrote:

I'm still suspicious about the readings on pins 1 and 16. It will be
interesting to see if they change when biasing 2 and 3 with the resistor.
Since between pins 2,3 and pin 1 there's only the base-emitter junction of the IC's input transistor... if the input is at 0,7V or so, it's expected that pin 1 will rise to about 0V, instead of staying at the nominal -0.8V or so... This even testimony in favor of the IC, that its input transistor isn't defective.
As for pin 16, since this is a differential input pair, the upper side being heavily (postively) biased, it's natural that it will bring the emitter of the complementary xsistor up... In fact, this is exactly what the differential input pair is meant to do.
One may be fooled by the fact (so often found on Oscilloscope's Vertical and Trigger amplifier circuits, ICs or discrete) that the transistors don't have their emitters tied together to a single "long tail" resistor and thus, not spotting right away a differential amp., but Y connected emitter resistors and Delta connected are mutually convertible and the set is indeed a differential input pair.
If one is heavily driven into conduction, the other will be driven into cutoff from its emitter.

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