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Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)

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joeqsmith:
I just finished tracing it out when I noticed your post.  Indeed, it seems very close if not the correct one.   I suspect I made a few mistakes tracing it out.  I was jotting down the part values and part numbers as I was going through it and measuring them.  Nothing seems out too far. 

Thank you very much for finding this. Time to double check my work.   

SilverSolder:
The circuit for flipping relay K1 is very psychedelic - not sure I understand how it actually flips it, rather than just pull in both directions at the same time! :D

[Edit]  Does it flip ON with a negative going edge, and OFF with a positive going one (from U3)?

joeqsmith:

--- Quote from: SilverSolder on January 24, 2021, 07:34:07 pm ---The circuit for flipping relay K1 is very psychedelic - not sure I understand how it actually flips it, rather than just pull in both directions at the same time! :D

[Edit]  Does it flip ON with a negative going edge, and OFF with a positive going one (from U3)?

--- End quote ---

Matches.  Maybe my hand drawn schematics will help.

joeqsmith:
I wonder why they added the bootstrap for Q3 but nothing for Q2.  The circuit all appears fine.  Nothing damaged.  It's almost like the slow CMOS switching times won't provide enough drive to the transistors to turn them on.   Time for a little tweak to their design.   

SilverSolder:

--- Quote from: joeqsmith on January 24, 2021, 10:52:54 pm ---I wonder why they added the bootstrap for Q3 but nothing for Q2.  The circuit all appears fine.  Nothing damaged.  It's almost like the slow CMOS switching times won't provide enough drive to the transistors to turn them on.   Time for a little tweak to their design.

--- End quote ---

I take it that "Set" is working, but not "Reset"?

C24 losing capacitance? 

Could the relay be getting "stiff", so the pulse isn't big enough to flip it over any longer (it needs a longer pulse)?

The relay may not completely symmetrical in how much pulse energy it takes to set vs reset, necessitating the difference in circuitry?

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