HP3455A DVM Repair

I finally got around to working on this HP3455A DVM, which I did a teardown video of back in 2018. After some bumbling around, I managed to repair it.

[@bassobalalaikka5005]

great video. a notice on 5V TTL logic levels: 0 is any value from 0V to 0.4V in outputs 0.8V in inputs and 1 if from 2.4V to 5V in outputs 2.0V to 5V in inputs giving 400mv margin for the circuitry to drop (not to mention the 5V supply being from 4.5v to 5.5V or 4.75V to 5.25V depending on device family and civil vs military ratings) greetinngs from ex military sergeant doing repairs of military test equipment in 1979-1982 and if you remember how tube triodes work, Nch JFETs behave similarily you need to have VP/VGSoff amount of negative gate vs source voltage to drop channel current to zero (D-S is the channel) in P channel JFETs polarities are reversed.

[@bertoid]

Thanks! Yes, I know that the lower level from the diode-OR circuit was valid, but just noted it as a possible area of trouble. As I'd never done any serious work with JFETs, this was a good learning experience. I've also never done any serious work with vacuum tubes, but I have some large tetrodes that I want to experiment with, one day...

[@gordonwedman3179]

Good job on that repair. I admire your persistence. I had a 3455 for while and then bought a 3456 which I liked better as it had a built-in calibration routine. Eventually sold that and got a 3457a. I will NOT be getting a 3458a, 😁

[@bertoid]

Thanks! I'm very happy with my old boat anchors. I like the LED displays - none of that VFD rubbish! And you'll have very high confidence in your 3457's measurements with those 2 extra digits to check it against!

[@sebastian_harnisch]

I always enjoy repair videos 👍🏻 Keep it up! One thing to mention: the teflon standoffs indicate that this must be a really high impedance node, because why bother if it’s not?

[@bertoid]

Glad you liked it! Yes, it must be very high impedance - when Q1 is on, the multiplex node is the input to the meter, which for the 10v range and below is >10GOhm. (So I should not have been surprised that Q2 could pull it to ground with as high as 100K...)

[@ricksampson6780]

Greetings Bertoid, have you considered selling any excess test equipment?

[@bertoid]

Hi, not at this time - I'm still collecting it!

[@alexgabriel1069]

Thanks! Have a good day! Easy video ranking - P R O M O S M !!!