On my Keithley 177 bench DMM, there's a 3.75-turn pot on the front panel for zeroing, that feels like it's either broke/bent or that there's action happening like on a rotary dial switch, turn-lock/turn-lock. It doesn't seem normal, or at regular interval as such, and is kind of hard to turn. This is a 1982 DMM, so that's plenty of time for it to get damaged. Physically it looks brand new, the plastic lid seems glued or wielded, so IDK. There's an outer ring w/retaining clip that turns a bit too if the shaft is turned. Again its a 3.75turn pot, so it has some "gears" or something........and they feel horrible, and tight too. It sort of looks bent, too late properly check now, I already put it back in the DMM for now.

In the datasheet it says continuous turn, this very much likes to rest in spots, and again it feels horrible. It seems to zero ok (on the ranges it's meant too)
https://www.bourns.com/pdfs/385126.pdf

 

Hello,

You can find some manuals and pictures on this page:
http://www.jabse.com/L0tlaXRobGV5LzE3Nw==

Bertus

 

Well I'm really proud now because I used the right tools (almost) and was able to make a continuity buzzer mod with a DPDT SW, a green LED to let me know it's on and a bright blue LED for continuity. And a piezo-buzzer with volume pot. And it looks beautiful, just like if it left the factory with this built in.

When all the selector switches are "off", the + term. is connected by DC to the 10Meg R. So I just tap an on/off SW right onto that node w/ all SW's "off". And I leave the 10Meg connected. For power I connected to the DMM's +5V rail which is a 3-terminal regulator.

I used a basic positive feed back, level shifting LM358 circuit. It drives the blue LED BJT, and also a BJT controlling the base voltage supply, of 2 BJT's in a multi-vibrator, that drives another BJT for the buzzer.

I made it on breadboard, then proto-bb. Then either had a problem I never calculated for, or had a bad connection, and I think it was I had a 10uF cap on the 5V reg. and that didn't work well for the buzzer. I wondered if I should have used the +15V rail, or if I had a short.

Then I BLEW UP the op-amp, dangling wires hit the live -15V rail. So I fixed the circuit and used a 220nF cap as my filter. And when the oscillator runs, it works perfect again, and also reduces the resultant spikes.

So then I finally put it all in place, but never glued anything, I plan to mod this a lot more.

I set the threshold with a 1k ten turn pot. And I can get it set so that with probes, it can buzz off 3R but not 4R. So if I have bad aligator probes w/ 1R, I should still be OK and not have to open the dam thing again.


Now after building it as an engineer this time, afterwards, I calculated a few pages and for the op-amp I found the open loop gain, the 2 closed loop states, and now I'm working the doing that for the oscillator. And it all more or less matches what my equipment says and LTspice said, so that's great.

I added a reverse polartiy diode across the Vcc too, maybe the opamp will survive next time. I should add a fine resolution pot, across the main 1, for the threshold on the outside....excellent.

Now I want to experiment with this DMM more and make a circuit that will scale the signal and feed it into the ADC so I can see how close I am to a short.

Then I want to add a diode tester circuit, or use the ohms range and do it.

I also want to either make analog circuitry to control the switching, w/ logic gates or chips, or later use a MCU

Then I want to experiment with Arduino on this thing. If that's goes well, I could add a whole new bigger screen, although the 1 that's on here is great, it's a big bright red 4-1/2d 20,000 count LED display.