Keithley 177 DMM bad 20k problem

Thread Starter

DarthVolta

Joined Jan 27, 2015
243
I have this meter and noticed that when on ohms, 20k, when no load is attached, the reading is not off limit like the other ranges are, it's near zero, wandering a bit, so it's reading near a short. And when I attach a resistor, the reading drops to practically zero, with hardly any wandering, so is some transistor or op amp not driving something ? I looked at the schematic, I went cross eyed with the switches.

When on volts, 20V works, so it's probably not the switch. I haven't checked the ampmeter yet, now I have a bench PSU so it should be easy, I forgot.
 

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dl324

Joined Mar 30, 2015
9,986
When on volts, 20V works, so it's probably not the switch.
The V and R range settings probably use different contacts in the switch, so it could still be the switch. One of my DVM's has a switch that's only flakey in the 20VDC position.
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
243
You know I was probably reading it wrong, not really remembering how a 20,000 count display changes. Now it seems to be working. It wanders by 1-3counts on most ohm's and voltage settings. It seems good at VDC and ohms, so far bad on amps tho.

I had this for ages but barely touched it because I never had a good PSU. So I calibrated some of it.

Anyone know if the cuurrent zeroing knob on the front left of the LCD, is suppised to turn smoothly or should it feel 'notched' at odd intervals? Or is my knob broke?. I don't own any other test equipment or poteniometers that turn weird like this one, maybe it's bent.
 

Thread Starter

DarthVolta

Joined Jan 27, 2015
243
The 200uA and 20uA range are both off by 1/2, so right now 8mV/1kohm= 8uA but on this the burden voltage is 4mV and it reads as 4uA.

I reads pretty good as 80uV on the higher ranges. I need to use some deoxit on the switches, but IDK if that really gets in these type or not. There's a big bank of them, looks hard to take apart. I had the pin for the - terminal crack off. I had to crave into the hard plastic to get enough lead to solder onto.


So I had the 20mA range zeroed as it says, what would make the 20u and 200uA range off by 1/2 ? It should have a way lower burden voltage, no way did it sell like it
 
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