Keithley 197 no oscillator ?

[MathWizard]

I had a problem with the LCD zebra strip for ages, and this winter I put a paper spacer in to help, and it did. And then over the past week or 2, the display would cut out, usually after a while, as if the DMM had a sleep mode. I didn't try pushing on the srceen, I just turned the DMM off/on, and sometimes only part of the screen would do anything, and then off/on again might make it work. Until a while later, it would go fully blank again.

So I was planning to upgrade the LCD display anyways, to something with a backlight, and I have some dot-matrix type display, that is almost the same size and the LCD dis. So I soldered up a bunch of wires to see how all that will work.

But I see the display problem is not just a bad zebra strip connection.

The main DMM voltage rails are working, but for the LCD board, I have no 82kHz clock. It's supposed to come from another chip, and ultimately from the main Osc Y101, connected to the MC146805E2 microcontroller. Looking at the Osc in/out pins , there's no osc happening. Ages ago I replaced the 10Meg carbon resistor with a nice 1% one, so it's not the resistor.

The MCU has Vdd, it's reset is high for normal running. But I guess without the osc, not much would be running.

So besides reflowing the solder joints, or trying some 8-12MHz crystals, or I could feed a 10Mhz signal in from my signal gen, and see if that gets it's going.

What else might this be ? The main rails are all nicely in spec, and I recapped it 2 winters ago.


U124 is the MC146805, it and the crystal are on page 69
https://www.tek.com/en/manual/keithley-model-197-autoranging-microvolt-dmm-instruction-manual-rev

[coromonadalix]

small schematic from xdevs

Find more on X'devs ... free registration


and you have display substitutes   with oled or others

https://www.eevblog.com/forum/testgear/modding-a-keithley-197/


https://www.eevblog.com/forum/projects/keithley-197-led-display-hack/

[MathWizard]

Ok it was just the quartz crystal that failed. It was ~3.3MHz, and I had a 3.6MHz there, and it worked right away. Thank god it wasn't a rare chip.

Yeah I've been looking at a few upgrade threads on these. I might have tried a VFD display I toyed with, but it uses too much current. I have some common display from an Arduino kit, for now that's what I'll try, but I want something bigger. I should load test the 5V rail, I wouldn't mind a big red LED display, like from old style alarm clocks, or some other Keithley meters., even if I make it myself with single LED's