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Keithley 197A owners' corner
ecclectrics:
Here is another happy new owner of a Keithley 197 (without the A). Got it from an ebay auction. Upon first tests it seems to work fine. Don't have the means (yet;) to verify its full specs.
There is just one thing I'd like to ask about: As soon as I plug in the power chord, even before I push the power-on button, it starts to hum quite noticeably. It doesn't sound really bad,
but I find it a bit annoying. Is this normal according to your experience? Is there anything one can do about it? Short of pulling the power plug or adding another switch in in the power
chord of course.
Thanks for all feedback,
Roland
indy:
Hi all,
I hope can find some help here.
I've a Keithley 197 which shows an always positive drift on the last 2~3 digits, It's seems to stop after ~1 hour if I don't touch any of the front panel controls.
This happens on any range.
I've started to do check of internal nodes and the only weird thing I've found is -V node at -14.5V, it's showing ~200mv of 100hz rectifier cap ripple. This node, from schematic, should be -15V see R132 and VR105 (which should be a 15V zener)
R132 is at nominal value and VR105 was tested good, so I guess there is some unexpected current request from -15V that is able to bring the node too low to make VR105 work.
All other voltages seems clean and at nominal value (-9V, -6.4V, +5V A and D, +10V)
Moreover I've was not able to spot any issue in the measure path or A/D waveform.
Please let me know if -V is expected to be -15V & clean, and any of your thoughts on this issue.
Regards,
Fabio I.
AVGresponding:
--- Quote from: indy on October 24, 2024, 03:29:57 pm ---Hi all,
I hope can find some help here.
I've a Keithley 197 which shows an always positive drift on the last 2~3 digits, It's seems to stop after ~1 hour if I don't touch any of the front panel controls.
This happens on any range.
I've started to do check of internal nodes and the only weird thing I've found is -V node at -14.5V, it's showing ~200mv of 100hz rectifier cap ripple. This node, from schematic, should be -15V see R132 and VR105 (which should be a 15V zener)
R132 is at nominal value and VR105 was tested good, so I guess there is some unexpected current request from -15V that is able to bring the node too low to make VR105 work.
All other voltages seems clean and at nominal value (-9V, -6.4V, +5V A and D, +10V)
Moreover I've was not able to spot any issue in the measure path or A/D waveform.
Please let me know if -V is expected to be -15V & clean, and any of your thoughts on this issue.
Regards,
Fabio I.
--- End quote ---
The manual states a 1 hour warm-up time, to rated accuracy.
Kleinstein:
For the ripply at -V one could check / replace C127. It is possible that the capacitor lost capacity has high ESR. -V does not look like it is used in critical positions and some ripply there may not have any effect. -14.5 V is close enough. It could be just tolerance in the zener, that is likely used with less than the nominal current.
Brian of Romsey:
Hi,
I haven't really been keeping up with this stuff for a while. However today I saw a YT on a display replacement that, while clearly NOT the 197A, is perhaps more generally applicable to display replacement in many older instruments.
It will be interesting to see if other people are able to build on Ian's work and what other instruments he covers.
Cheers, Brian.
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