Keithley 192 DC Volts issue

[mbear2k]

Hello all.  I have a newly acquired Keithley 192 meter that has an issue with the DC volts function.  I am just starting the troubleshooting process, so bear in mind I'm still sorting...

Software version: E7
The meter has the 1923 IEEE-466 Interface option, but no AC Volt option.

The self test (program eight) passes the Display, Ram and Rom test.  I have tested power supply voltages and all are within spec.  All front panel functions/buttons work and select the functions as they should.  I just realized I have not done a ripple test though...

So - the issue.  Firstly, the Ohms functions work okay.  I have not checked for accuracy, but my sampling of precision resistors indicate it's not far off.  However the DC Volts function is not working.  Measuring a battery shows no indication or change.  Zeroing the meter on ohms works.  Zeroing the meter on DCV works, but even with leads shorted it climbs from zero rather quickly and then jumps all over.  I think an indication that there is imposed DC somewhere is that at .2 and 2 settings, the meter reads OVLD.  Above that it indicates random voltages - not wildly random, but jumping around.

I'm just digging in to the service manual and the tests they outline, but wanted to bounce this off you folks to see if there are any general issues with this model to look for.  A little reading may have me start at the Mux Fet (Q328) and the protection Fets (Q329, Q330)?

Thanks - Mark

 

[mbear2k]

I've made a little progress.  The main 5v digital PS filter cap was bad - all kinds of leakage.  My 5v is now rock solid.  So now I can read a voltage on the DC volts scale, but it is still jumping around and is not accurate.

Note: if I zero the DC volts, it does zero, but immediately starts climbing.  My test battery that is around 1.2v reads 2.2 volts with the trailing digits jumping around.  Applying 10vdc, reads the same thing - 2.2v.

[VintageNut]

Download the instruction manual. There are procedures to diagnose your DMM.


http://www.keithley.com/support/keidoc_searchresult?keyword=192&item_type=All

[wiss]

There will always be averaging as long as the counter reading is within 6/60 counts, on the ohms range noise might not show on the display.

If the reading is consistently low or high I should check for leakage in any of the switching jfets. The voltage over the gate resistors should be single mV or so.

Any chance you could send me a copy of the eprom?

[mbear2k]

The Ohms scale seems to be fine.  The DCV scale does not measure.  I do have the Keithley manual and have been using it to troubleshoot.

One question - on my "20v Reference" I see 28vdc.  All supply voltages check ok.  The right side of R339 is my 2V reference and that measures correctly.  U315 is an LF351.  Passive components measure fine, including R361 and R369 sections.  W304 and W305 jumpers are cut, W303 is intact.



Also - I'd be happy to send the eprom, but need to know how to copy.  I don't have an eprom burner handy.

[wiss]

What are the input voltages to U315?

[mbear2k]

Very late follow-up to this one... I found two leaky FET's as mentioned by wiss and the meter works very well now.

[wiss]