blown keithley, not sure how  - Page 1
Forum section: electronics / beginners

author: gmac34
author_rank: Regular Contributor
timestamp: September 14, 2023, 11:49:19 pm
content: I'm writing this here since I guess only a beginner can make this mistake. But what mistake? I'm not sure.I was trying to repair an hp 3403C rms voltmeter and I needed to have a higher ac voltage than the 10vpp my signal gen can do to make a calibration.So I took one of my audio amplifiers and used it, connected to a laptop, to generate 25-27v RMS 30hz. The keithley, a lovely 2000, had the duty to measure the ac voltage coming form the amplifier connected to the rear, in parallel with the 3403C, as well as measure a dc voltage on a test point on the 3403C, connected to the front. both referenced to the same ground.At some point during the procedure I noticed the AC voltage reading was out of whack and all of a sudden the keithley cannot pass the selftest, with a lot of errors.Now, I'm not sure how I managed to kill it, but 2 things come to mind:there might have been a moment where 27v RMS were present while I was by mistake in resistance mode, as well as I might have switched between front and rear inputs with a signal present, I don't usually do this, but it was late...I didn't think you could kill one of these bench meters so easily, but indeed I did. what do you think?

author: bdunham7
author_rank: Super Contributor
timestamp: September 15, 2023, 01:03:17 am
content: Quote from: gmac34 on September 14, 2023, 11:49:19 pmCan you describe in detail how that connection was made?Also, you may as well do the basic troubleshooting on the Keithley, sometimes 'lots of errors' translates into a simple problem.

author: gmac34
author_rank: Regular Contributor
timestamp: September 15, 2023, 06:48:28 am
content: this was the setupamplifier and laptop where not connected directly to earth, the amplifier only has a 2 prongs plugbut the chassis ground of the amplifier was still grounded trough it's cold output being connected the hp3403.The test point inside the hp3403 is the output of the AC converter 1v at full scale, referenced to the hp3403 analog ground, in this configuration it is connected to earth and the rest

author: Kleinstein
author_rank: Super Contributor
timestamp: September 15, 2023, 07:12:36 am
content: The protection in the K2000 is not very robust. I part it relies on MOSFETs to block the signal. The circuit shown should not cause problems even if using wrong settings at the meter. The tricky part may be when conncting the parts with possible electrostatic charge - this may damage the protecton and than later overload may cause the damage to the main circuitry so that errors come up.To find the cause of the damage it may help to know what is actually damages. The error messages may give a hint.

author: gmac34
author_rank: Regular Contributor
timestamp: September 15, 2023, 07:40:54 am
content: Sometimes it fails only the 400 bank and sometimes it fails almost everything....Functionality wise- it measures DC in all ranges, a bit noisy but accurate- ac measure consistent (same value) in all ranges but its way off (0.38 vrms instead of 1)- DCI measures consistently across ranges with a 2.5mA offset- resistance seems ok, maybe also a bit noisy but accurateI guess I should ignore the rest of the error for now and check the ac path, as well as voltages around

author: gmac34
author_rank: Regular Contributor
timestamp: September 17, 2023, 10:45:26 am
content: Quote from: Kleinstein on September 15, 2023, 07:12:36 amI took a look around and checked the voltages, they seem ok, but I did find that U105 a switch in the ac circuit is getting very hot (>70deg) so that's faulty, I will order a few DG211BDY to replace it.Given that AC is the only part not working this might be enough to fix the meter. But I wander how should I check the input protection so that this doesn't happen again.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 17, 2023, 12:19:07 pm
content: The DG211 may cause more trouble from too much current drawn from the supply. So there is a chance it could also be responsibel for some other errors.For the protection it may be wirth checking Q101 and Q102 (high voltage MOSFETs) if they are fused short.With a closed case one could measure flowing into the meter when in the 1 or 10 V DC range and applying something like +-30 V externally. Ideally this should be less than some 100 µA. With bad FETs Q101/Q102 this current would be more like in the 1.5 mA range.

author: gmac34
author_rank: Regular Contributor
timestamp: September 17, 2023, 01:57:09 pm
content: I measure less than 20uA with 30v on 1 and 10v ranges

author: gmac34
author_rank: Regular Contributor
timestamp: September 21, 2023, 06:55:02 pm
content: I replaced U105, the symptoms are gone. Still not too convinced by the protection circuit… but I’m also not too eager to stress test itThanks for the help

author: Kleinstein
author_rank: Super Contributor
timestamp: September 21, 2023, 07:02:28 pm
content: 20 µA for the input with 30 V is perfectly OK, especially if that way in both polarities. This means the MOSFET(s) and optocoupler in the protection work as intended. This is about what can be tested for the protection. SO I would not worry very much about the protection.It is not that rare that old CMOS switches like DG211 fail in different circuits - AFAIK they are somewhat prone to latch up, e.g. during turn on / turn off or similar transients and this may destroy them.There is a slight chance that newer DG211B are a bit more robust.