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Old 19th Jul 2017, 1:46 pm   #1
MrBungle
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Default HP 427A resistance drift diagnostic strategy?

I've got an HP 427A voltmeter. It's absolutely spot on accurate on DC ranges. However if I leave it to settle for an hour, cal the ohms range with a decent quality 0.1% 1k RN55D and leave it a couple of hours, the calibration drifts off. The DC cal remains fine. I'm not talking about the mid point calibration either this is the range as set by the ohms calibration pots (A1R18, A1R20)

Does anyone have any ideas on how to diagnose this or have any ideas on what to test? I've tested the resistors in the range selector and they are spot on, the entire DC metering circuit works and the power supply seems to be fine too.
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Old 19th Jul 2017, 2:49 pm   #2
kevinaston1
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Default Re: HP 427A resistance drift diagnostic strategy?

According to the service manual, the d.c./ohms circuitry remains the same, only the ohms range attenuators are changing.

So, provided there is no drift on the d.c ranges, there must be a change in R12 to R25, or the switching contacts.


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Old 19th Jul 2017, 2:51 pm   #3
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Default Re: HP 427A resistance drift diagnostic strategy?

I've measured them with hair dryer on and off and all the resistors are fine. switch contacts all read milliohms. I'm stumped.

Is it possible the ohms cal trimmers are knackered?
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Old 20th Jul 2017, 9:53 am   #4
dave cox
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Default Re: HP 427A resistance drift diagnostic strategy?

Guess you've check the power supply is not drifting ?
Its only 1mA (?) into 1K so self heating seeems unlikely !
I would connect a 'known to be good high impedance dc meter' accross the test R - this should at least narrow it down.

dc
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Old 20th Jul 2017, 10:39 am   #5
MrBungle
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Default Re: HP 427A resistance drift diagnostic strategy?

Yeah the power supply is surprisingly rock solid. Not even noisy.

Good idea. That should at least identify current changes. I've got an HP3478 which has a 3v range impedance of 100M which is considerably higher than 1k. Will do that. Thanks again.
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Old 22nd Jul 2017, 1:55 pm   #6
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Default Re: HP 427A resistance drift diagnostic strategy?

I did what you suggested and found a tiny amount of drift! Turns out the power supply was faulty, I had just failed to measure it being trouble. I have replaced the main smoothing capacitor and it's fine now. Thank you
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Old 22nd Jul 2017, 3:00 pm   #7
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Default Re: HP 427A resistance drift diagnostic strategy?

Definitely not drifting now. After an hour with a 1k 0.1% RN55C from Dale:

Click image for larger version

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This is rapidly becoming my favourite meter.
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