34401A ACI Fault

[WernerFalken]

Good day! I have THREE 34401As with the exact same overarching fault: ACI doesn't work. Tested 6 digit range w/1mA, 100Hz source.

I've tested the following on all three:

• Self Test: Pass
• ACV & DCV @ 5V and 50V
• DCI @ 1mA
• Freq @ 100Hz
• Resistance @ 100, 1000, 10,000, and 100,000 ohms
• Front & Rear Jacks for consistency
• Checked R120 & R121 for grins

All of the above tests had great results... Tried all the different filters for AC... same issue.

I also tested the resistance across Lo & I input terminals: about 0.65ohm resistance when in ACI (all three of the 34401A)...0.75ohm on a 34461A in 3A range

Looking through the schematics and theory of operation, I am not seeing a clear failure mode for this situation - it's not a relay, as all of the other working modes exercise each of them successfully

All I can think of is that *I* am doing something wrong, but I've checked myself with:

• 34461A reports ACI @ ~1mA (that's what I am giving it)
• 3458A reports ACI @ ~1mA
• Fluke 189 reports ACI @ ~1mA

I know I went overkill on validating my source, but really... I have three bad ones, so might as well prove it's not *ME* three times! I mean, how on earth can I end up with three of these things (from different sellers) with the same symptom?

Any words of wisdom on this?

Thank you!

[WernerFalken]

Foul language is coming out of my mouth right now.

Problem solved. It wasn't *ME*, but it was my equipment. My source, apparently, doesn't like 34401As - not 100% why... maybe the 0.65ohm is a deal breaker.

Right before I shut it all down to hit the sack (2:30AM here), I glanced over at my 33522B and thought.... "Hmmmm, I won't really *KNOW* the current level for it w/o doing some math I am not inclined to do right now, but I can see if the 34401As can measure it...."

2Vpp @ 100Hz w/HiZ load gives ~14mA on all three 34401As and my 34461A.

Problem solved. Never mind. Continue on your way... thank you for listening!

Cheers!

[WernerFalken]

Although... 0.65ohm is nowhere near HiZ... still, problem solved.

[JohanH]

34401A doesn't specify lower than 1% or 10 mA as the minimum current on the 1A AC range (footnote 4 in data sheet).

There is an undocumented 10 mA AC range on which you can measure down to 100 µA AC. This can be turned on over the remote interface: DIAG:POKE 25,0,1

See the following forum post for further details:

https://www.eevblog.com/forum/repair/hp34401a-1ma-calibrator-and-10ma-hidden-range/msg2439456/#msg2439456

[WernerFalken]

Those darned footnotes!

Thank you!