HP 3400A - indicator needle behavior on range change - Page 1
Forum section: electronics / testgear

author: macfly
author_rank: Contributor
timestamp: November 08, 2015, 11:03:42 am
content: Hello,some weeks ago I bought a HP 3400A TRMS-Millivoltmeter.Yesterday I put power on and looked what happened. The indicator needle runs over the scale and settled then on zero. Okay, as expected.But when I changed the measurement range, the needle jumps from full left to full right and then back to zero. This repeats on every range change.Is this a normal behavior of the 3400A ?RegardsMacfly

author: calmtron
author_rank: Regular Contributor
timestamp: November 08, 2015, 12:03:19 pm
content: I believe that this is normal behaviour, I have two 3400A:s behaving similarly.

author: Gyro
author_rank: Super Contributor
timestamp: November 08, 2015, 01:50:21 pm
content: Yep so does mine, normal behavior.

author: macfly
author_rank: Contributor
timestamp: November 08, 2015, 05:53:33 pm
content: Hi calmtron, hi gyro,thanks for your answers. These are good newsRegards,Macfly

author: Rupunzell
author_rank: Frequent Contributor
timestamp: November 08, 2015, 06:19:04 pm
content: Keep in mind the hp3400A is a thermal converter based RMS voltmeter. Know the thermal converter and related system needs some time to settle into the measuring value. This is one of the classic thermal converter based RMS voltmeters with spec'ed accuracy to 10Mhz and up to 100:1 crest factor. There are no hand held DVMs and very, few bench DVMs with this capability.Bernice

author: Gyro
author_rank: Super Contributor
timestamp: November 08, 2015, 06:44:08 pm
content: One potential trap for the unwary though (no pun intended). The HP3400A has an input impedance of 10M ohms, not the 1M ohm that you might be expecting. Beware of this if you try to hook a scope probe to it, your reading wont be what you expect (10:1 probe will act more like a 2:1 probe but the capacitance matching will be all wrong).

author: dacman
author_rank: Frequent Contributor
timestamp: November 08, 2015, 08:49:23 pm
content: If the 3400A is jumping between every range change then the range switch would (usually) need to be cleaned. (See table 5-5 in the manufacturer's manual, although it points out something else.) I use DeoxIT and LPS-1 (and foam swabs). (LPS-1 1st, then if that doesn't work, DoexIT and LPS-1.) It doesn't surprise me that all your meters are jumpy, if the range switch has never been cleaned.

author: SteveH
author_rank: Contributor
timestamp: November 09, 2015, 08:00:27 am
content: I have a NOS 3400B with relay range switching that does the same. The swing to FSD is very slow and well damped as is the return back to zero.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: November 09, 2015, 07:18:58 pm
content: Quote from: dacman on November 08, 2015, 08:49:23 pmI'd be damn careful not to soak my switch wafers with anything other than IPA. DeOxit will leave a nice permanent load of oily stuff in there. Might affect your 10mhz readings. just a thought...

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: November 09, 2015, 07:33:52 pm
content: 100% IPA... not rubbing IPA!

author: dom0
author_rank: Super Contributor
timestamp: November 09, 2015, 09:20:46 pm
content: Quote from: Gyro on November 08, 2015, 06:44:08 pmIt also has a somewhat high input capacitance (60 pF or so). Measured with a 50 ? source - as usual - the -3 dB bandwidth (the 10 MHz spec refers to +- 10 % iirc -- -3 dB bandwidth is more interesting for noise measurements) of my units are 25 and 27 MHz, respectively. It's frequency response is roughly 1st order to at least 50 MHz.

author: dacman
author_rank: Frequent Contributor
timestamp: November 10, 2015, 02:23:31 am
content: Quote from: SoundTech-LG on November 09, 2015, 07:18:58 pmIt has been my experience that if I need to repair the switch, and I just clean and lubricate it, then the unit will not pass frequency response that day, but it will the next. I've also been taught that if IPA or contact cleaner is used, then it should be lubricated afterward else it can be destroyed due to lack of lubrication.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: November 10, 2015, 08:06:13 pm
content: As long as you limit, and keep the oily stuff on the actual contact itself, and not filling up the porous insulation of the wafer, you should be good to go, but how to successfully accomplish that task is another good question. :-//If you just use the meter for below 20kHz, in low impedance tests, probably not a huge issue.

author: dacman
author_rank: Frequent Contributor
timestamp: November 11, 2015, 01:31:09 am
content: You seem to be concerned with loosing the calibration. Where I work, we have the equipment to calibrate the 3400A. The switch can be drenched in LPS-1, blown out with compressed air, let dry (I like to leave a small fan blowing on it), and the switch should then give no trouble for many years. I don't like to leave DeoxIT on a switch, because it can turn the contacts green, so I flush the DeoxIT with LPS-1.http://www.lpslabs.com/product-details/560

author: r6502
author_rank: Regular Contributor
timestamp: February 01, 2020, 10:16:44 pm
content: Hello all,I just bought also a 3400a, and found a strange behaviour, in the 1mV / 3mV range of the meter.When I apply a frequency, close to the line frequency, that is also used for the internal chopper amplifier - here in germany it's 50Hz - the pointer of the meter is oszillating a lot. In the 1 mv range up to +/- 0,25 mv. When I change the frequency away from the line frequency, this phenomen decreases. The phenomen is alo present on the 1st harmonic, so at 100Hz, but not that mutch.In the 10 mV range this phenomen is not so mutch present. From the 30mV range and higher, it is no more visible.Is this normal behaviour or is there something defective in the device?The RMS reading of the 3400a is very simmilar to the measurement of my TEk oszilloscope.Kind regardsGhido