3421: any tips on calibrating?


Hi, all;

after a while waiting (some years...) I got my 3421, at last! My friend that had it actually took good care of it and had some personal issues (it took him about seven years to send me the unit... no problem, I can handle it once knowing we are dealing with friendly human beings that may surely have their reasons and circumstances), but anyway I finally have my own 3421!

These 'beasts' are timeless...

Before trying anything, I opened it and saw a 6.3V/2,4Ah sealed lead-acid battery. I also noticed that it came with a multiplexer/actuator assembly board. Needless to say the battery was dead: 1.8Vcc. I did not even try to charge it; the poor thing has already been sent to a proper recycle container, only for batteries (I do my part...). Then I measured the small battery that keeps RAM data and found 3Vcc! Amazing! That one was alive.

Before plugging it into the AC line (I found the mains supply voltage selection quickly), I replaced the lead-acid battery for another (taken from a 9114 battery pack) and the 3421 reacted with a small 'click' from a relay, I guess. I tried the POWER switch and it woke up with ERROR 0 (no RAM data). After plugging it into the AC line, the lead-acid battery voltage raised from plain 6.12Vcc to 7.01Vcc: the power supply seems OK.

I plugged it to an HP41 with MLDL2000 previously loaded with the X-IO and Data Acquisition ROM, and I tried some HPIL commands. The 3421 responded to all acceptable commands as expected, and generated some error messages, also as expected. I also performed the fastest measuring test described in the first pages of the manual: shorting the resistance input terminals and turn the unit ON. Both errors 0 and 7 (shortened input) showed up in the display. INSTAT returned a number to the X-register and set related flags.

Because I never had any HPIL measuring device before, I am still 'trying it out' with a lot of reasonable care. I will take some specific time this weekend to read some material about it: user manual, HP41 Data Acquisition ROM manual and four application notes related to calibration. The 254-page manual will give me the basics, I hope, along with the HP41 data acquisition ROM manual, but if there is something else I should know about, I'd appreciate any info/advise you can give me.

I remember seeing some posts about the 3421 at this very forum many years ago, but back then I neither have my HPIL, nor many stuff I have now (9114 disk driver, MLDL2000, Clonix/NoVRAM, documentation, knowledge basis... thanks to many of you, guys!), so I just read those messages briefly. I am now collecting many of them and I'll read them prior to go ahead.

I am positive it will take me some time, but hey! Better now then never. This is my first HPIL measuring device, I really want to go ahead using it accordingly.

Thanks a lot, from a 3421 'newbie'.

Luiz (Brazil)

Edited: 23 July 2010, 11:22 p.m.


Hi Luiz,

Indeed, those HP3421A's are timeless.
I have two of them, that I frequently use, most of the times as MUX in front of an HP34401A,
under HP-IB control. But sometimes on Battery under HP-71B control (battery charge measurements)
One of them also has the Self Test Failed #0 (Calibration RAM Checksum error).
I have so far not been able to find out what the cause is, nor how to fix it.
There is a remark in the Service Manual "Error #0 will occur if an option 442A 10 channel multiplexer
Assembly is added or moved to another slot after the 3421A was calibrated".
I added a second MUX to slot 1, still #0, and a third to slot 1, still #0.
So I only used a MUX in Slot 0, and did the Temperature Calibration (in a faint attempt hoping that due
to this the correct Checksum would be stored..) but to no avail.

To do a full Calibration you will need at least a 6 1/2 Digit DAM, and a lot of time.



Dank je, RamLab!

I was afraid I'd have to spend some considerable amount of time calibrating this 'little' beast. I began reading the maintenance and calibration manual but I decided to ask if there is anything about it that was not written there. No problem, I'll just go ahead reading it.

I do not have a 6 1/2 digit measuring device, but I'll find a way to get this done. There is a 17-page operating note (03421-90011), page #5 and ahead, describing some calibrating procedures that predict the possibility of using the 3421 internal voltmeter to do so, but there is a Temperature Calibrator Board (03421-66505) that must be plugged onto the Multiplexer/Actuator Assembly. I do not have it and I am not aware of what such temperature calibrator board has in it. Given that currently available components and devices may achieve accuracy better than the ones form the 80's, I thought I could at least try building an equivalent calibrator board even if I do not achieve the same HP standard. Would you have any info about it?

Thanks again.

Luiz (Brazilie)


Hello Luiz,

It all depends on what you want to do with the 3421, and what causes the #0 Self test error.
Are you intending to measure Temperature with it, do you have Thermocouples, the correct connecting block ?

What do you get back from the 3421 when you send "REF" ??
This should return the temperature of the temp sensor on the MUX board in Slot 0.
Do you get Temperature reading, an error, other than #0

Doing a Temperature Calibration is actually simple if you have a Fluke 51/2/3 or Fluke 16 with a K-couple.
Place the K-couple as in the manual described for the Thermistor.
Simple follow the instructions : "REF--" followed by "C00----" and fill the Temperature reading of the Fluke on the blacks.



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