Check your 34401A multimeter

[dimmog]

I picked up another 34401A from ebay. This unit was rejected by the cal lab with 'failed 1000V adjustment’ sticker on it. It passed the self-test and all modes and ranges worked fine.
So I hooked it up to the calibrator and fed 500V. The overcurrent protection tripped immediately in my calibrator and multimeter went out completely.
I cycled the power and now it would not turn on at all. After digging quite a bit I found 5V VCC rail almost shorted to ground. Ok FLIR time and sure enough the U150 was about 170°C.
There was no more obviously bad or fried components or anything like that so I replaced the U150. The DMM worked again. I hooked it up to the calibrator once again and started
increasing the voltage bit by bit. At around 450V it happened again with exactly the same result - the U150 was dead. Then I started a complete and thorough check of the frontend
and I found the culprit. This was the first time in my experience.

The K101 had a very small burnt spot on the case where it touched the E100 surge arrestor (with one side of it connected to the HI terminal).
I checked the K101 relay and metal shield / housing had resistance to the coil about 14Ω. So it was a double breakdown between the E100 and relay housing and also between relay housing and coil!
Perhaps the prime cause was too little clearance between the E100 and the K101 and some sort of vibration rubbed the paint of the relay housing and initial high voltage breakdown occurred.
Now I will check the clearance in each 34401A and encourage you to do so as well!

[AVGresponding]

My Agilent branded one with a build date of January 2013 has a much better clearance:

[dimmog]

My Agilent branded one with a build date of January 2013 has a much better clearance:

Yes, they slightly shifted by 3mm the K101 and K104 in the latest board revision. But in this revision K101 now touches R122. 

[AVGresponding]

[Miti]

I have two 34401A and both tested positive for this issue, one worse than the other. Unfortunately there's not much I can do about that, the surge arrestor is jammed between the relay and the switch. I wonder if a piece of Kapton tape would help.

[Kean]

I wonder if a piece of Kapton tape would help.

Kapton tape (at least the genuine 3M 3mil thick stuff) is rated for 7.5kV, so it should be effective if you can slide it in.
You can get thicker non-adhesive Kapton/Polyimide sheet as well.  I have some 6mil/0.15mm thick sheet which I bought on AliExpress.

[dimmog]

Confirmed.

And now I'm really worried. I just opened a random 34401A and K101 relay has similar damage.

[McJim56]

By golly, I had best check mine now. Thanks for the tip.

[artag]

> 34401A Facebook group
> http://www.facebook.com/groups/34401A

Multimeters have a facebook group.  What's wrong with an IEEE-488 bus like any other self-respecting instrument ?

[factory]

I don't use facebook multimeters, too much noise in the measurements.

David

[coromonadalix]

  

[Swake]

Verified an HP with serial US3606xxxx and revision 09-05-02 and same thing. Have added a double layer of kapton tape in between. Could not see if the coto had any damage without removing it because there might have been room for a third layer of kapton but certainly not a forth.

[robert.rozee]

just added insulation to my HP34401A (sn 3146A78567).

i used some 0.2mm thick insulating sheet that had been recovered from the HV backlight driver board on an old LCD TV. from this i cut a strip 15mm wide and 71mm long, folding into a "U" shaped with 35mm at one end, 28mm atthe other end, and 8mm between the two. this slipped down around E100 and was held securely in place just by friction. the 35mm end was against K101 so provided insulation for E101 as well, while the 28mm end provided extra insulation from the adjacent switch spring.

no disassembly was required beyond the three screws holding the back and case sleeve on. see below.


cheers,
rob   :-)

[factory]

Just got round to completing the repair, cleaning & reassembly of my early 90s HP 34401A, I've found the same clearance issue between the relay & GDT, a piece of ex-Tek waveform monitor insulation sheet was cut to size and fitted.

David

Edit: Just had a quick look inside the 1999 edition, it has the same lack of clearance.

[dimmog]

BTW, added another one to my collection.

[BennoG]

My 1997 model has the same problem.
Put some nice isolation between it.
with some glue so it wont shift due to vibration.

Benno

[Sensorcat]

Same here, Agilent model from the early 2000s. Zero clearance.

Does anyone know more about the root cause? Does it start with vibration, which causes the insulation to be damaged, and only then the high voltage creates the failure? Or is high voltage enough to make it fail because of the lack of clearance?

And if it's vibration, transport or operation?

I ask because it is not yet clear to me if fixing this requires only mechanical protection or an insulation for at least 1kV. It is only clear if the insulation of K101 is damaged already.

[Gyro]

Any solid dielectric has a finite life, this depends on aging, voltage stress etc. and it's only a matter of time. By keeping voltage stress at a low percentage of its breakdown voltage the MTBF can be extended to the point that it doesn't cause failure at anything like the expected life of the component or product that it is installed in. Class Y caps are a good example of this.

The ADuM4160 USB isolator datasheet has a nice concise summary of this in the Insulation Lifetime section (page 12 and Table 8 )...  https://www.analog.com/media/en/technical-documentation/data-sheets/adum4160.pdf

On that PCB revision of the 34401A, you have a an uncharacteristic HP major cock-up. The GDT is in contact with the metal case of K101, separated only by a poorly controlled layer of paint, which might even have pinholes. If it is never exposed to more than, say, 20V the MTBF of that paint insulation layer could well be so long that it is never noticed (just think how many 34401As there are out there that will never see a problem). Start exposing it to hundreds of volts and that MTBF of the paint layer will plummet.

Yes, things like vibration, pressure of the GDT against to paint will affect things (although the flow soldering process would have limited this) may affect things, but ultimately it is just a matter of voltage dependent time. The important thing is to make sure that the breakdown voltage of any added insulation fix is a decent multiple of the measurement voltage it will see.