Agilent U1242B Multimeter Problem

[bsgd]

Hi guys,

So, I just got myself a second hand Agilent U1242B multimeter, but it doesnt work as expected. To better understand the problem, I decided to shoot a video and upload it to youtube. Basically, the meter cannot autorange for some voltages and gets stuck in some weird loop while measuring DC Volts.

https://youtu.be/Ipqk

Any ideas what to look for? I opened it up but couldnt see anything obviously damaged.

[bsgd]

No ideas? 

[BravoV]

I own same meter, and mine doesn't show the problem as yours.

Not sure if this helps, have you try set "Default Factory Settings" ? (read the manual on how to do it)

[AndersJ]

Is there a electrical field of some sort in your environment?
Do the same test outside.

[bsgd]

I own same meter, and mine doesn't show the problem as yours.

Not sure if this helps, have you try set "Default Factory Settings" ? (read the manual on how to do it)

Just tried it but it made no difference

[bsgd]

Is there a electrical field of some sort in your environment?
Do the same test outside.

I have tried it on a few different places (not because of an electrical field, I just have been doing these testes for days now everywhere) but I walys gate the same problem, no matter where I ty it.

[bitseeker]

If you manually range it for that higher voltage, does it measure properly? If not, there may be a problem with that particular range, causing the auto-ranging to fail and retry.

[BravoV]

..... got myself a second hand Agilent U1242B multimeter

I opened it up but couldnt see anything obviously damaged.

If I were you, I will open and pull the main PCB off, and do a thorough inspection probably under high magnification at solder joints at both sides, and try to measure obvious soldering points to see any weird readings.

Who knows it's history as its bought used, probably had been abused like dropped or experienced quite strong G shock.

[bd139]

I noticed that the later model U1241C which I bought is significantly more ruggedised and they rate it for 3m/10 foot drops. U124xB they are silent on. Wonder if this was a common failure mode of the U124xB meters.

Worth contacting Keysight.

Also might be a firmware bug and why that is a second hand meter. Think you can update the later ones. Not sure how though.

[BravoV]

I noticed that the later model U1241C which I bought is significantly more ruggedised and they rate it for 3m/10 foot drops. U124xB they are silent on.

That is interesting, thanks. 

[Fungus]

Hi guys,

So, I just got myself a second hand Agilent U1242B multimeter, but it doesnt work as expected. To better understand the problem, I decided to shoot a video and upload it to youtube. Basically, the meter cannot autorange for some voltages and gets stuck in some weird loop while measuring DC Volts.

It might happen if you're right on the edge and the meter changes impedance between ranges (as some meters do).

[bd139]

This is quite interesting. If I compare to my U1241C, there is a huge amount of hysteresis on range switching. I'd expect the same from that meter. This should eliminate any "hunting". Wonder if there is something dead in the range sensing or some leakage somewhere. Doubt this is impedance related as they are constant 10M meters (C revision has programmable impedance however)

1. If I start at 12V, it jumps into 100V range as expected.
2. If I then slowly decrease voltage, it switches down range to 10V only when the source voltage drops to 9V.
3. Going the other way, it only switches up range to 100V when it hits 10.999 volts.

I only use mine in manual range though to be honest. Same with my bench meter.

Video to compare (excuse potato quality - youtube didn't want to upload high quality from my phone for some reason):



Edit: I suppose the question is: does it work in manual range?

[Fungus]

Hook a potentiometer up to a 24V power supply and find the exact voltage where it happens. That should give you a starting point.

[bsgd]

If you manually range it for that higher voltage, does it measure properly? If not, there may be a problem with that particular range, causing the auto-ranging to fail and retry.

bitseeker,

Looks like there is a defect somewhere, but I have no idea where to start looking. Is anyone familiar with the schematics and workings of these Agilent meters?

https://youtu.be/U_D0DC

[bsgd]

This is quite interesting. If I compare to my U1241C, there is a huge amount of hysteresis on range switching. I'd expect the same from that meter. This should eliminate any "hunting". Wonder if there is something dead in the range sensing or some leakage somewhere. Doubt this is impedance related as they are constant 10M meters (C revision has programmable impedance however)

1. If I start at 12V, it jumps into 100V range as expected.
2. If I then slowly decrease voltage, it switches down range to 10V only when the source voltage drops to 9V.
3. Going the other way, it only switches up range to 100V when it hits 10.999 volts.

I only use mine in manual range though to be honest. Same with my bench meter.

Video to compare (excuse potato quality - youtube didn't want to upload high quality from my phone for some reason):

Edit: I suppose the question is: does it work in manual range?

As you can see from my last video, id does not work in manual range.

Hook a potentiometer up to a 24V power supply and find the exact voltage where it happens. That should give you a starting point.

I noticed that the later model U1241C which I bought is significantly more ruggedised and they rate it for 3m/10 foot drops. U124xB they are silent on. Wonder if this was a common failure mode of the U124xB meters.

Worth contacting Keysight.

Also might be a firmware bug and why that is a second hand meter. Think you can update the later ones. Not sure how though.

I suppose its no a firmware bug but some hardware problem... meter is not under warranty any more

[bsgd]

UPDATE: I just tried with no leads connected and have the same peoblem

[bd139]

I'd check under the switch assembly. Look for worn traces and contacts etc

[bitseeker]

I was afraid that it was a range problem. It could be that a component, such as the resistor network, got damaged while in that range. Modern equipment tends not to have schematics available and I haven't dug into any of my Keysight handhelds to know more specifics.

Perhaps someone such as Kleinstein might know more. Maybe High Voltage would have some input, although I've mostly seen his posts regarding repairs on HP/Agilent bench DMMs.

In any case, hang tight and see who else pops into the thread. At least we've started to narrow down the problem.

Oh, it just occurred to me to also check an AC signal while manually set to the 100V range. The RMS converter makes the AC signal into DC, so I'd expect it to fail as well, just as another point of verification since we don't have a schematic.

[bsgd]

I'd check under the switch assembly. Look for worn traces and contacts etc

I did and the traces look pristine... I cleaned them but it made no difference.

[bitseeker]

OK, interesting that it's fine with the AC. Maybe the TRMS converter also drops the signal down based on the range, thereby eliminating the need to send it through the same range mechanism that the DC goes through.

See if the manual has at least a block diagram. Maybe you can see what's different in the AC vs. DC paths to further narrow down where the problem lies. It kind of sounds like the 100VDC range isn't dividing down the input signal properly and, hence, results in OL.

[bsgd]

OK, interesting that it's fine with the AC. Maybe the TRMS converter also drops the signal down based on the range, thereby eliminating the need to send it through the same range mechanism that the DC goes through.

See if the manual has at least a block diagram. Maybe you can see what's different in the AC vs. DC paths to further narrow down where the problem lies. It kind of sounds like the 100VDC range isn't dividing down the input signal properly and, hence, results in OL.

Thanks bitseeker,

I couldnt find any info about the meter workings, neither a block diagram. Too bad, as I thinks this could be a simple problem but narrowing it down wont be any easy by randomly testing parts on the board 

If anyone has any info that could help, please share 

[Alex P]

My U1242B (from a second hand store) does not show the problem, it works exactly as written here:

...my U1241C...
1. If I start at 12V, it jumps into 100V range as expected.
2. If I then slowly decrease voltage, it switches down range to 10V only when the source voltage drops to 9V.
3. Going the other way, it only switches up range to 100V when it hits 10.999 volts.
...