Agilent 34401A Ohms does not Zero with a short across terminals -FIXED !!!!

[daveyk]

Agilent 34401A Ohms does not Zero with a short across terminals.  A short reads about 10 ohms.  All resistance readings are 10 ohms high. 

If I do a NULL it reads zero, but the resistance drifts or bounces.  Turn off and back on the NULL is no longer. 

This one came back from calibration as not good.  I swapped the main PCB with another one I had with a dimmer display (wanted to keep same external SN).  That is working and is back off for calibration again.  I put the bad board in the one with the dimmer display and it has the same problem.  Ohms does not zero with a short.

All other functions seems to be fine.

I'm looking at Q203->Q210.  All the PNPs seem to check okay in-circuit.  The manual points to a failed Q202 or Q211 (I can't figure out what that one is doing other than used for gate leakage current).   

Q202 is a MMBF5461 that I can not find anywhere. 

Is this a common failure, and of so, what JFET is a good substitute.  If that would not fix it, I don't know what else to look at.

[bdunham7]

I'm not sure why you are looking at the ohms current source for this fault without checking the test current in ohms mode.  If you select the 2W 100R range manually you should see ~1mA and about 6.8V open circuit.  If you get that, the entire ohms current is not your problem.  If you're reading 10R in the 100R range with a short, that indicates that there is about 10mV on the sense circuit. 
 
Try testing with a 4W short in 4W mode (no need to get fancy, test lead will do) and see if you get the same result.  Also, check the 100mV range and see what you have with the input shorted.  Unfortunately, you may have an issue with U101.

[Martin72]

I'm looking at Q203->Q210.

 

[daveyk]

"I'm not sure why you are looking at the ohms current source for this fault"

Cause I don't know what I am doing trying to trouble-shoot this problem.  That is why I am here. 8-)

" without checking the test current in ohms mode.  If you select the 2W 100R range manually you should see ~1mA and about 6.8V open circuit. "

in 100R manual range, I am seeing .997ma and 6.85volts.  This is what you say to expect.

"If you're reading 10R in the 100R range with a short, that indicates that there is about 10mV on the sense circuit. "

Where do I start looking for that?  C102 (on HiSense) has 31.2mv on it (no resistance load).

The reference voltages are all good.

Does the HiSense go to AMP+ in to the input amp on the 3rd page of the schematic (same page with the resistance current source)?

[daveyk]

P.S. 4-wire mode works perfect.

https://www.utsource.net/sch/1SK6-0001.html   $25

[daveyk]

More info that may help:

As measured from the negative input to U101-5:  the resistance value that is floating around matches the voltage reading, in mv, at U101-5

ex: input shorted, meter reads 4.6 ohms, voltage at U101-5 reads .0046volts.

[Kleinstein]

If the 4 wire mode works perfect, the problem is likely with the front-rear switch or maybe a realy that is in the path for 2 wire ohm. A first try / check would be repeated (e.g. 10-20 x) switching and look if the reading with a short gets lower. I would be as little as dirty contacts that can clean from use.

[bdunham7]

Does the HiSense go to AMP+ in to the input amp on the 3rd page of the schematic (same page with the resistance current source)?

No, look at the previous schematic, 9-9 Function Switching.  You'll see that the HI_SENSE and LO_SENSE are both switched by U101.  It measures both and then calculates the difference, or IOW the autozero uses the LO-SENSE rather than AGND.  So the input (sense) circuit for 4W is completely different than the 2W.  There's also a function switching chart on p.95 of the service manual, Ch. 5 (Theory of Operation) that explains this.

As measured from the negative input to U101-5:  the resistance value that is floating around matches the voltage reading, in mv, at U101-5

If you short the inputs in the 100mVDC range, what do you get?  If it is acceptably close to zero, then you may just have some resistance somewhere in the circuit that shouldn't be there.  The Front/Rear switch and K101 are probably your main suspects at that point. 

[Dr. Frank]

Yes, at the very first, please check the front / rear switch!
Put a short on the rear INPUT jacks as well, select 2W Ohm, and switch to REAR.
If there's a significant difference, or if the reading changes strongly when you actuate the front/rear switch, then latter is the culprit.
Cleaning / Repair has been described already, somewhere here.

Next candidate would be Relay K101, whether pins 3/4 are lo Ohm.

Meanwhile, please hands - off the transistors and the current source!

Frank 

[daveyk]

I think I have it.

If I short out the contacts of K101, the problem goes away and it measures resistance correctly in two wire mode.

Thank you kindly for your assistance.  Now to find another relay or see if I could do some surgery (doubtful).

[daveyk]

If I short out the contacts of K101, the problem goes away and it measures resistance correctly in two wire mode.

Thank you.

[daveyk]

After some research on the innerwebs, I ordered a Koto 3501­-05­-511 from Mouser to replace K101.  It was the closest I could find that I think the pin pattern will match.  We'll see.  It is only rated at 200v, where as the one in the service manual shows 500volts.

I can find a "pull" on ebay, but what's the odds it is good?

[bdunham7]

I can find a "pull" on ebay, but what's the odds it is good?

There is some risk with a used relay, but I'm not sure a lower voltage replacement is a good idea either.  You can try few alternatives:

-contacting Coto (p/n 3200-0121) and Keysight (p/n 0490-1914) directly

-contacting the NSN resellers (this might be expensive) with NSN 5945-01-396-3010

-wait for one to show up on eBay

[AVGresponding]

After some research on the innerwebs, I ordered a Koto 3501­-05­-511 from Mouser to replace K101.  It was the closest I could find that I think the pin pattern will match.  We'll see.  It is only rated at 200v, where as the one in the service manual shows 500volts.

I can find a "pull" on ebay, but what's the odds it is good?

You'd be better off finding one that matches coil current and voltage, and contact ratings. You can always make an adapter board, if you can't source a new COTO one.

[Dr. Frank]

After some research on the innerwebs, I ordered a Koto 3501­-05­-511 from Mouser to replace K101.  It was the closest I could find that I think the pin pattern will match.  We'll see.  It is only rated at 200v, where as the one in the service manual shows 500volts.

I can find a "pull" on ebay, but what's the odds it is good?

If I remember correctly, we had this exact same discussion recently, with the same error of K101.
The HV version of the relays was unobtainium, no chance asking COTO, and the other guy asked, if the degradation of the voltage rating would be OK.

My opinion: For personal / non professional use, that's fine, if you don't use the DMM at higher voltages.
Don't sell it to other people, then it's also fine.

Frank

[daveyk]

"My opinion: For personal / non professional use, that's fine, if you don't use the DMM at higher voltages.
Don't sell it to other people, then it's also fine."

I do not read voltages about 16 volts with the 34401a.  I basically use it as a A/D converter, HOWEVER, it does get certified once per year.

I wonder how much voltage does show up ACROSS the contacts?  There is a lot of resistance, in the HIGH input network. In DC mode, it looses its ground reference with K102 open. In DC mode it looks like about 10Meg Ohm to ground.  Alas we are dealing with voltage and not current.  I don't know how much that 10MOHM resistance discourages arcing.

Someone else had a suggestion.  I am going to look for a reed relay that can handle some voltage and make an adaptor PCB.

[daveyk]

"You'd be better off finding one that matches coil current and voltage, and contact ratings. You can always make an adapter board, if you can't source a new COTO one."

That's a good idea.  Here is one possibility:
https://www.mouser.com/ProductDetail/MEDER-electronic-Standex/BT05-2A66?qs=Vo7e0yZOYdGseWOcqf%252B4Ow%3D%3D

Contact resistance is rate at 150mOhm.  I think the original had a rating of .200 ohms max.  So, it sounds like it should work with an adaptor pcb.  Will an adaptor PCB introduce any issues? (noise/oscillations)?

[daveyk]

"If I remember correctly, we had this exact same discussion recently, with the same error of K101."

Dr. Frank, what do you think about this one with an adaptor board?
https://www.mouser.com/ProductDetail/MEDER-electronic-Standex/BT05-2A66?qs=Vo7e0yZOYdGseWOcqf%252B4Ow%3D%3D

[alm]

I do not read voltages about 16 volts with the 34401a.  I basically use it as a A/D converter, HOWEVER, it does get certified once per year.

Is this certification limited to 16V? Normally a cal lab will test all the ranges up to their max value, e. g. 1000 VDC and 750V AC up to 100 kHz. I think you should at least clearly mark it for its limited input voltage on the front panel and notify the cal lab when you send it in if your argument is "I only use it up to 16V".

[daveyk]

"Is this certification limited to 16V? Normally a cal lab will test all the ranges up to their max value, e. g. 1000 VDC and 750V AC up to 100 kHz. I think you should at least clearly mark it for its limited input voltage on the front panel and notify the cal lab when you send it in if your argument is "I only use it up to 16V"."

Where does it get its 1000volt input now?  That relay may have two reed elements in series, but the relay, in the service manual, is only rated at 500v?

I am also trying a 500v reed relay with an adapter pcb.  We'll see how that goes.  Again the relay is only rated at 500volts, the same as the HP/Agilent but 1000volts in to the meter.

If anything, I would have the lab limit their voltage cal to 200volts DC.

Dave

[Dr. Frank]

"If I remember correctly, we had this exact same discussion recently, with the same error of K101."

Dr. Frank, what do you think about this one with an adaptor board?
https://www.mouser.com/ProductDetail/MEDER-electronic-Standex/BT05-2A66?qs=Vo7e0yZOYdGseWOcqf%252B4Ow%3D%3D

.. looks fine ... if some time in the future my own, 30 y/o HP34401A will show this error.. I'll remember this 

[daveyk]

".. looks fine ..."

Thank you.  It should be delivered Friday.  I will be anxious to install it and test. 8-)

[yo0]

same problem in the past, still working fine

https://www.eevblog.com/forum/repair/agilent-34401a-2w-drift-and-high-readings-when-power-up/msg1270277/#msg1270277

best regards

Pio

[HighVoltage]

Years ago, I was able to buy the original relay from Keysight.
Who knows, maybe it is still available, even if it is not listed anymore.
I would ask Keysight.

Coto will not sell it, because it was made for Keysight / Agilent.

[AVGresponding]

"You'd be better off finding one that matches coil current and voltage, and contact ratings. You can always make an adapter board, if you can't source a new COTO one."

That's a good idea.  Here is one possibility:
https://www.mouser.com/ProductDetail/MEDER-electronic-Standex/BT05-2A66?qs=Vo7e0yZOYdGseWOcqf%252B4Ow%3D%3D

Contact resistance is rate at 150mOhm.  I think the original had a rating of .200 ohms max.  So, it sounds like it should work with an adaptor pcb.  Will an adaptor PCB introduce any issues? (noise/oscillations)?

To some degree, it's a case of "suck it and see". If you keep the traces and leads as short as possible, it'll reduce the chances, at least.