Damaged 34401A 4-Wire function. OPEN reads as short, or floating resistance.

[Xepherrea]

Hello EEVBLOG, knew I was going to make an account here one day for repair help, but I did not think the road would start from my own fault.

Short story. Went to measure a voltage during an Arduino project, and while in 4-wire, I grabbed the wrong leads and my 34401A's autoranging started freaking out since I had it in some configuration with my power supply (sourcing 12V). I have been trying to assess the range of the damage and I seem to have gotten away with minimal damage.

I have been working on my repair and circuit analysis skills, but I have yet to tackle something like this.

--------------------------FINDINGS--------------------------

• Self-test passes with no errors.
• OPEN 4-wire resistance measurements now read 0 ohms, or floating sometimes when nothing is connected to the input.
  
• 1 MOhm and above float, while 100 kOhm and below short out.

However, 2-wire does work properly as it reads open when nothing is connected. The meter is still reading correctly (though the last digits even in 4-wire seem to be a bit more unstable, this could be attributed to the higher temp and humidity in my room right now as well as the meter not being warmed up fully). This is causing the unit to not auto range properly until a load is connected. I did check the source currents for each range using my 87V and it all checks out. I have some L&N standard resistors which agree very close to their last known values. The open and 4-wire short values are open = 3 mOhm, 4-wire short = 6 mOhm. 1 to 3 mOhm sounds about right with the short have been on and stabilized previosuly.

I believe something is primarily wrong with either the circuit protection or sense circuitry. I have thus far checked:

• CR202 - Acts as it should
• Q211 - I do get a 0.7V drop from Gate to drain/source as the drain/source are shorted in the schematic, but nothing with leads reversed. Approx. 600 kOhm reading from G to DS.
• Q201 - Essentially the same as Q211, with gate to drain, and gate to source reading 0.8V, and approximately 500 kOhm between G/D and G/S.
• Q202 - 0.7V drop from source(high) to gate(low), and drain to gate, but not from gate(high) to drain(low) or source. Approx. 450 kOhm from both drain, and source to gate.
• CR201 - When looking top down, so the single pin is at bottom, the right pin has a 0.7V drop, but the left pin is open in both directions. (Possible culprit?). However I can find no datasheet for it.

I am not really sure where to look from here as I have not found any posts with a similar issue as to mine, but it is always possible I was not using the right key-words.

Any help would be appreciated in narrowing this down. I have no experience with SMD components in my soldering ability yet, and nor do I have tools on me apart from an standard tip on a FX888D, so taking anything out to test individually is not ideal, though I understand I may have to.

I do work in a calibration lab, so I can bring this in this week to have the full verification run on it to assess if anything is out of tolerance, but all seems to be working for the most part right now outside the one issue.

Thank you,

[Patest]

What I have found while calibrating these units (and I have calibrated a lot of them over the years) is that the front / rear input select switch is always the culprit.
Hope this helps
Best regards

[Xepherrea]

I calibrated this unit a few weeks ago and the zeros were all well within tolerance. After my accident I can still manually range fine, and there is no difference in the open/short reading between front and rear, so its something further into the circuitry, just understanding what it could be is where I am getting lost.

[bdunham7]

Are you saying you damaged it with 12 volts?  Was there a lot of current, sparks or anything like that happening?

[Xepherrea]

I wouldn't exptect 12V to do any damage and I was running fairly low current. I had my max draw on my psu set to 100 mA for testing mosfet switching, and my circuit and psu are fine. All I know is that I heard the auto range going crazy switching relays really fast and now the auto ranging doesn't work and the input is now shorted somewhere when open, but seems to read perfectly fine.

[Kleinstein]

When having an open with 4 wire ohm one still needs the connection from sense H to drive H and sense L to drive L = COM. So nothing connected is not valid for 4 wire ohms and may give an undefined result (all the way from overload to negative values).

A realtively easy point to check is the test current in Ohm mode (e.g. use a 2nd meter), ideally with only the meter and maybe a LED in series to test with a little higher voltage (should get the same current).

[bdunham7]

I wouldn't exptect 12V to do any damage and I was running fairly low current. I had my max draw on my psu set to 100 mA for testing mosfet switching, and my circuit and psu are fine. All I know is that I heard the auto range going crazy switching relays really fast and now the auto ranging doesn't work and the input is now shorted somewhere when open, but seems to read perfectly fine.

I would also not expect 12V to do any damage.  Do you know which inputs were connected to the 12V when this happened?

Reading your description I'm not 100% clear what the symptoms are.  Let me restate what I think you mean.

The meter passes a self test and works properly in all ranges except 4W ohms.  In 4W ohms mode, the auto range hunts and any resistance over 1M results in a floating measurement and under 100K results in a near-zero reading.  Or did you mean the 1M and 100K ranges, not an actual test resistance?

You said that you tested the output currents of the resistance ranges with another meter and they are OK.  So if all that is correct, I think the problem has to be in the HI-SENSE or LO-SENSE circuit somewhere.  The first thing to try is to do all the same tests from the rear terminals. 

[Xepherrea]

I would also not expect 12V to do any damage.  Do you know which inputs were connected to the 12V when this happened?

Reading your description I'm not 100% clear what the symptoms are.  Let me restate what I think you mean.

The meter passes a self test and works properly in all ranges except 4W ohms.  In 4W ohms mode, the auto range hunts and any resistance over 1M results in a floating measurement and under 100K results in a near-zero reading.  Or did you mean the 1M and 100K ranges, not an actual test resistance?

You said that you tested the output currents of the resistance ranges with another meter and they are OK.  So if all that is correct, I think the problem has to be in the HI-SENSE or LO-SENSE circuit somewhere.  The first thing to try is to do all the same tests from the rear terminals. 

I unfortunatley do not fully know what was connected as I dropped it all when I heard the relays clicking and disconnected the meter.

The auto range seems to only want to be in the 100 ohm range as it reads zero ohms, but if I manually range its reads zero up to the 100kOhm range. Above that it reads a floating value.

When I am home from work I can report back with both the F/R range currents and compliance voltages. The sense circuitry was my second concern, but since what I have tested thus far matches their readings pre-damage to post damage, I am not sure where the issue directly lies.

I want to say that it possibly could have been connected from PSU high to sense low.

[Xepherrea]

I wouldn't exptect 12V to do any damage and I was running fairly low current. I had my max draw on my psu set to 100 mA for testing mosfet switching, and my circuit and psu are fine. All I know is that I heard the auto range going crazy switching relays really fast and now the auto ranging doesn't work and the input is now shorted somewhere when open, but seems to read perfectly fine.

I would also not expect 12V to do any damage.  Do you know which inputs were connected to the 12V when this happened?

Reading your description I'm not 100% clear what the symptoms are.  Let me restate what I think you mean.

The meter passes a self test and works properly in all ranges except 4W ohms.  In 4W ohms mode, the auto range hunts and any resistance over 1M results in a floating measurement and under 100K results in a near-zero reading.  Or did you mean the 1M and 100K ranges, not an actual test resistance?

You said that you tested the output currents of the resistance ranges with another meter and they are OK.  So if all that is correct, I think the problem has to be in the HI-SENSE or LO-SENSE circuit somewhere.  The first thing to try is to do all the same tests from the rear terminals. 

I have attached the bias current and voltages for each range, both 2W and 4W for front and rear in excel format. The 10/100MOhm ranges are reading low on my 87V in HiRES mode for some reason.

I've been looking around the SENSE Hi and LO traces in the schematic and thus far haven't figured out what could be causing the path to ground. I checked caps C102/103, and C111. I'm getting about 2 nF for each ones reading, so I'm figuring I'm getting some other capacitance in parallel with them that is causing it to read high at each probe point.

[bdunham7]

OK, but I haven't seen this said expressly so sorry if I missed it--does the meter behave the same way if you try to do 4W ohms measurements from the rear terminals?  Showing near-zero ohms in all ranges up to 100K?  Can you try this using actual resistors and a 4W setup instead of just letting it float open?

[bdunham7]

The 10/100MOhm ranges are reading low on my 87V in HiRES mode for some reason.

Don't let that worry you--those ranges are not constant-current source on the 34401A so a variance in the burden of the 87V would show a difference.

[Xepherrea]

OK, but I haven't seen this said expressly so sorry if I missed it--does the meter behave the same way if you try to do 4W ohms measurements from the rear terminals?  Showing near-zero ohms in all ranges up to 100K?  Can you try this using actual resistors and a 4W setup instead of just letting it float open?

See attached photos. Need to upload the last 2 separately. Rear reads the same thing.

[Xepherrea]

OK, but I haven't seen this said expressly so sorry if I missed it--does the meter behave the same way if you try to do 4W ohms measurements from the rear terminals?  Showing near-zero ohms in all ranges up to 100K?  Can you try this using actual resistors and a 4W setup instead of just letting it float open?

See attached photos. Need to upload the last 2 separately. Rear reads the same thing.

[bdunham7]

OK, so I'm still not getting something.  Do you have a 100R resistor connected for all except the last photo where it is a 1M?

[bdunham7]

OK, carefully rereading...

Is the problem that open circuit in 4W ohms mode the meter autoranges down to the lowest range and reads near zero, then if you manual range back up it reads near-zero until you get to the 1M range and above, where it 'floats' and reads some random values?  Again, all open circuit (meaning nothing connected) and the meter does read actual resistors properly in both 2W and 4W modes?

If that's it, it ain't broke.  And conventional wisdom is "if it ain't broke, don't fix it".   

[Xepherrea]

OK, carefully rereading...

Is the problem that open circuit in 4W ohms mode the meter autoranges down to the lowest range and reads near zero, then if you manual range back up it reads near-zero until you get to the 1M range and above, where it 'floats' and reads some random values?  Again, all open circuit (meaning nothing connected) and the meter does read actual resistors properly in both 2W and 4W modes?

If that's it, it ain't broke.  And conventional wisdom is "if it ain't broke, don't fix it".   

Yes the last photo had a 1MOhm resistor on it just to show the range was reading properly. The 100 ohm throughout was for showing linearity. I had to do a few tries to get the photos to be able to be uploaded, so the response got lost.

I looked at the higher ranges again, 1 MOhm+ and they float until draining to 0 as well, it just takes longer to do so, but once at zero it stays at zero. Something is wrong because no multimeter I have ever worked with has a shorted open reading. I don't like that the unit has an issue and doesn't seem to bee fixable, but I guess I'll live with it for now until I get a new meter sometime later next year.

[bdunham7]

I looked at the higher ranges again, 1 MOhm+ and they float until draining to 0 as well, it just takes longer to do so, but once at zero it stays at zero. Something is wrong because no multimeter I have ever worked with has a shorted open reading. I don't like that the unit has an issue and doesn't seem to bee fixable, but I guess I'll live with it for now until I get a new meter sometime later next year.

My 34401A does exactly the same thing and I can assure you it is not malfunctioning.  The 4W ohms function needs there to be continuity between the HI and HI-SENSE leads, as well as LO and LO-SENSE.  There's a specification for maximum lead resistance for the various ranges, so there can be a few ohms there, but if you leave it open there's just a separate current source and then a voltmeter with open leads--HI-SENSE and LO-SENSE.  Those two go through U101, the switching IC, in a different way than the regular DCV circuit, and while I couldn't tell you exactly how that works, it results in a reading that doesn't float like a high impedance voltmeter might.  In 2W and in 4W with the proper leadset connected (even with no resistor, or one too large for the range) you should see "OVLD" in the display.

[bdunham7]

In case anyone cares about this rather arcane topic, bench meters with 4W ohms do vary in their displays when put in 4W mode with no leads connected.  Only my old 8500-series Fluke bench monsters seem to take note of the fact that they aren't connected properly and display "Error H".  All the others display nonsense on certain ranges and OL (or equivalent) on others.  However, the 34401A is the only one I have that seems to eventually settle around zero on the display when in this state.

If you look at the schematic, specifically U101, it looks to me like they are reading HI-SENSE and LO-SENSE alternately, using LO-SENSE as the autozero rather than AGND.  Since there's no reason to use the precharge feature when measuring ohms this way, if the connections are open and there's no leakage, any residual charge on HI-SENSE will be pumped out to LO-SENSE eventually as the small capacitance of the input amplifier (AMP+ output on U101) is successively charged by HI-SENSE and then discharged through LO-SENSE.  This matches up with the OP's observations and mine--it's a feature not a bug.