Author Topic: HP/Agilent/Keysight 34420A NanoVolt/Micro-Ohm Meter repair (Read 21259 times)

plesa · « **on:** January 02, 2016, 12:05:16 am »

Multiple months has been under my bench waiting for repair, so lets started.
Meter is quite old ( according to serial number made 1995 ).
This meter is quite popular for ration measurement compare to 34401A it has low voltage preamplifier which enable 1mV DCV range and low ohm measurement with high precision.
- 7½ digits resolution
- 100 pV, 100 n? sensitivity
- 1.3 nVrms, 8 nVpp noise performance
- Built-in low noise 2 channel scanner
- Direct SPRT, RTD, Thermistor, and Thermocouple measurements
Thanks to absence of fan unit is clean after several years in operation.
Compare to competitor (Keithley 2182A Nanovoltmeter) has own current source which enable Ohms/RTD measurement.
Meter is still in active production (end of this year will be not possible to ship to EU - RoHS)
Price $4445 new one, $1538k unit exchange by ~~Agilent~~ Keysight. Used working unit can be found on eBay for quite high prices ( similar to 3458A), working unit in uknown state should be about $500, with calibration $700-1k.
Like 3458A it has been sold by Fluke as http://assets.fluke.com/manuals/hp34420xiseng0000.pdf for temperature measurement and calibration.

Initial inspection:
- Power on and passed self test
- VFD display is little bit dim
- Input connector has bend pins
- DCV channel 1 - not working
- DCV channel 2 - OK
- 2W Ohms - OK
- 4W Ohms - OK
- RTD - OK

Schematic: http://www.ko4bb.com/manuals/download.php?file=HP_Agilent/Agilent_34420A/Agilent_34420A_SCHEMATIC.pdf
User Manual: http://literature.cdn.keysight.com/litweb/pdf/34420-90001.pdf?id=1000003601:epsg:dow
Service Manual: http://literature.cdn.keysight.com/litweb/pdf/34420-90010.pdf?id=816987

When I tried to fix bend pin I found they are made from pure copper ( not like the regular gold plated LEMO pins ) and are quite brittle

So I needs to order replacement.
Replacement we discussed https://www.eevblog.com/forum/repair/looking-for-the-input-connectors-for-agilent-34420a/
and after this excersise I can confirm that the connector housing is EVP.1S with custom made pins.
I do not want to stop repair by this, so I desoldered connector assembly and solder insted of it 50cm of PTFE insulated silverplated wires (made by Habia).

After repair meter can be handy for low EMF measurement and ratio measurement of LTZ1000 boards.
Any experience with DCV Channel 1 repair may be helpful.

ManateeMafia · « **Reply #1 on:** January 02, 2016, 01:11:48 am »

I bought a 34420A that had a defective input channel. I cannot remember which one was defective, but I traced it straight to an optoisolator that wasn't functioning. Looking at the schematics, I think mine was U125. I could see the enable signal being applied, but the ground connection wasn't completing. I was able to jumper pins 4 and 6 to get the DC function working.

I would have to open it again to verify which one it was but I would start with those. The replacement was bought at Digikey, I will have to search through previous orders to find it if needed.

plesa · « **Reply #2 on:** January 02, 2016, 02:03:49 am »

Thanks for tip, it is the same failure U125. When shorted pin 6+4 Channel 1 DCV reading is OK!!
If you can find replacement of HSSR8400 it will be appreciated.
It was much faster repair than I expected, now it will be little bit tricky to find connector replacement.

ManateeMafia · « **Reply #3 on:** January 02, 2016, 02:16:20 am »

That's great. My repair was also quick and it took longer to track a replacement.

I ended up using this part http://www.digikey.com/product-search/en?keywords=LH1510AAB-ND

plesa · « **Reply #4 on:** January 02, 2016, 12:27:06 pm »

Thanks!!

HighVoltage · « **Reply #5 on:** January 02, 2016, 01:16:03 pm »

First time I see the inside of a 34420A.
Congratulations on the fast repair.
I am looking forward to some measurements you will perform.
Now you need a real SPRT sensor, like this one, to have one of the greatest thermometer.

https://youtu.be/zQ5_oJI1cJM

plesa · « **Reply #6 on:** January 04, 2016, 08:20:36 pm »

Quote from: HighVoltage on January 02, 2016, 01:16:03 pm

Now you need a real SPRT sensor, like this one, to have one of the greatest thermometer.

I was quite surprised that they are using 25 Ohm.

But back to the repair.
In service notes I found that there are lot of issues related to older version of firmware and communication.

http://servicenotes.literature.keysight.com/litapp/SearchSN.do?method=openExternalSNSearch&prodNum=34420A

- RS-232 communication can be solved by replacing U700 ( Intel N83C51FA with 34401 marking )chip, fortunately my unit is OK, otherwise it will require return to Keysight. Chip is in socket so I do not know why

. Maybe that's why there is attached sticker on instrument.
BTW the version of this chip is the middle number in revision menu.

- Firmware is stored in U502 (27C020 in PLCC32, Keysight blank can be ordered 1818-5187) in my unit is version 3.0 (first number in revision menu).Chip is also stored in socket so I do not understand why unit needs to be returned to vendor. That's a big difference compare to 34401A where both chips are soldered to board, where I understand that it is not customer job.
Please send me PM if you have more recent version of firmware ( >4.0).

plesa · « **Reply #7 on:** January 07, 2016, 09:18:40 pm »

After replacing obsolete HP HSSR8400 by Vishay LH1510AAB channel 1 is working properly.
What I found as a new issue for solving is permanent overload on lowest 1mV range on both channels.
I make low thermal short and check it on 10mV range, readout is below spec. 120nV on 10mV range and on 1mV is still overload

So it must be connected to low voltage amplifier and to 10000 gain setting only.Gain in this range is switched controlled by ASIC U101, resistor networks is OK.

Kleinstein · « **Reply #8 on:** January 07, 2016, 11:05:44 pm »

I first thing to check is if the amplifier is somehow oscillating (ouput of U306). The feedback path is quite long and thus many places thing can go wrong. Candidates for failure are U303C and U303D.

p.s. Funny how they made a low amplifier with an LM358.

ManateeMafia · « **Reply #9 on:** January 08, 2016, 01:32:03 am »

Sounds like my recently repaired K2182. It had a bad resistor network. A couple of the resistors were open and all the low ranges were in overload.

Good luck. If you need a comparison measurement, let me know.

plesa · « **Reply #10 on:** January 08, 2016, 07:20:14 am »

Thanks, analog switches U303, U304 (DG411,DG211) looks OK. Both resistor networks as well.So I will check oscillations.
Is anybody interested in posting old firmware?

ManateeMafia · « **Reply #11 on:** January 08, 2016, 11:28:19 am »

I will check my versions and upload what I have to xdevs. It won't be until later today.

ManateeMafia · « **Reply #12 on:** January 09, 2016, 01:26:40 am »

Bad news, my rev 5 eprom is soldered to the pcb. It looks like it may have been replaced at one time. Another one has rev 6. I will open it later as I was gathering data for the Fluke 8808A thread.

ManateeMafia · « **Reply #13 on:** January 09, 2016, 03:32:16 am »

Second 34420A with Rev 6 eprom is also soldered to main board.

plesa · « **Reply #14 on:** January 10, 2016, 05:05:11 pm »

Thanks for check. It was similar on 34401A that some units has socket and some directly soldered EEPROM to PCB. Version 6 of firmware seems to be the most recent one. I will run lon term GPIB test and we will see if there will be some GPIB communication issues related to older firmware.

When I inspected units I found that someone tried to fix channel 1 issue and desoldered Q128/Q129 FETs, I correct it and clean PCB. And I removed wire connections U313 pin 2 and C451 bypass cap, which looks like from unsuccessful repair.
But it was mistake after putting the wire back the overload on 1mV range disappeared.

Another issue what I found was on channel 1 and 10V range on autorange, sometimes the readout was 20-50mV.
After replacing U120 HSSR8400 optocoupler by Vishay LH1510AAB everything works like charm.
To prevent other issue I replaced all opto-couplers, but two additional without further improvement.

On 2W ohm measurement seems to be offset 0.1 Ohm, but this I hope will be removed during calibration (still in spec) . 4W Ohm works ok ( 20uOhm stable readout, spec is 17uOhm).Quite good results for unit which was calibrated 20 years ago

With inputs shorted there is 48.5 uV on Channel 1 and 82uV on Channel 2 readout, which is according to spec OK.

ManateeMafia · « **Reply #15 on:** January 10, 2016, 06:01:47 pm »

This is good news. After a long burn-in, we can collect some data on your 34420A. I can also do the same on mine as my 3458A testing has collected enough data to show how they are drifting. 3 of 4 tested fine. The last one is almost as bad as TiN's high plains drifter.

plesa · « **Reply #16 on:** January 10, 2016, 07:46:26 pm »

I just assembled unit and overnight I would like to take first data set.

plesa · « **Reply #17 on:** January 17, 2016, 09:38:15 am »

Data collected by BenchVue. Channel 1 noise 13.1nV, offset -8.5nV, Channel 2 noise 8nV, offset 28nV.
Also I receive confirmation from Lemo about connectors:

For cable FVN.1S.304.CLYC37Z price 50 EUR
meter input connector is EVP.1S.304.CLY price 40EUR
both with copper contacts not gold plated.

EZGPIB scpit for some reasons seems to not work with my unit,old firmware issue maybe.

HighVoltage · « **Reply #18 on:** January 17, 2016, 10:57:36 am »

Did the 2W Ohm reading correct the offset after calibration?

plesa · « **Reply #19 on:** January 17, 2016, 11:33:21 am »

Quote from: HighVoltage on January 17, 2016, 10:57:36 am

Did the 2W Ohm reading correct the offset after calibration?

Yes, all offset corrected after performing Zero calibration. Now the offset in 2W Ohm measurement is 125 uOhm!!
Also the offset in DCV were corrected to -3nV on both channels. Currently I'm working on LabView program for data collection.
I must say that I'm impressed by this meter.

alanambrose · « **Reply #20 on:** March 16, 2016, 05:58:42 pm »

Re noise results - this seems to be the spec (rms at 10nplc / low impedance source):

That is, lowest ppm noise on 10V range. 2-3x on other ranges except 1mV which is ~30x higher.

And here are my early results - with a dodgy short as I don't have any proper connectors yet:

Interestingly, lowest on 100mV range, with other ranges <1.8x that. The 1mV range seems good. My 1V and 10V ranges look out of spec, curiously.

alanambrose · « **Reply #21 on:** March 17, 2016, 01:00:10 pm »

BTW anyone have an actual 34103A short that they don't mind taking the case off? I'm reluctant to pay $100 extra to have someone solder a couple of copper links into a FVN.1S.304.CLYC37Z Lemo. I was expecting this to be a crimp connection, but it looks like 'solder bucket' to me.

TIA, Alan

Vgkid · « **Reply #22 on:** March 17, 2016, 02:04:16 pm »

I believe in the Keithley info sheets, it says that they are soldered. I believe it was SN95AG5 solder, apparently what they seitched to after cadmium.

alanambrose · « **Reply #23 on:** March 17, 2016, 03:36:42 pm »

Ah yes thanks...

Model 2188 Low-Thermal Calibration
Shorting Plug: A low-resistance, low-thermal
shorting plug required to calibrate the Model
2182A Nanovoltmeter. Made of a LEMO connector
with its pin connectors shorted together
using copper wire and Sn/Ag4 silver solder.

p.s. for the Europeans, the best place I've found for 96/4 is:

http://www.reichelt.com/Solder/2/index.html?&ACTION=2&LA=2&OFFSET=500&GROUPID=557&SORT=artnr&SHOW=1

i.e. the Felder Sn96 Ag4 F-SW32

alanambrose · « **Reply #24 on:** March 17, 2016, 05:17:23 pm »

OK my first cut of the noise floor. My meter reporting firmware versions: 9.0-5.0-2.0 btw.

(edit) Added the correct analysis.


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Author Topic: HP/Agilent/Keysight 34420A NanoVolt/Micro-Ohm Meter repair (Read 21259 times)

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