[SOLVED] HP 34401a - Error 612, 613, 615, 617, 618, 619, 621

[Zucca]

Hi there,

Check this last mail email from a cal lab (see full story here https://www.eevblog.com/forum/testgear/34401a-cal-error-707/):

We recently received another 34401A in that has the same issue with the ohms function as your did. Would you be insterested in trying to fix it for us. If you do we can work out some kind of deal with the calibration. Let me know if this is something you'd be interested in? If not, I understand, I will just send your unit back.

After a quick check on this unit, it seems there is a lot to do here, see below the self test results:

Code: [Select]

-> SYST:ERR?
<- +612,"Ohms 500 nA source failed"
-> SYST:ERR?
<- +613,"Ohms 5 uA source failed"
-> SYST:ERR?
<- +615,"Ohms 10 uA source failed"
-> SYST:ERR?
<- +617,"Ohms 100 uA source failed"
-> SYST:ERR?
<- +618,"DC high voltage attenuator failed"
-> SYST:ERR?
<- +619,"Ohms 1 mA source failed"
-> SYST:ERR?
<- +621,"AC rms full scale failed"
Now I have to start the investigation and post my findings here...

Z

[free_electron]

That looks like the thick film resistor network is shot. check the 9 megaohm resistor in it. most likely it is open.

[Zucca]

Now I have time to follow this up, so let´s understand what those code means:

612 Ohms 500 nA source failed
This test configures to the 10 V dc range
with the internal 10 M 100:1 divider  U102A connected across the input.
The 500 nA ohms current source is connected to produce a nominal 5 V
signal. A 20 ms ADC measurement is performed and the result is
checked against a limit of 5 V  ± 1 V.

613 Ohms 5 uA source failed
This test configures to the 10 V dc range
with the internal 10 M 100:1 divider  U102A connected across the input.
The 5  µ A ohms current source is connected. The compliance limit of the
current source is measured. A 20 ms  ADC measurement is performed
and the result is checked against a limit of 7.5 V  ± 3 V.

615 Ohms 10 uA source failed
This test configures to the 10 V dc range
with the internal 10 M 100:1 divider  U102A connected across the input.
The 10  µ A ohms current source is connected. The compliance limit of the
current source is measured. A 20 ms  ADC measurement is performed
and the result is checked against a limit of 7.5 V  ± 3 V.

617 Ohms 100 uA source failed
This test configures to the 10 V dc range with the internal 10 M 100:1 divider  U102A connected across the input.
The 100  µ A ohms current source is connected. The compliance limit of
the current source is measured. A 20 ms  ADC measurement is
performed and the result is checked against a limit of 7.5 V  ± 3 V.

618 DC high voltage attenuator failed
This test configures to the 1000V dc range. The 500 nA ohms current source is connected to produce a
nominal 5 V signal. A 20 ms  ADC measurement is performed and the
result is checked against a limit of 5 V  ± 1 V.

619 Ohms 1 mA source failed
This test configures to the 10 V dc range
with the internal 10 M 100:1 divider  U102A connected across the input.
The 1 mA ohms current source is connected. The compliance limit of the
current source is measured. A 20 ms  ADC measurement is performed
and the result is checked against a limit of 7 V  ±  3.5 V.

621 AC rms full scale failed
This test configures for the 100 mV ac range.
The 1 mA ohms current source is switched on to charge the ac input
capacitor  C301 . This produces a pulse on the output of the rms-to-dc
converter which is sampled 100 ms after the current is applied. A 20 ms
A/D measurement is performed and checked against a limit of 10 V  ± 8 V
into the  ADC

618 and 621 are respectively due to 612 and 619, so we can ignore those two.

Yes free_electron, the U102A is the first suspected, but in R I have no ohm current source.

What if the ohm current source is fried?

mmmmm... I will find out... 



 

[free_electron]

like i said : check the 10 meg divider. pin 1 to pin 12 i believe of that ceramic hybrid. imost likely it is open.
i have seen those fail two times. that hybrid is very expensive... like 250$ ..

[Zucca]

Thanks free_electron to stay with me!

I did after I posted the previous one and it was not open.

I connected the bad 34401a HI to the I 3A current on a 34461a, and then I connected the two LO between 34401a and 34461a.
With 2WR set on the 34401a I got no current flowing in the 34461a.

In this set up the 10MR divider doesn't play any role, please tell me if I overlooked something.

Thanks a lot!

[Zucca]

I activate the 2WR with a shortcut between HI and LO.

Yep... something is wrong here [everything in V]



I need to replace the U201, IC-Op Amp LP Dual 8-Pin AD706JR

http://www.mouser.com/ProductDetail/Analog-Devices/AD706JRZ/?qs=sGAEpiMZZMvtNjJQt4UgLalm6EXZly%2fyYoythdEg1pg%3d

How can I understand if also Q201 is bad?

[Dr. Frank]

U201-A and Q201 should be ok.

Both stabilize the 7.012V reference over R202 correctly, and the gate voltage on Q201 is 3.637V only, i.e. lower than source voltage, therefore the necessary current of 175µA must be delivered correctly over the drain, from the 11.444V node. All voltages around Q201 are quite reasonable.

It is strange, that you measure 4.920V over R201, as both FETs in U101-D should be shut. Maybe U101 is corrupt. But I could remeasure on my unit..
Edit: No, that's also fine, mine shows 4.9115V there. Obviously the third FET inside U101-D biases that point to VDD = Vcc ~ 4.937V
Also, all other voltages around Q201 and U201-A are equivalently set in my unit.. besides Drain voltage, which is 9.725V vs. 14.812V above 28.75k inside U102-D, which gives correctly around 5V in my unit.

These 11.444V are too high, should be 13.853V-5V = 8.853V instead.
So, U201-B may be defect, as it may source current into that node.
Also, as pin5 (+) = 11.444V, pin6 (-) = 12.292V, therefore output pin7 should go towards AGND, and not to 14.297V.

Therefore, U201 is the most probable error, and I would start with its replacement, which should fix the problem.

Frank

[Zucca]

Thanks Frank to double check with your device   !

...and the gate voltage on Q201 is 3.637V only, i.e. lower than source voltage, therefore the necessary current of 175µA must be delivered correctly over the drain, from the 11.444V node...

In University they told me in a n-ch FET if (V Gate - V Th) <= V Source the FET is open. In this case is acting as a 24.5KR resistor. That´s why I was a little confused. Anyway I should have read the Q201 datasheet.

So, U201-B may be defect, as it may source current into that node.
Also, as pin5 (+) = 11.444V, pin6 (-) = 12.292V, therefore output pin7 should go towards AGND, and not to 14.297V.

I love when another person thinks the same way I do  .

Let´s oder some of them now...

Not sure why it failed like that. When I saw the ohm current source was dead I thought most likely the protection diode or BJT transistors did their job... and some smart user was testing R live on a very high voltage circuit...

[Zucca]

U201 Replaced. Solved.

[mzzj]

Year ago I had a 34401a also with several error codes. Same problem, after checkin input and outputs of the u201 i came to conclusion that it has to the problem. Replaced and good to go.

[Phiveaces]

Just wanted to say that thanks to Zucca, I now have a working 34401A

Mine only had errors 613, 615, 617, 619 and 621.

Setting the Ohms range to Ohms, and checking the output of U201 compared with its inputs showed  ~14V when it should have been closer to AGND.

Replaced with a new one from Digikey and now all tests passed! 

[Zucca]

That was my Xmas present to you! Enjoy!!!

[chihaxinh]

I have same problem but replace U201 not help ! 

[Dr. Frank]

I have same problem but replace U201 not help ! 

You mean, you have the same Error messages?

To change this OpAmp simply on this, was obviously the wrong action.

You also have to check the voltages in the Ohm circuitry, as zucca, Phiveaces, and me have done. This is urgently necessary to identify the really corrupt component.
A useful help is a diagram with the correct voltages vs. the erroneous ones, you have to measure on your own.

Please come back when you have made these measurements!

Frank

[Zucca]

Please come back when you have made these measurements!

+1

[chihaxinh]

Hi All

I have some free day for prepare new lunar year , i making measurement and give you information. I replace 3 opamp 706 but not help , again and try again . So i have only one new opamp 706 , pls help me

I checking Q202 and Q211, Q211 part exactly in service guide but Q202 have marking 1671 .
In service guide, Q202 it's N chanel JFET 2N5461 , i cheking pinout:
Q202 - 1671 marking have  D S G
2N5461 have S D G
i think 1671 like japan jfet . I change pin 1 2 for matching like schematics but not help !

Can you guide me how to check opamp live or die !

Thank you so much !

[TiN]

Did you measure output current from ohm source?
Use external DMM to measure current from marked node to 34401's LO terminal.

[chihaxinh]

Hi Tin

i have 3 another 34401a, but i not pro. can you guide setup for measurement ! you mean checking voltage or current?

[TiN]

Current. You should read correct currents with red lead connected to CR202 anode, and black lead connected to 34401 meter's LO (black input jack is ok).

[chihaxinh]

Hi Tin

I have some mistake , Q202 it's P chanel JFET - so i checking picture in internet for sure pin out connect in diagram .

You can see picture below for setup measurement current like you said ?

[TiN]

Yes, that's correct connection.
Use manual ranges for ohms on bad DMM, and measure current. Change to every range and check all current source levels.

[chihaxinh]

Hi Tin

I checking and give new information, vol dc and ohm function not woking in 2W and 4W mode

I check current like your said and have

000.000 Ohm - 1mA
0.00000 KOhm - 1.006mA
00.0000 KOhm - 1.002mA
000.000 KOhm - 0.010mA
0.00000 MOhm - 0.005mA
00.0000 MOhm - 0.000,6mA
000.000 MOHm - 0.000,6mA

And voltage measurement after replace U201 and swap pin Q202 for correct pinout in schematics.

[TiN]

Then your ohms current source working fine, and problem is somewhere else.
Did you check all zener voltages and power rails? I think you saw my 34401 repair? Maybe yours have similar problem.

[chihaxinh]

Hi Tin, i checking power supply rails , everything ok !

Voltage DC and Ohm function not work ( 2W or 4W mode )

[TiN]

Did you check all the zeners? There are many of them.

[chihaxinh]

Hi Tin

Ok let's me check all zenner and report for you soon !

Update: I check all zenner have voltage drop like datasheet, but CR201A (  4.7V ) in Ohm current source have osic from -0.9V to 1.4V .

And voltage drop on pin 7 U201-B have only 4.6xxV

[robjpiel]

I have been working along with this thread trying to solve a similar issue that I have with my 34401a. 

Error codes only:
612 Ohms source current failed,
618  DC HV attenuator failed (no surprise here- I hit it with between 1100~1250VDC while in the DCV AutoRange, yes this was dumb but we got a new 34461 and now I'm trying to repair this 34401 for fun/learning experience/I want it for myself)

Sorry to dredge up such an old thread but it has so much relevant information in it, and you all seem to be the only resource I could find out there with this kind of information.

The meter does work in several other functions such as the DCV all except the 100V and 1kV range so I figure it is worth some time and attention.  When it is in these two fried ranges it is not accurate at all on low V inputs and only registering 70% of actual higher up ie. a 1000 volt input will only read 700v.  (It will not accept a new calibration this far out fyi- I tried this first)

I first thought I blew up the 9.9M and 100k ohm voltage divider resistors in u102-A, but they are fine so I checked all the rest in:
u102-C: 2k, 2k 2k-All Good
u102-D: 20.57k, 5k, 50k, 500k, 1M-All Good.

I checked all the voltages that I could (attached).  I highlighted red only values with significant deviation from what the OP had (Zucca I believe) and in black I have additional readings in case they were to help me down the road, but most of my values turned out to correlate with TiN's working meter leading me to think this IC u102 may be OK??  Hoping anyway.  Where my confidence stops is after checking u102-B I get 2k Ohm (pin11-14), 17.3k (13-14), and 84.8k (13-15)instead of 2k, 18k, and 180k.  But I didn't really expect any values to be in tolerance since its still in the circuit and I'm measuring resistance which I know is fundamentally wrong to begin with.

The second issue I found here is that my output Ohms Current test did actually have 0 current from the low terminal to the anode of CR202.   I was measuring with a 77IV so I will try it again to confirm with a 3458 tomorrow I guess just to be sure. 

If anyone would like to call me an idiot for looking where I have been,  and point me in the right direction that's obvious to you that would be much appreciated, or tell me those resistance readings in u102-B are OK since they are still in the circuit that would be cool too.  Otherwise my next step was going to be to see if RV102 was having an issue only when it sees higher voltages and maybe the rest of the meter is working just fine after that point - but I will say I dont know how to check a -"Diode-Varistor 1.1kV" yet.   Time to pull out my textbook I guess.

Thanks in advance for any input you may have!!!   

[Dr. Frank]

618  DC HV attenuator failed (no surprise here- I hit it with between 1100~1250VDC while in the DCV AutoRange, yes this was dumb ...

I don't understand this expression..
What exactly happened?  Did you apply 1250Vdc when in Ohm mode, or simply when in DCV / Auto mode?
The instrument should withstand such overvoltages to some degree in both modes.
 

The meter does work in several other functions such as the DCV all except the 100V and 1kV range so I figure it is worth some time and attention.  When it is in these two fried ranges it is not accurate at all on low V inputs and only registering 70% of actual higher up ie. a 1000 volt input will only read 700v.  (It will not accept a new calibration this far out fyi- I tried this first)

Are 10Vdc, 1Vdc, 100mVdc accurate?
I would have expected that you damaged these lower ranges by applying the overvoltage..

Anyhow, the overvoltage might have damaged the input path, somewhere in between the input jack, ESD circuit, relays K101, L101, K104 , towards  U102-A, that is the 100:1 divider.
I propose that you chose 1000V DC range, apply 10V to the input jacks, and measure with your 34361 in high Z mode at first the input voltage to U102-A, i.e. pin 1.
If you don't see 10V, you have to check the components and PCB traces of the mentioned input path.
If 10V are present on the input, check pin12, if the divided 100mV are correct. If not, the input multiplexer, pin 8 of U101-B may have been damaged.

I first thought I blew up the 9.9M and 100k ohm voltage divider resistors in u102-A, but they are fine so I checked all the rest in:
u102-C: 2k, 2k 2k-All Good
u102-D: 20.57k, 5k, 50k, 500k, 1M-All Good.

I assume, that you unsoldered U102, before making these measurements?

I checked all the voltages that I could (attached).  I highlighted red only values with significant deviation from what the OP had (Zucca I believe) and in black I have additional readings in case they were to help me down the road, but most of my values turned out to correlate with TiN's working meter leading me to think this IC u102 may be OK??  Hoping anyway.  Where my confidence stops is after checking u102-B I get 2k Ohm (pin11-14), 17.3k (13-14), and 84.8k (13-15)instead of 2k, 18k, and 180k.  But I didn't really expect any values to be in tolerance since its still in the circuit and I'm measuring resistance which I know is fundamentally wrong to begin with.

Correct, you will only get the proper values by unsolder U102...

Very probably, U201-B is damaged, pushing current from its + input into the node between Q201 and pin4 of U102-D.
This low bias OpAmp in many cases already, fails after years, or maybe the overvoltage killed it, as the 100:1 divider is sitting in the same package U102.
Anyhow, replacing U201 probably will solve this problem.

The second issue I found here is that my output Ohms Current test did actually have 0 current  from the low terminal to the anode of CR202.   I was measuring with a 77IV so I will try it again to confirm with a 3458 tomorrow I guess just to be sure. 

That may also be caused by a failed U201-B

Frank

[robjpiel]

Thank you Dr. Frank,

Yes- To cause the problem I applied 1250VDC to the meter while in DCV Auto range.
(I was testing the 1KV output of insulation resistance testers (meggers), normally they only put out 1050vdc and when I tested a dozen of the new fluke 1507 they were putting out over 1200vdc and without thinking twice I had tested 8 in a row and then they started reading 800vdc.  Went back and checked the units that had just read 1250 and they too were reading 800vdc.  Verified the problem with my meter calibrator, and tried to recalibrate the 34401 with no luck.  Then I ran the self test and got the codes.)

Yes- the 10VDC, 1VDC and 100mVDC all are 100% accurate (0-full scale). 
-Every ACV range is 100% accurate as well. 
-Resistance ranges are all good until I get up to the 10MOhm range (which reads 9x higher than it should)

Believe it or not I actually did not de-solder the IC for any of these measurements.  I Do not have the equipment yet to do so.  I am ordering some soon so that I can enhance my repair capabilities to SMT and ics.  But while I had it open I probed it anyway and all those resistance values were dead on until I got to u102-B.       As soon as I get some equipment to desolder this IC I will check u102 properly.

Current Output Over the Resistance Ranges  All check Good
100Ohm    1.010407mA/7.90V
1kOhm   1.010403mA/7.912V
10KOhm    100.71440uA/8.185V
100kOhm  10.09959uA/8.422V
1MOhm 5.060033uA/8.439V
10MOHM    0.505691uA/5.0719V
100MOhm  0.505714uA/4.820V

Below is what I have gotten so far for voltage readings going to the divider.  The voltage up to R122 is 10.000V after it drops to .8V  going into u102 pin 1 then then STEPS UP to 8V. ?  I feel that means R122 may be open?  I checked the Resistance across it (while in circuit) and it reads .22GOhm. 

I piggybacked a 6.19K decade resistor over R122 and This 100V and 1kV range all works fine.  (I'll need to recalibrate it when I get the replacement R122 installed) 

Excellent!! THANK YOU VERY MUCH FOR YOUR HELP SIR!

I'll post back after if it is not 100%


-Rob

[Kleinstein]

R122 might need to be a special resistor, e.g. fusible or higher peak voltage rating. If this resistor burnt, the root cause might have been arcing in the relays directly following it (e.g. during switching).

Trouble with high Ohms readings somewhat points to possible excessive leakage. So it might be a good idea to check that too (e.g. 1 to 10 M resistor across input in the 1 V or 100 mV range, alternatively observe charging of a small (e.g. 1 nF) low leakage cap).

[robjpiel]

The two upper Ohm ranges are working now too, with me holding that substitute decade resistor in place of R122 it is surprisingly stable even with my shaky connection.

Thank you for the info.  If it does not stabilize and pass calibration I will definitely look into that.

You guys have been a big help

[Samogon]

R122 made my day. Another 34401A is back to work.

[robjpiel]

Just be sure to run through all the performance verification steps after.   Mine was out of tolerance across several ranges.   I just recalibrated all of them and this thing is working great.

[Samogon]

Sure it will be calibrated.

[Satbeginner]

Hi all,

This thread now seems to be a 34401A repair source, so sorry for me joining as well:

I acquired a 34401A that has the following errors: 612,613,615,617,618,619 and 621.
Both DCV and ACV readings seem to be OK.

Based on the information here, I did some checks and it seems that the second half of u201 (b) has died.
The 7V on r201 and/or r202 when changing ranges is still there, so I ordered some ad706's.

Will keep you posted, in saludo,

Satbeginner (Leo)

[Satbeginner]

Hi all,

This thread now seems to be a 34401A repair source, so sorry for me joining as well:

I acquired a 34401A that has the following errors: 612,613,615,617,618,619 and 621.
Both DCV and ACV readings seem to be OK.

Based on the information here, I did some checks and it seems that the second half of u201 (b) has died.
The 7V on r201 and/or r202 when changing ranges is still there, so I ordered some ad706's.

Will keep you posted, in saludo,

Satbeginner (Leo)

Hi all,

Today the AD706's arrived, so I could replace U201.

Result?? Yes!! One 34401A back to life, and compared with my HP 3457A well within specs.

I am one happy bunny!

Un saludo,

Satbeginner (Leo)

[Samogon]

Good job!
Welcome to the club:)

[Zucca]

Satbeginner nice!

When I stared this I had not idea this failure was and is so popular... I have no clue why the U201 is dying so often...  meh... lesson learned always post a fix in the EEVBLOG to help the world.

[HighVoltage]

I have replaced a few U201 already and when I get another broken 34401A, this is a first go to check for me now, right after testing the power rails.

With all the information on eevblog about the 34401A repair, it has gotten much easier to fix these great DMMs.

[Satbeginner]

I don't get it either, because the "A" half was still working fine. It must have something to do with somebody having applied a (really) high voltage while on ohms setting??

Un saludo a todo.

[Andrejka]

Hello colleagues

Can someone suggest me, what components to check if:
Self test shows errors: 612, 613, 615, 617, 618, 619, 621, 622
Ohm current source is working - I got correct 500nA,  5uA, 10uA, 100uA and 1mA
Only DCI mode seems to working correctly, on other modes only chaotic readings.
Comparing to other meter there is no input impedance (infinite input impedance reading). In working meter on DCV mode I can measure 10Mhom input impedance.

If Ohms current source is working then custom resistor assembly is ok?

Thanks

Andrej

[TAMHAN]

Likely one of the SMD inductors close to the input jacks. They, um "levitate away" during shipping and handling. Sadly, I lost mine...and getting that Gowanda part is difficult. Which is why I currently dont have a working 34401A...but it is very low on the repair priority list.

[Andrejka]

Likely one of the SMD inductors close to the input jacks. They, um "levitate away" during shipping and handling. Sadly, I lost mine...and getting that Gowanda part is difficult. Which is why I currently dont have a working 34401A...but it is very low on the repair priority list.

Tamhan You are 100% right - cracked small inductor. Absolutely invisible with naked eye, but signal path tracing helps. Now looking for suitable replacement...





Thanks

Andrej

[Resonant]

HP 34401a repair
I recently acquired a nice HP34401a, an early one made in the USA and I thought I would do some checks against my calibrated meter and left it on for a while, tried various voltages and they were close but generally a little high. This meter had been in private ownership and had very little use and probably not been calibrated since new.
After trying various voltages and putting the meter through its paces it came up with an error. So I re-ran the Test All which had passed OK earlier and it came back with the 612, 613, 615, 617, 618, 619 error codes. A quick scan on the net brought me to this thread where my heart sank when I read of all the possibilities and expense this could be.
I ran a few tests Voltages above 100V DC were Ok all AC ranges worked and there was still output current for the ohms readings although there were no readings. I tried the input impedance on most ranges it was the expected 10M ohm but on 100mV 1V and 10V it was only 66K so out with the circuit diagram of the input circuit.
It was late so I pondered it overnight and thought there must be a leak/short on the input. Looking at the resistor ladder R104 to R109 6X 13K  looked close to the value I was getting so I checked C100 470p and sure enough it measured 1 ohm, so I lifted one end and ran the self-test and it passed again all OK, so I will be ordering a cap and checking out the resistors. I will post again when completed. I assume the cap is a filter to remove noise on the input along with C101.
A request, I was intending to calibrate it but the manuals explanation makes it look complex compared with a Keithley 2000 which I have done previously I found after entering parameter step 5 pressing Auto/Man exited calibration, so how do you move on to the next value without exiting?
Chris

[HighVoltage]

Nice that you found the problem.

Some 34401A also suffer from blown tantalum capacitors.
When I have repaired these DMM, I replaced all of them.

If you do manual calibration, you have to do it for each value separately.
There is no next "value"

There is one "shortcut". Instead of pressing SHIFT and MENU ON/OFF just press SHIFT RECALL and it brings you back to the calibration menu.

[Resonant]

Well its all fixed a replacement input cap from CPC (CA0708) and some Panasonic FR series 105C Caps for the PSU and all working properly again. I checked all the 13k input resistors to make sure they had not been stressed and they were all in spec. So after a calibration all working well and stable so I'm very happy.
HighVoltage thanks for the tip on moving through the calibration sequence

[free_electron]

Yup. those blue stinkers short out sometimes.
same for the craptalium capacitors. i knock those out by default.

[abnormalalien]

Hi all,

I'm also working on a faulty 34401a and was wondering if you could explain some of the stuff you've mentioned earlier in this post?

I (guess incorrectly) posted my own thread explaining the particulars of my machine's situation (https://www.eevblog.com/forum/repair/hp-34401a-repair-fails-st-606-609-610-612-613-615-617-618-help/) but long story short, there's something wrong in the Ohm's Current Source part and prob elsewhere too. On here, I've seen several people print out Dr. Frank's schematic and write in their own measurements but I'm a bit confused. I've been assuming the boxed values are what we want; was this run through a circuit analysis software or did you all figure that out by hand (shudders)? Mild curiosity aside...I'm quite unfamiliar with electronics, especially at this level. Can someone let me know exactly what voltage drop I'm supposed to be measuring?

For example, on the far left of the diagram, there's a +7REF line, which you have measured to be 7.012 or 6.994V. You plugged high on a working meter into pin 3 of U201-A? Or somewhere else? Lo is then ground via pin 4 on U201-A? Or lo is the white power line ground from the power schematic? (Does it matter?) What about for the boxed value 12.750: how is that done?

Any advice is much appreciated! Thanks.

[Zucca]

I've been assuming the boxed values are what we want; was this run through a circuit analysis software or did you all figure that out by hand (shudders)?

The boxed values were from my bad unrepaired unit, the one written by hands were from the good Dr. Frank's unit. All of them were measured values from real life.

Beside that, it looks like you have problems to understand a schematics, where the pins are and where to put your DMM probes. I could give you more advises but I am worried you will understad them in a wrong way so please post pictures of what you are measuring, how you do it and where you put the DMM probes.

Worst case we will tell you: You are wrong.

As long you keep your DMM in V reading mode you and the circuits are safe. So you could even do some tests by yourself and with your active brain you will be surely understand some electronics black magic secrects.

There is main voltage in the 34401a, please be very very careful to stay away from that corner.

PS: Do you have hot air (de)soldering equipment? Probably you need to replace some SMD components and without it you will be stuck.

[abnormalalien]

Ah ok that makes sense, I apparently missed that. Written values = functional, got it.

I appreciate the concern; while I'm a bit  on the electronics background, I have no intention of dying for my thesis! I once tried to design/implement an electron beam evaporator which involves a high voltage (1500-3000V) but low current circuit using thoriated tungsten wire. This combined two of my biggest discomforts in the field: electronics and radiation...I wound up burning some hair off my arm at 3000V. Yea, that's when that project stopped being a priority.

I understand some basics; I can identify most of the circuit symbols including ground. I think ground is just something that's never really clicked in my brain. Talked to the advisor about it last night; I've been informed there's two, analog and digital, but that in general when I'm troubleshooting, I can probably just use the green line directly connected to the case. Somewhere that ground is connected to power ground (white line in power).

I've attached a couple pictures. In the first you can see I've got a (working) Keysight and a Fluke meter. I'm mostly using the Fluke but occasionally checking results with Keysight. In this one you can also see my lo on the fluke connected via an alligator clip to case ground. I've got a red probe for the positive terminal and have been using that to check voltages. In the other picture, I've circled the U201. As I've understood from this and other sources, that's a common failure. To measure the U201, I would just use my positive probe to check each pin directly?

Ah I'm not sure about the hot air soldering gun. I've got both large and micro soldering stations and I can probably borrow one from another lab in the department. (Most everyone is DIY here, someone's got to have one.) Or I might have one stored in my Dad's old tools. Can check this weekend.

[Samogon]

If current source opamp is kaput, sometime you can measure voltage between pins 2-3 and 5-6 of u201
Should be zeros. If not, change opamp

[abnormalalien]

Yes, it does seem to be the case that there's something off at U201 (see attached schematics with measured voltages in green). However, there is also a problem with the input voltage to the op amp. Instead of 7V, I'm getting around 3-4V. I've tried following the schematics to find out where the 7V comes from but I couldn't find it. Any ideas where that comes from so I can trace what's wrong there?

Thanks!

[Zucca]

Nice!

We are talking the same language see... a little bit of effort and you are with us.... very cool.

Again a shot U201, replace it.

Regarding the 7V, anything getting hot on the board? It could be a bad device (maybe U201?) pulling down the 7V rail.

I've tried following the schematics to find out where the 7V comes from but I couldn't find it. Any ideas where that comes from so I can trace what's wrong there?

In the manual sheet 5 of 8, 9-12, page 160 of 165 in the pdf here:

https://www.keysight.com/upload/cmc_upload/All/34401-90013-mla2.pdf

there is a nice page with title REFERENCES: 7V is generated by U400-B. I did spent 5 minute on this so sorry if I was wrong... but it should help you anyway.
EDIT: Yes the 7V are 1:1 the output from U403 (LM399). The U400-A takes the 7V from there to the U400-B which is just a buffer for the rest.   

ahhh look here too:

https://www.eevblog.com/forum/repair/repairing-an-hp-34401a-dmm-51092/msg714640/#msg714640

EDIT2: check the supply voltage rails 15V/18V for the OPAMPs too.. just to make sure...

[abnormalalien]

Excellent, thank you! I was completely skipping over the word References in favor of the schematic titles at the bottom right. I will check out this one next!

I am still a bit confused about ground. Earlier, I mentioned my advisor said I should be able to directly measure voltages by putting lo at case ground and high at whatever pin/location I'm trying to measure. I tried that and it did not work; everything around U201 was 0V. Then I checked: the resistance between case ground and the white line behind the display ribbon is OVLD. Thus, I've been using the white line. On the power schematics, this is labelled GND. But on the References (AD Converter) and the Ohm's supply schematic, many of the grounds are labelled AGND (analog?). Can I still use the white GND line or is there another ground that is more appropriate for AGND labels?

[Kleinstein]

For most measurements (especially DC volts) in the analog part the common input terminal should be a sufficient ground point.

The outer case is connected to protected earth and the interfaces and digital section. So there are two essentially isolated parts.

[abnormalalien]

Ok then! I think I might see the (a?) problem: the output of my U403 is not 7V, it's around 5.7V. However, I noticed as well that the -15V inputs to U403 is also not correct(-14.4V). If I'm understanding right, that comes from CR402A. So, if I replace CR402 and it properly outputs -15V, will U403 give me the correct 7V? Or are these two separately problematic components?

[Zucca]

U400-A needs also a funeral in my eyes... the output should go 15V and not 6.35V.

Regarding the -15V being so high -14.4 remember it could also be too much current on the innocent CR402A.
You could also desolder CR402A and test it outisde....

My next steps would be:

1) Search for hot spots on the board... as I said before too much current can offset the power supply voltages.
2) Remove U400 and check voltages again
3) Remove U201 and check voltages again

There is not too much to measure further, grab the iron ehm... hot air gun and act.

BTW something does not make sense, PIN 1 U403 is 5.67V but PIN 3 U400-A is 5.90V unfortunately they should be connected and have the same voltage...  is that voltage changing in time? Time for getting a scope?

PS: Congratulations for your work, I wish all the beginners would be like you! 

Z

[Dr. Frank]

U201 is defective, because it doesn't regulate the constant current through the 28.75k hybrid resistor correctly, and its offset between + and - is way too high.
U400-A is defective, as it does not amplify correctly to 10V, (and its offset is way too high, also), which in turn doesn't feed enough current to the LM399, its output is probably oscillating.
So the -10REF voltage of -6.13V might differ that much from the +10REF value of 6.35V, due to the oscillation also, but U401-A might as well be defective.
This can be decided only after replacement of U400, when the +10REF is ok again.

You should order 3EA of the AD706.

This -14.4V supply voltage is probably ok, as the value of CR402-A and also CR401-A  probably have been changed to 3.6V at some time.
Take notice, that the +15V supply is ALSO about 3.7V lower than these 18.73V, instead of the nominal 3.3V zener voltage.

Frank

[abnormalalien]

Thanks you guys! I'm trying really hard on this one. One thing I'm a bit perplexed on...how do you mean is anything getting hot? Uhmm, I'm not real comfortable sticking my hands in there and that's about as hand-waving an estimation as I could make...is there some professional way of telling what the component temperatures are? Like an IR camera or something?

[Zucca]

If you don't know where to put your fingers just don't do it.
Order some AD706s and start replace the failed components, we will see what happen then.

[abnormalalien]

Took some time to get the AD706s from Newark but I finally replaced both U201 and U400 and now the multimeter passes all self-tests. I also tested various functionality: DCV, ACV, and 2WR are all great. Tested 4WR with a handful of resistors and found that the values seem reasonable. The only oddity is that when nothing is connected to the inputs and 4WR is selected, it does not read Overload. I believe it should read OL when there's no inputs right? Is that a sign of another problem?

Haven't tested ACV yet (too many researchers not enough machines) or ACI (how does one even go about sourcing ACI?).

Thanks again for all your help!

[Zucca]

BOOM, Enjoy your repaired meter.

What does it read then in 4W with nothing connected? Remember the floating Voltage on Sense + and sense - is interpreted by the DMM as a resistance value.
I'm out from my lab so I can not check, but to me it's not a problem.

Now you should calibrate that puppy, self test passed doesn't mean it will works 100% right.

In USA a 34401a can be calibrated for cheap, do it.

[abnormalalien]

When I first turned it to 4wire, it was around 0.2Mohm but it looked like it was climbing. Now, it seems to be varying between 0.37Mohm and 0.38Mohm. You mean it is better to pay to get it calibrated rather than do it myself?

[bitseeker]

abnormalalien, any stray voltage on the sense inputs will register as a resistance value. Remember that the DMM sends current through the device under test, measures the voltage across it, and then calculates the resistance. Rub your finger across the input jacks while in 4W mode and you may be able to alter the apparent resistance that the meter sees.

On my 34401A in 4W mode, sometimes it goes up to around 0.3M and stays for a while or goes down to zero. However, even if it goes up for a while, it eventually ends up at or near zero. Switching to other modes and returning will display a resistance for a little bit and then returns to zero.

It's best to check that it's zero with a 4-wire short and measures known resistors correctly.

[Zucca]

Thanks bitseeker, that's what I was thinking.

You mean it is better to pay to get it calibrated rather than do it myself?

If you have some calibrators and you trust them, well good luck.
I will put 75$ now 99$ on the table and considered it done, no brainer:
https://www.eevblog.com/forum/metrology/$75-calibration-with-adjust-and-data-for-a-variety-of-multimeterscountersetc/

[Dr. Frank]

Took some time to get the AD706s from Newark but I finally replaced both U201 and U400 and now the multimeter passes all self-tests. I also tested various functionality: DCV, ACV, and 2WR are all great. Tested 4WR with a handful of resistors and found that the values seem reasonable. The only oddity is that when nothing is connected to the inputs and 4WR is selected, it does not read Overload. I believe it should read OL when there's no inputs right? Is that a sign of another problem?

Haven't tested ACV yet (too many researchers not enough machines) or ACI (how does one even go about sourcing ACI?).

Thanks again for all your help!

Ok, so my error prediction was correct.

The behavior of the open 4W mode is fully normal!

In this configuration, the Input jacks (the right ones) work as a constant current source and the Sense jacks (the left ones) operate as a voltmeter input with high impedance.
There is no internal connection between the current source jacks and the sensing jacks, therefore, there is no overflow indication and display simply indicates the floating voltage on the sensing jacks, as if you would put the 34401A into DCV volt mode, with High Impedance mode and jacks open.

So if the sensing jacks accumulate enough charge by its leakage current, the voltage might drift up or down, and finally might lead to an overflow.

Frank

[abnormalalien]

Awesome, thank you guys so much! I talked to my advisor about calibrating and he didn't seem that concerned. He had me test it against our Keithley sourcemeter and it is accurate over a large range of voltages (our primary measurement).

So thanks again for helping me prove two people - department electrical engineer and thesis advisor - wrong when they said they didn't think I could get this thing up and running again!

[Zucca]

Don't tell them we helped you

 

[bitseeker]

So thanks again for helping me prove two people - department electrical engineer and thesis advisor - wrong when they said they didn't think I could get this thing up and running again!

Hehe, that's excellent.

Beware you don't get drafted into fixing everything that goes wrong from now on.

[bobof]

Hahahahaha... if you don't laugh you cry.  This time the joke is on me!

Saw an auction come up for a load of test gear with a local auction house.  3x 34401A units (1 HP, 2 Agilent) in what looked like nice condition.  Got them for what I thought was sensible money for 3 good instruments...  Of course, no opportunity to test them.

All 3 have significant issues; 2 have the various combinations of codes which have often been fixed by U201 replacement.  I figure I'm likely to need one at least by odds so why not order one.  Might as well open them up firstly to see what is going on inside...

First one I open...


The other two at least don't have any physical damage and measure small DC volts OK, though resistance isn't right on either.  Projects for another day I guess... ho hum!
On a 3 out for 3 basis I certainly can suggest where to think twice about buying test gear from from in future...

[Zucca]

bobof, you can do it. Open a separate threadand start with little steps and patience.

[tggzzz]

Hahahahaha... if you don't laugh you cry.  This time the joke is on me!

Saw an auction come up for a load of test gear with a local auction house.  3x 34401A units (1 HP, 2 Agilent) in what looked like nice condition.  Got them for what I thought was sensible money for 3 good instruments...  Of course, no opportunity to test them.

Phew. I'm glad I didn't bid on those I decided to spend my money on a 19" rack full of vaguely similar equipment which I haven't checked yet. If and when I can figure out how to use it (!), I'll let people know. If not, I believe contains many many Vishay foil resistors

One of the previous items I bought from that auction house went into the rubbish bin, but fortunately I was able to rescue a functional HP OCXO. Another item was only fit for stripping for parts.

OTOH I've had some good things from that auction house, but it is very hit and miss. At least at this time of year there won't have been too much time for stuff to have become rusty.

[bobof]

Well, one of the other units did have a bad B side on U201, and rather helpfully my BBQ'd unit's U201 appeared to be OK at a cursory glance, so it got swapped in (beats waiting for Digikey), and Ta-Da! At least now I have one unit that appears fully working.  The BBQ unit display was brighter too, so I swapped that over while I was at it.

Phew. I'm glad I didn't bid on those I decided to spend my money on a 19" rack full of vaguely similar equipment which I haven't checked yet. If and when I can figure out how to use it (!), I'll let people know. If not, I believe contains many many Vishay foil resistors

One of the previous items I bought from that auction house went into the rubbish bin, but fortunately I was able to rescue a functional HP OCXO. Another item was only fit for stripping for parts.

OTOH I've had some good things from that auction house, but it is very hit and miss. At least at this time of year there won't have been too much time for stuff to have become rusty.

It looks like in this case I've literally bought someone's bone pile... Knowing this now next time I'll temper my bidding accordingly.  Hope you have better look with the gear you picked up!  At least with one working with a nice bright display it doesn't stand me at much of a loss.

bobof, you can do it. Open a separate threadand start with little steps and patience.

Thanks! Will do.  I've stripped the burnt corner of the board including the relay.  PCB has been properly carbonised in the area so I guess the only way this could be made functional again would be to re-build this area off the main PCB.  Board at least looks OK around the relay site, and there isn't much to the circuit so I think it has to be quite do-able (assuming nothing downstream has taken a dirt dive).  Maybe with a slim repair board made up with the relay footprint on it to tie the repair board to the mainboard.  Oh, and it will want a U201 now it has been scavenged...

The last remaining faulty unit had a VFD which had some leakage issue (extraneous dots) and has errors 744,745 for the calibration data.   More research needed...

[ua4yhz]

Hi everyone. My device has the same errors during testing. Tell me, what voltage should be on the U101-B? I have 0V on all pins. New chip 1SK6-0001 PLCC44 has not yet ordered, decided to consult here.

[Dr. Frank]

Hi everyone. My device has the same errors during testing. Tell me, what voltage should be on the U101-B? I have 0V on all pins. New chip 1SK6-0001 PLCC44 has not yet ordered, decided to consult here.

Hello,
your failure description is meaningless and unspecific.
U101-B is the input multiplexer, and w/o any signal applied, its input pins, and the ouput pins will be of course at or around 0V.
U101 itself very rarely fails, if it turns out later that it's affected, then check if its +5V, +18V and -18V supplies are ok

But please first describe, which modes and ranges are failing, and which are working, then we might track down the error properly.

Frank

[ua4yhz]

The measurement of resistance and "cont)))" does not work. I checked your voltage map and all the voltages were approximately the same as yours. Those that are written in black squares.

When the inputs are disabled, the readings slowly show increasing resistance. The reading stops at about 800 ohms over the entire range of the resistance measurement.

[Dr. Frank]

The measurement of resistance and "cont)))" does not work. I checked your voltage map and all the voltages were approximately the same as yours. Those that are written in black squares.

When the inputs are disabled, the readings slowly show increasing resistance. The reading stops at about 800 ohms over the entire range of the resistance measurement.

The faulty values are inside the black squares, my correct ones are the hand written values. That's a first indicator that this OpAmp U201 is defect, again.

Maybe you can copy your own measurements inside this graph, also, but please apply a resistor and state, which measurement mode you're using.

I do not understand what you mean with  "inputs are disabled".
Are you using 4W Ohm mode and simply leave the jacks open??

Please make measurements only in proper configuration.   

Uh yes, and of course all this has nothing to do with U101, up to now.

Frank

[ua4yhz]

I just leave the nests open.
OK, thanks Frank.
I will look for the u102 chip and change it.

[Dr. Frank]

I just leave the nests open.
OK, thanks Frank.
I will look for the u102 chip and change it.

nests?
If you use 4W Ohm and let the jacks open, then the sense lines will simply drift to a random bias voltage, so that's no correct modus operandi.

Although U 201 (!) is a good guess,I would first make sure, that it really fails, by posting your voltages here.

Frank

[ua4yhz]

I just leave the nests open.
OK, thanks Frank.
I will look for the u102 chip and change it.

nests?
If you use 4W Ohm and let the jacks open, then the sense lines will simply drift to a random bias voltage, so that's no correct modus operandi.

Although U 201 (!) is a good guess,I would first make sure, that it really fails, by posting your voltages here.

Frank

I repaired the device today. Q201 and Q202 are broken. U201 does not seem to be working either. I will order these three items and then unsubscribe after replacing them.

[ua4yhz]

Today I changed Q201 (MMBF4392), Q202 ( MMBF5461) and U201 (AD706). Tests-pass. 2W mode worked. 4W mode worked.

[bitseeker]

Good progress.

[calin]

Well, i have to thank everyone  .. or brag about it
Thanks to this thread I revived my "gamble" 34401A that I scored on ebay for 180$ (shipped !!!)  . At arrival it was not looking very bad, power supplies all dandy but it was showing the usual errors 612, 613, 618 and 619.

After I replaced the famous U201 and Q201 bah still no luck .. same failures. I started to go component by component and after super careful inspection of the mainboard (under microscope) i found the culprit .. R122 which had left the magic smoke escape. Most likely it was fried by some voltage spike applied at input since that seems to be on the DCV measurement path if I read the schematic correct.
Anyways after that no more errors, all self tests pass  with success. I tested as much as i could and everything seems to be working correct as far as I can tell. It even looks like still being in calibration even if it seems that was last calibrated in 1995. I need to search for a cal lab to get it calibrated. I kind of remember that I saw somewhere on the forum someone saying that there is a cal lab in Phoenix, AZ (my town) which does not charges an arm and a leg for a calibration. Does anyone know the name of the company ? or find that post somewhere ?
Again, thanks all for the good info on this thread !!!

EDIT: I found them !!! https://4gte.com/products/75-calibration-special/
The thread is here - https://www.eevblog.com/forum/metrology/$75-calibration-with-adjust-and-data-for-a-variety-of-multimeterscountersetc/

[Dr. Frank]

Hello,

I bought this defect 34970A with the same error codes.
It has the same DMM circuit inside, like the 34401A, but on a separate board.
In the picture you see the infamous U201, AD706, which also was defective in my case.
The measurement on the constant current circuit revealed that the 2nd OpAmp, pin 5, 6, 7 broke down as usual.
I've written down the voltages before and after  '=>' repair .

What I was not aware of before is the fact that the bipolar input transistors break down in such a manner, that they source fault currents from the positive supply (+18V) into the +/- inputs.
The -input therefore drives an additional current via pin 40 backwards into the FET switches of ASIC U101, which raises the voltage on the Source of j-FET Q202 to these 12.6V.
One can measure this excess current of several mA by attaching a current meter in its mA range from the Source of Q202 (pin 1) to AGND, when selecting a low test current of 5µA or 500nA in the 1 or 10MOhm ranges.

After repair, the DAQ 34970A operates correctly again. It's from 2000, has still its first factory calibration (CAL COUNT = 32), and deviates by about +30ppm in all DCV and Ohm ranges, pretty good for that time period.
What's really nice, compared to the 34401A, that the Firmware features temperature measurement, and OFFSET COMPENSATION for the Ohm mode, which I'm missing on my old 34401A from 1993.

Frank

[bitseeker]

Thanks for including the analysis schematic, Frank. My 34970 just needs a new battery, but I'll know where to come back to if it misbehaves beyond that.

[Dr. Frank]

Thanks for including the analysis schematic, Frank. My 34970 just needs a new battery, but I'll know where to come back to if it misbehaves beyond that.

No problem.
Nobody asked about the T-sensor, so far.
Frank

[JohnAH]

Hello Gentlemen,
  Thank for all the great information about the Hp34401 errors 612, 613, 615, 617, 619 ect. But so far none of the solutions have solved my meter's failure.
Hoping you can help me or at least point me in the right direction. 

I have swapped out U201, q201, and q202. Looked for shorts and opens but have found nothing out of order. I have a good meter to compare it to but still can't seem to figure it out. Attached is Dr Frank's Current Source sheet with my values in blue. 

Thanks for any light you can shed on the problem.

John

[Dr. Frank]

Hello Gentlemen,
  Thank for all the great information about the Hp34401 errors 612, 613, 615, 617, 619 ect. But so far none of the solutions have solved my meter's failure.
Hoping you can help me or at least point me in the right direction. 

I have swapped out U201, q201, and q202. Looked for shorts and opens but have found nothing out of order. I have a good meter to compare it to but still can't seem to figure it out. Attached is Dr Frank's Current Source sheet with my values in blue. 

Thanks for any light you can shed on the problem.

John

John, your voltages are partly ok, if you compare it with the latest diagram I have published for my 34970A. The slight differences probably occur due to different supply voltages and a different zener diode.
It's not clear, what mode you use, and in which state you took the measurements.. i.e. open circuit in Ohm modes, or 4W, or which resistor was attached?

Which error messages do you get, when you are doing the full self test?

Please measure the constant current values e.g. in 4W mode out of the "Input Hi / Lo" jacks, for 100 Ohm to 10MOhm ranges.

Frank

[JohnAH]

Error codes 612, 613, 615, 617, and 619
blue ink measurements were 2 wire ohms auto

4 wire in respective ranges against high accuracy resisters .1%
R            I             Range
100ohm .998mA    100 range
1k ohm  .998mA    1K range
1k         .o991 mA  10k range
1k         .0096 mA  100k range
80k       .0095mA   1M range
80k       .0046mA   10M range

The ohm readings were quite close. I didn't calculate them out but all probably within 1%.

[Dr. Frank]

Error codes 612, 613, 615, 617, and 619
blue ink measurements were 2 wire ohms auto

4 wire in respective ranges against high accuracy resisters .1%
R            I             Range
100ohm .998mA    100 range
1k ohm  .998mA    1K range
1k         .o991 mA  10k range
1k         .0096 mA  100k range
80k       .0095mA   1M range
80k       .0046mA   10M range

The ohm readings were quite close. I didn't calculate them out but all probably within 1%.

1M and 10M ranges have the wrong currents, if you didn't mix up the ranges / measurements.. should be 5µA and 500nA instead of 10µA and 5 µA.. a pity that you can't measure the low currents with higher resolution.
Anyhow, this would partly explain errors 612, 613 but not the others, as these currents or compliance voltages seem to be ok.

Another question:
What have you done with Q201, Q202, U201:
Did you swap  them, i.e. really swap Q201 and Q202,  or have you replaced all 3 components with new, compatible ones?

Would it be possible for you to check all these ranges at near Full Scale, i.e with the appropriate resistors, like 10k, 100k, 1M and 10M?

Did your instrument fail, or have you bought it as defect?

The current sources seem to be work, maybe partly, but something is wrong at the output stage Q202 and behind.
Maybe the switches inside the ASIC are steered incorrectly...
I have to think a bit more, in the meantime.

Frank

[JohnAH]

yes, i just checked, i mixed them up
R            I             Range
100ohm .998mA    100 range
1k ohm  .998mA    1K range
1k         .o991 mA  10k range
1k         .0094 mA  100k range
80k       .0043mA   1M range
80k       .00012mA   10M range

u201 was new. QS

[Dr. Frank]

All right, now the 500nA seems NOK, but already the 5µA might be just an error from your other DMM, due to lack of resolution. What other DMMs do you have?
Please try to measure with an appropriate  1% shunt, to decide, whether the sources are ok, or not.
What about Q201, 202? Have you changed anything with them?
Or what's the meaning of to "swap out"?

In which range and mode did you measure the voltages in your diagram?
Open jacks, or something connected?

[JohnAH]

Sorry it looks like half the message I sent last was truncated.

We don't have any better DMMs.
both Qs were known good used stock. They were not swapped with each other.

The diagram was 2 wire auto range, open jacks. 
I can probably get high ohm precision resistors but it going to take a little time.

The meter has been working for a long time. It died when a customer borrowed it. It is being used to verify accuracy in a ICT machine via IEEE program.
 

[Dr. Frank]

OK, under these circumstances, at first please measure the voltages with a load connected, e.g. 1k resistor in 1k range.

I assume, that U201, Q201, Q202 are fine, as the currents might work ok, as far as one can tell from your measurements.
If you have a 2nd 34401A, then you might measure the 80k resistor precisely, use it as a shunt, and measure 10µA and 500nA, using the 2nd DMM in its 1V and 100mV range, i.e. about 800mV @ 10µA and 40mV @ 500nA, but with much higher resolution. Simply use a calculator, and set the 34401A to R > 10¹⁰ Ohm. The absolute accuracy is not important, 1% is fine, but I just want to get rid of offsets and alike.

I assume that the transistor array might be damaged, or the 3rd FET which goes to GND.

Didn't we have that here, recently?

Sorry, need to sleep a bit now, continuation by tomorrow.
Frank

[Dr. Frank]

Another idea: Please check if a 10V dc signal is read correctly in the 100V range, compared to the 10V range. maybe the relay is damaged, as it's used also for all these failing tests.


I assume, that the Ohm circuit was not damaged at all, initially (have you checked the correct function before you changed the components?).

Only the 5 tests for current source compliance failed, but that could also be caused by components in a completely different section, like this relay, or other components inside the DCV path.
Therefore please check all DCV ranges also.

Frank 

[JohnAH]

That might be it. The 10V scale is slowly to rises to 9V. It really never gets there.
The 100V scale measures 10V.

Using Fluke Calibrator.

[JohnAH]

Got it! C

[Dr. Frank]

Got the fault?

[JohnAH]

I don't know what is going on with my posts but a lot of them are getting truncated.

Yes found it. R105 had a cracked solder joint. It is passing everything now.

Thank you Dr Frank.

[shaohu.tang]

In a few days I will also plan to repair a few of them, and then release them to ask everyone!

[spicytofu]

I started repairing a unit with similar error codes. I have repaired units in the past by replacing U201 as most have done here. However, this one is a no-go. All DCV, ACV ranges work and spot on. DC-I, only the 10 mA and 100 mA are accurate. The 1 A and 3 A ranges are way off. Now lets get into the interesting part: Ohms source.

All opamps have the normal +/- 15 V and 18 V, so voltages and zeners look good. When in ohms mode, no current comes out from the TERMINALS, no readings either. HOWEVER, when I measure the current from the anode of CR202, I am getting the CORRECT currents on all ranges......huhhh? The 9.9 M resistor divider measure good. I measured most of the U102 hybrid but it was not removed from the PCB, anyways I dont suspect U102 to be bad based on the current source is working fine. The only problem is there is no current coming out from the HI terminal. Any ideas? must be a faulty contact or relay?

[HighVoltage]

Did you check the front / rear switch?

[Dr. Frank]

Yes, the front-rear switch might be polluted by its own plasticizer from the housing, but this would not explain the current mode failure.
Anyhow, check front/rear jacks for Ohm, and especially if the 4W mode is working somehow.

As both modes, Ohm and current are affected, K102 is under suspect. It may be stuck, shorting poles 9-13  so that only the low current range is available.
That would imply (or maybe not?) that it would always short the OHMS_CURRENT to GND, via poles 4-8, but you are able to measure the current behind CR202.
Maybe this part of the relay is completely open. Actually, I'm not aware how bistable relays are working mechanically.
Check for continuity on this relay, and also the resistance of the current range shunts, when you switch from 100mA to 1A. The difference 0.1 to 5.1 Ohm should be clearly detectable.

Frank

[spicytofu]

[0quote author=Dr. Frank link=topic=34104.msg4355395#msg4355395 date=1660376375]
Yes, the front-rear switch might be polluted by its own plasticizer from the housing, but this would not explain the current mode failure.
Anyhow, check front/rear jacks for Ohm, and especially if the 4W mode is working somehow.

As both modes, Ohm and current are affected, K102 is under suspect. It may be stuck, shorting poles 9-13  so that only the low current range is available.
That would imply (or maybe not?) that it would always short the OHMS_CURRENT to GND, via poles 4-8, but you are able to measure the current behind CR202.
Maybe this part of the relay is completely open. Actually, I'm not aware how bistable relays are working mechanically.
Check for continuity on this relay, and also the resistance of the current range shunts, when you switch from 100mA to 1A. The difference 0.1 to 5.1 Ohm should be clearly detectable.

Frank
[/quote]

Thanks Frank and HighVoltage. I did see a small PCB trace burnt off at the switch. The trace was so small, I wouldnt have known it was there. I tried connecting back the trace but still had the same errors as the title of this thread. Measuring the voltage of the anode of CR202 I get .55V, and then at the cathode is basically 0V with some uV variations. (if I got my terminology right, cathode is the white line end of the diode, just making sure) I tested the diode and works fine with a FV of .55V

I removed the CR202 and shorted it and still no current comes out of the terminals. (rear and front) Measuring the resistance from the CR202 cathode to LO terminal, I get 9.99 Megs. I tried following the schematic to where the ohms current goes after CR202 and I get lost at all the relay switching. I was attempting to follow the schematics to locate the path where the readback goes into the A/D to see if there are any problems there.

An idea: What if I push a external 1 mA source through a resistor and then connect the HI and LO terminals to read the voltage? Like a 4W mode using two DMMs, but the faulty one would be in 2W mode. Although no current will come out from the HI and LO terminals, I expected it to be able to read the voltage and it will translate into ohms. Then we know it has to be some thing physical blocking the current source.

Update: the 1 mA current source worked. Pushing the correct range current using an external source gives the correct ohms reading on the DMM. Also, 2W and 4W does not work (but does work if you have an external current source). Furthermore, in the DCI, normally I would hear the relay click when it switched to the 1A and 3A range, but there is no click. Measuring the shunt shows 5.5 ohms only, I do not see the 0.1 ohm shunt. Maybe it is a relay problem?

[Dr. Frank]

..

As both modes, Ohm and current are affected, K102 is under suspect. It may be stuck, shorting poles 9-13  so that only the low current range is available.
That would imply (or maybe not?) that it would always short the OHMS_CURRENT to GND, via poles 4-8, but you are able to measure the current behind CR202.
..
Check for continuity on this relay, and also the resistance of the current range shunts, when you switch from 100mA to 1A. The difference 0.1 to 5.1 Ohm should be clearly detectable.

Frank

Thanks Frank and HighVoltage. I did see a small PCB trace burnt off at the switch. The trace was so small, I wouldnt have known it was there. I tried connecting back the trace but still had the same errors as the title of this thread. Measuring the voltage of the anode of CR202 I get .55V, and then at the cathode is basically 0V with some uV variations. (if I got my terminology right, cathode is the white line end of the diode, just making sure) I tested the diode and works fine with a FV of .55V

I removed the CR202 and shorted it and still no current comes out of the terminals. (rear and front) Measuring the resistance from the CR202 cathode to LO terminal, I get 9.99 Megs. I tried following the schematic to where the ohms current goes after CR202 and I get lost at all the relay switching. I was attempting to follow the schematics to locate the path where the readback goes into the A/D to see if there are any problems there.

An idea: What if I push a external 1 mA source through a resistor and then connect the HI and LO terminals to read the voltage? Like a 4W mode using two DMMs, but the faulty one would be in 2W mode. Although no current will come out from the HI and LO terminals, I expected it to be able to read the voltage and it will translate into ohms. Then we know it has to be some thing physical blocking the current source.

Update: the 1 mA current source worked. Pushing the correct range current using an external source gives the correct ohms reading on the DMM. Also, 2W and 4W does not work (but does work if you have an external current source). Furthermore, in the DCI, normally I would hear the relay click when it switched to the 1A and 3A range, but there is no click. Measuring the shunt shows 5.5 ohms only, I do not see the 0.1 ohm shunt. Maybe it is a relay problem?

Hello,
you obviously also didn't read and understand what I have already written: Check relay K102!

It switches the current ranges and as well switches the driving current for Ohm mode, so might explain your double trouble.
As you now additionally describe, that the cathode of CR202 is somehow grounded, this is another confirmation, see schematic!

Please describe in more detail, which signal trace has been burnt! Pictures are welcome!
That could have been another reason for the failure as well, but we need to know, what caused this burning, and what further consequences this might have.

Frank 

[spicytofu]

I was getting to the relay next after my first post. I already checked it and it was stuck, but before I saw your reply. The problem is fixed now, just banged on the relay when I took it out and it is working again. My guess is that a very large current was pushed through burnt some traces and possible arched the relay. The fuse couldnt save it in time. I repaired two burnt traces and everything seems to be working now.

Another note: it seems that the transformer wiring was wrong from the factory. (this unit was never opened) I noticed a very darkened and hot PCB area at the regulators. Checking the AC volts, I saw 28VAC and DCV unreg was 33V (total was 66V!) No wonder the vregs were burning up. I swapped the blk/red and blk/org xformer secondary wires since they were reversed and now I am getting the correct 27VDC and 23VAC at the regulators.