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

[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