HP 34401a - No DCV/Ohms issue

[n7gtb]

Greetings all,

I have a HP 34401a that fails 612, 613, 615, 617 and 619. It also does not respond to DCV or Ohms applied voltages/resistance measurements. ACV appears to work as expected.  I have not yet tried amps.

What I have done so far:
I searched this forum for all references to the above mentioned codes, and while very informative, did not quite match or solve this particular problem.  I checked the supply voltages, voltage across 2W Hi/Lo in the different Ohms ranges, and all voltages appear within range.

Studying the schematics and block diagrams shown in the service manual, I thought it odd that the unit would display/read ACV but not DCV, as the ADC receives only DCV (i.e., RMS ACV are converted to DCV per the manual).  So I decided to do the following:

Applied approx. +5VDC from my benchtop adjustable regulated power supply, and (using a handheld DMM or 'HHDMM') measured between the large PCB ground pad located near CR202 and R104 (on the end that connects to K103).

With the 34401a in DCV mode, it displayed correctly, but only after I touched the probe tip of the HHDMM.  The HHDMM also confirmed that the applied volts were present at R104.  I also could hear what sounded like one of the relays 'click' at the same time the 34401a correctly displayed the applied voltage (hope all that makes sense...).

So I'm wondering if this is a logic problem, bad relay, or ...?  After writing all this, it looks as though the relay coils are supplied from separate +- 15VDC supply? I will be attempting to investigate that next.

The error codes are perplexing, as it appears the 34401a is unable to read the Ohms compliance voltage (something Dr Frank mentions in a related post...).  This caused me to suspect the DC amp, but now in light of the above, I'm uncertain what's causing them.

Any suggestions/advice on what to check, would be greatly appreciated... I think I might be getting close to zeroing in on the culprit, but not quite certain where to look next.
Thanks.
Vern

[n7gtb]

More playing...  While switching ranges, and modes, something in the 34401a suddenly changed. I found that ACV works, both in manual and auto ranging. DCV only appears to work on highest (manually selected) volt ranges (2 and 3 digits). Anything lower and it simply displays 'OVLD'. 

In auto ranging, DC volts just float randomly at or below a few hundred mV. Shorting the leads does nothing, nor does applying a voltage.

Resistance, continuity and diode now display OPEN or OVLD on all ranges, both manual and auto range. I'm still measuring the correct voltages between Lo and Hi. But before all this happened, I was fooling around with 4W mode, and trying to get some sort of meaningful response. While trying to measure a length of bare wire, the 34401a suddenly began displaying OVLD, back to some random set of digits, then all zeros, then back to OVLD, where it permanently settled.

More weirdness:  I just repeated the complete self test, and it now only reports error 612.  Continuity and diode test display erratically, with diode displaying the "mVDC".  I checked the voltages in Ohms mode, with a separate DMM, and get 2.6, 6.5 and 7.6 VDC for the different Ohms ranges.

I'll go back and check the power supply voltages... 

[Dr. Frank]

Hi, and welcome to eevblog forum.
Low DCV ranges, Ohm and diode modes are routed via relay K103 into the DCV_Low channel of the MUX U101B.
100V and 1kV are fed into divider and into DCV-HI, ACV from ac/DC converter chip into AC of the MUX.
That explains, why both modes work.

I suspect at first K103, pin 13 to 9 not working. Set DCV, 10V range, and apply e.g. 10V to input jacks.
With a 2nd DMM, test e.g. at R104, if about 10V are present, and at pin5 of u101B. You can use ground of Lo input jack.
If no signal at pin13 / left side R104, replace K103.
If no signal at pin5 of u101b, search R104.. R111 chain, and L110, L111.


t.b.c.

Frank

[n7gtb]

Dr Frank,

Thank you for the welcome, and the assistance!

I did as you suggested, and...

Results...
Applied volts: 10.106 (read with Brymen eevblog dmm)
volts at R104: 10.104
volts at U101B.p5: 0
if no volts at R104 and U101B.p5, then
volts at R104 - R111 chain, and L110, L111: signal found at R104, but not between R105 and R106.

Visual examination of R105: Solder joint on downstream end of resistor looked suspicious, so heated the joint and added small amount of fresh solder, then retested. Still no signal downstream of R105.  Ohms measurement confirmed resistor was 13k. Tried again to detect signal at R105/R106, but still nothing found.

On a hunch, light pressure was applied to middle of R105.  A visual check of the 34401a display found that it registered correctly (i.e., 10.10?,? VDC - close enough to applied voltage) in 10 Volt range.  For a short time, auto range even appeared to work as well, but the moment was fleeting...

I don't have SMD resistors in this physical size, so will need to order; but looks very much (to me) like this may be at least part (if not all) of the issue...

I will report back once the resistor is replaced, with additional test results.  Will also test Ohms function(s) again as well...

Again, thank you for taking valuable time to help!   

-Vern

[Dr. Frank]

Good news.
Cracked solder joints (due to large package size of these resistors), or crack inside resistor is an already known defect for the 34401A, if I remember correctly.

For testing purposes, simply desolder this R105 and replace by either short, or leaded type.
This will only delimit the over voltage protection, but would let you check for root cause, until new parts arrive.

Please check solder joints of other resistors by magnifying glas, and individual resistance values for drift / damage. Maybe you'd like to replace them all.

Frank

[n7gtb]

Are there any problems with replacing thin film resistors with thick film of same values/ratings?  I can find 2512 series SMT resistors in thick film, but not thin (except for Keysight, but cost is large)...


-Vern

[Kleinstein]

The resistor string is there for protection. So the accuracy and resistor excess noise should not matter. So I would think thick film resistors should be OK too. The main parameters to look for would be the maximum voltage and maybe thermal EMF. With not so much temperature gradient in the area thermal EMF should not be such a big deal. So thick film or even carbon film resistors should be OK.

[Dr. Frank]

Big case sizes, even down to 1206 are continuously vanishing from the market.
Especially the expensive Thin Film technology poses additional price pressure to terminate these big sizes.

Thick Film might even be the better choice for this use case, as they might have better overload pulse resistibility compared to Thin Film, due to the literally much thicker film, and simpler structuring / trimming method, but latter being manufacturer dependent.

T.C., tolerance, drift parameters are of no importance, but noise is higher than Thin Film. At 12kOhm value, this does not play a significant role.

So go for Thick Film.

Frank

[n7gtb]

...a quick update:

After re-reading Dr. Frank's prior comments, I decided that I had not fully done 'due diligence' where the resistor in question was concerned.  I re-soldered the other end of R105, and the 34401a now appears to be fully functional (without pressure from above - so to speak).  Ohms, DC Volts and auto ranging for both now function as I would expect them to.  Continuity and Diode test now functions again as well. 

I executed the 'TEST ALL' from the menu, and it passed with no errors reported.

I will likely still order replacement resistors to have on hand (thick film Vishays are cheap enough), and sweep through to check other components (like tantalum caps) for failure or drift.

Vern

[Dr. Frank]

Thank you. 
Another rescued 34401A.
Frank