Electronics > Repair

HP34401A Ohms function problem

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WaveyDipole:
I recently acquired a HP34401A with a failed Ohms function. The DC and AC voltage and current functions work fine. The ohms function shows approximately 3 ohms when the probes are shorted, but resistance readings are all over the place. I researched the problem online and on this site and found that the usual culprits are either the COTO9947 relay contacts or the AD706 op amp in the Ohms current source circuit. The relay is good and I have just carefully replaced the AD706, but the problem persists.

The usual reports of an AD706 failure indicate that there is no current present on the terminals when Ohms mode is selected. In this case, there is approximately 10ma present rather than the expected 1ma, both before and after the replacement of the AD706 so clearly something is still wrong with the current source.

Voltages around the AD706 with reference to AGND (pin 4) terminals open / shorted are:

1.  4.09  / 4.07
2.  6.92  / 6.92
3.  6.92  / 6.92
4.  0 / 0
5.  8.28  / 8.76
6.  7.65  / 8.78
7.  13.25 / 12.44
8.  18.88 / 18.88

The 18V line looks a bit high but other than that I am not sure where to look next and any insights would be appreciated. There is some suggestion on one thread that the problem might be U101 (custom HP chip 15K6-0001), but how do I determine this?

Dr. Frank:
Hello,
you certainly refer to e.g. this thread here: https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg489687/#msg489687

I you don't mind, please get a copy of the schematic, measure all these voltages indicated in the diagram, write them down and copy this here.

Your description of the test current is not clear, i.e. is it faulty, or not? Do you use 2W or 4W Ohm, and what's the difference in behavior?

Please chose 4W Ohm mode and measure all different test currents in each range with another DMM, from 100 Ohm up to 10MOhm. The nominal values are listed in the service manual or in the specification.

In first place, I would not suggest that U101 is faulty, there are some other components around which might cause trouble, more probably.

Frank

Kleinstein:
It looks like there is more current flown than supposed too. So the first suspect would be Q202, as this is the transistor (P-JFET) to control the current. With some 12-13 V from the AD706 the JFET should be essentially tured off all the way. So the voltages around Q202 would be interesting.

WaveyDipole:

--- Quote from: Dr. Frank on May 04, 2021, 01:21:11 pm ---Hello,
you certainly refer to e.g. this thread here: https://www.eevblog.com/forum/repair/hp-34401a-error-612-613-615-617-618-619-621/msg489687/#msg489687

--- End quote ---

Thank you for that link. I hadn't come across that particular thread yet.


--- Quote from: Dr. Frank on May 04, 2021, 01:21:11 pm ---I you don't mind, please get a copy of the schematic, measure all these voltages indicated in the diagram, write them down and copy this here.

--- End quote ---

Ok, I will work on that and post later.


--- Quote from: Dr. Frank on May 04, 2021, 01:21:11 pm ---Your description of the test current is not clear, i.e. is it faulty, or not? Do you use 2W or 4W Ohm, and what's the difference in behavior?

Please chose 4W Ohm mode and measure all different test currents in each range with another DMM, from 100 Ohm up to 10MOhm. The nominal values are listed in the service manual or in the specification.

--- End quote ---

I have measured on both 2W and 4W settings and the results are approximately the same.

Auto   7.861  mA
100Ω   7.924 mA
1kΩ      8.516 mA
10kΩ   8.541 mA
100kΩ   8.494 mA
1MΩ      8.490 mA
10MΩ   8.509 mA
100MΩ   8.507 mA

The current readings are approximate as the current starts high and gradually sinks by a few 10ths of uA.
The following errors are reported:

+612,"Ohms 500 nA source failed"
+613,"Ohms 5 uA source failed"
+615,"Ohms 10 uA source failed"'
+617,"Ohms 100 uA source failed"
+618,"DC high voltage attenuator failed"
+619,"Ohms 1 mA source failed"


--- Quote from: Dr. Frank on May 04, 2021, 01:21:11 pm ---In first place, I would not suggest that U101 is faulty, there are some other components around which might cause trouble, more probably.

Frank

--- End quote ---

I am certainly hoping that the problem is elsewhere and not U101....


--- Quote from: Kleinstein on May 04, 2021, 01:34:57 pm ---It looks like there is more current flown than supposed too. So the first suspect would be Q202, as this is the transistor (P-JFET) to control the current. With some 12-13 V from the AD706 the JFET should be essentially tured off all the way. So the voltages around Q202 would be interesting.

--- End quote ---

Yes, the current is much higher than it should be on all ranges. The voltages around Q202 measured with reference to AGND are:

Q202 (01H - MMBF5460 - P-Channel general purpose amplifier)
G   12.80
D   8.15
S   4.38

So yes, there is some 12-13V from the AD706 but evidently the FET is not turned off. The 18V feed is present at R290. There is a 0.78V drop across the diode and the R290 tests good. Its a P-Channel FET so as you point out, a positive voltage at the gate should turn it off. Looks like Q202 ought to be the next candidate for replacement.

Kleinstein:
The gate voltage at Q202 still looks quite low. It looks like the current from R290 frows towards the gate. The zener diode should not conduct much with a low voltage, at least not to lower the voltage.

Before odering new part, I would also check Q203-Q210 in circuit. There is a chance they can fail from too high external voltage.  This should not cause the extra current from the ohms source, but it could be a path on how to damage Q202.

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