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
Thank you for that link. I hadn't come across that particular thread yet.
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.
Ok, I will work on that and post later.
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.
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"
In first place, I would not suggest that U101 is faulty, there are some other components around which might cause trouble, more probably.
Frank
I am certainly hoping that the problem is elsewhere and not U101....
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.
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.