| Electronics > Repair |
| Agilent 34401A Bubble on PCB ... can it be saved ? (SOLVED) |
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| Dr. Frank:
--- Quote from: The_Spectrum.A_idiot on September 19, 2024, 06:22:13 pm --- Used a 9V PSU Tried every range on VDC and values don't change. re measured μΑ on ranges after cleaning the flux and doing the range change. Results : 000. ohm range -999.1 μA 0.1k range - 999.3μA 00.1k range -99.46μA 000.1k range -9.84μA 0.1M range -4.83μΑ 00,1M range - 0.36μΑ ** 000,1Μ range -0,36μΑ ** --- End quote --- The reference currents are all still n.OK, i.e. about 140..180nA too low, which is clearly visible for the 500nA, 5µA, 10µA and probably also for the 100µA reference currents.That's the reason, why you get ERR 612, 618, which both indicate a failure of the 500nA current, i.e. out of tolerance. U201 was really defective, as initially none of the expected voltages @ pin5, 6 were ok, instead showing the usual faulty values for too high bias currents, i.e. broken input stages. Now you have to search, where these about 180nA are flowing to GND. Hopefully your new U201 is OK, then Q211 would be the next suspect. So let's at first rule out both these failure modes: Please desolder CR202, and then measure the reference current directly in situ towards AGND, i.e. from pin 3 / collector of Q209 to AGND, which is @ INPUT LO. you might check for the lower currents of 500nA, 5µA and 10µA only, so you have to select 10M, 1M and 100k ranges (see page 98 of service manual). An external resistor is not needed. Please compare these three currents with those you have measured in the above table. Frank |
| Dr. Frank:
For evidence, I measured the currents of my 34401A: 100, 1k range: 1.0003 mA (+0.03%) 10k range: 99.898 µA (-0.10%) 100k range: 9.9858 µA (-0.14%) 1M range: 5.0198 µA (+0.4%) 10M range: 501.66 nA (+0.3%) Therefore, the 3 highest currents are about 0.1 .. 0.2% within nominal, and the 2 lower currents are about 0.5% within nominal, so that's the ballpark what to expect as being OK or nOK reference currents. To be more precise: The nominal values of the currents should be within about 0.5% consistent, relative to each other, as an additional +/-2% for the LM399 voltage reference (6.95V nominal) has to be taken into consideration. For my 34970A, I found these currents: 100, 1k range: 0.9877 mA (-1.2%) 10k range: 98.712 µA (-1.3%) 100k range: 9.8962 µA (-1.0%) 1M range: 4.9341 µA (-1.3%) 10M range: 494.3 nA (-1.1%) Frank |
| The_Spectrum.A_idiot:
CR202 Removed; Measurements still the same : 00,1M range - 0.36μΑ 000,1Μ range - 0,36μΑ next step ? |
| mawyatt:
Out of curiosity we took a quick look at our HP34401A and AG34401A {readings}. These were acquired years ago with unknown history and aren't used much these days, and never properly calibrated since acquired. 1.001484mA {1.001779mA} 100.3637uA {100.0724uA} 10.00111uA {10.04446uA} 4.99031uA {5.026583uA} 497.396nA {503.761nA} BTW the above Dr Frank post obviously should be 501.66nA not uA on the 10M range. Best, |
| Dr. Frank:
what about 1M and 100k ranges? Next step: Please remove Q203 and measure these 3 currents from pin 2 of Q202 to AGND / INPUT LO. Frank |
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