Electronics > Repair
Agilent 34401A Bubble on PCB ... can it be saved ? (SOLVED)
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The_Spectrum.A_idiot:

--- Quote from: Kleinstein on September 20, 2024, 10:49:06 pm ---The parts that were replaced don't need to be paired and most of them should not even effect the calibration / scale factors. From the changed parts it is only the offset of the OP-amp that has a small effect on the scale factor of all resistance ranges. The OP-amp bias and leakage currents can also effect the high resistance ranges (10 M + 100 M and a little 1 M) . The others should not be effected enough to get much of an error.

It looks like the PCB is not yet fully clean / dry. If there is some conductive flux residue or similar it can take quite some cleaning to get rid of the leakage.
Worst case there is leakage from the overheated PCB part or dirt inside the bubble.
one may have to close the case for the tests, so that light would not effect some of the SMD parts - some can be a bit light sensitive.

Changing all the parts was not such a bad idea. The OP-amp was very likely bad from the tests and the other parts are not that expensive and possibly damaged.


The voltage and current ranges should not be effected by the repair, unless there is PCB contamination.
There are a few additional tests that one can do with relatively little effort / instruments to check a few more points no checked so much in the self test.

--- End quote ---

okay , I will try to clean as much as possible under the IC and around it ( Isopropyl and contact cleaner might do the trick ? )

I guess giving the whole PCB an ultrasonic bath would be a big no. no I assume.

I will get more resistors and see if I can get the reset fo the tests done.

Thank you very much


coromonadalix:
same here, you solved many errors,  maybe another cleanup with better products,  contact cleaner must not leave any residue or oil / lubricant

and maybe, just maybe  try to get  better resistors values  as kleinstein wrote to tests all possibilities, and slowly redo all the tests / current generator  etc ..

this meter has gone thru a lot,  be sure the front and rear switch is good too, it can play some tricks, there was some who had relay problems  on one coto  model ...

be patient and do things methodically, you'll get there,  and lucky you  schematics are known ...
The_Spectrum.A_idiot:
You can say that again indeed this poor instrument had a very harsh life.
But yes step by step hopefully I can get it back to spec.
Dr. Frank:
sorry to reply so late.
The current source still might have a problem gor the high Ohm ranges. That should be the 500nA source current, affecting both highest ranges.
Please apply a 1M or 10M resistor in 2W autorange  mode, and fill out the diagram, which shows the crucial voltages in this circuit, once with the correct ones as measured by me.
Most suspect is this dual OpAmp with very low bias current, which fails quite often.

Frank
The_Spectrum.A_idiot:

--- Quote from: Dr. Frank on September 21, 2024, 02:37:46 pm ---sorry to reply so late.
The current source still might have a problem gor the high Ohm ranges. That should be the 500nA source current, affecting both highest ranges.
Please apply a 1M or 10M resistor in 2W autorange  mode, and fill out the diagram, which shows the crucial voltages in this circuit, once with the correct ones as measured by me.
Most suspect is this dual OpAmp with very low bias current, which fails quite often.

Frank

--- End quote ---

1M resistor : 0.9953mohms on fluke 289
on instrument :  0.959,20 =mohms

R201 4.993v
R202 6.972v

U201-A 3 - 6.972v
U201-A 2 - 6.972V
U201-A 1 - 3.8189V
+7REF - 6.972V

Q201-1 - 8.896
Q201-2 - 7.113
Q201-3 - 3.8189

CR203-A 1 - 13.974
CR203-A 3 - 18.810

1SRC Q202 - 1 - 8.920
Q202 - 2 - 5.965
R290 - Q202 - 3- 10.406
 
CR201-A -3 - 10.406
CR201-A -1 - 5.667

U201-B - 7 - 5.670
U201-B - 6 - 8.926
U201-B - 5 - 8.901

Thank you in advance
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