Electronics > Repair
Agilent 34401A Bubble on PCB ... can it be saved ? (SOLVED)
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chilternview:
Lets recap, you say DCV is OK, and DCI appears to be so, but continuity is flaky and ohms not at all?
And self test gives errors 621 , 619 , 619 , 617, 615, 613 ,612 (you've got two 619s, I guess a typo) - which are AC RMS failing full scale, and all ohms current sources failing? Any more error numbers?

That tells you there are two problem areas:
- ohms after U101/2. So check Q202-211 and U201,CR201/202 (the +7V ref must be OK if DCV reads all OK).
- ACV. Could be anything on the AC schematic page 9-11, but you can apply an AC signal and check with a scope its following the circuit path.

It is possible someone's been in there and tried to repair it and not succeeded/given up, so the visual damage may have been caused by the initial fault and has been fixed, or by someone trying to fix the problem.
The_Spectrum.A_idiot:

--- Quote from: coromonadalix on September 12, 2024, 11:39:24 pm ---if you have them   that's ok

the worst from the bubble would be vias, they could have snapped open, and be very careful not to overheat parts before removal


i've seen some,  drilling an hole where there is no parts and filling the space with superglue  while giving some pressure on both sides, to bring back layers together and slimming down the bubble expansion  ... but not sure if i would try this,  it does seem recoverable, and it could continue to work fine

patience and be meticulous ... you'll get it repaired

i hope the special asic has not been damaged  ....

--- End quote ---

I've checked all of them with my Multimeter they seem ok.

if I missed any we will find out soon enough.

Now I need to find out a way to test the rest :D

I have this but I have no idea under what conditions the system is being tested. 10Mohm in the input ? 
The_Spectrum.A_idiot:

--- Quote from: chilternview on September 13, 2024, 12:57:51 am ---Lets recap, you say DCV is OK, and DCI appears to be so, but continuity is flaky and ohms not at all?
And self test gives errors 621 , 619 , 619 , 617, 615, 613 ,612 (you've got two 619s, I guess a typo) - which are AC RMS failing full scale, and all ohms current sources failing? Any more error numbers?

That tells you there are two problem areas:
- ohms after U101/2. So check Q202-211 and U201,CR201/202 (the +7V ref must be OK if DCV reads all OK).
- ACV. Could be anything on the AC schematic page 9-11, but you can apply an AC signal and check with a scope its following the circuit path.

It is possible someone's been in there and tried to repair it and not succeeded/given up, so the visual damage may have been caused by the initial fault and has been fixed, or by someone trying to fix the problem.

--- End quote ---

Thank you very much for your time and support.
Yes you are right there is a typo.
errors 621 , 619 , **618** , 617, 615, 613 ,612

next how should I test said components properly ?
Check Q202-211 and U201,CR201/202 ?

Are there some expected values ? Do I have to desolder them ?


Thanks
Kleinstein:
One can do a first test in circuit. A common failure mode for diodes and transistors is a short. The DMM diode test usually works OK for this and should also work in circuit here.

Form the meaured voltages things are not totally off. There may still be an issue with the protection, especially the diode used JFET.
It would make sense to measure the test currents to see which ranges are effected. Ideally one would measure the current directly with just a 2nd DMM and also with something like a LED or 2 or 3 diodes in series to see if it changes with external voltage.
Dr. Frank:
Hello Unknown,
it's a pity that some people don't even reveal their first name. I don't like to cite SUCH alias names, in first place  :palm:

From a closeup of the damaged PCB, I strongly assume, that somebody tried to repair the instrument, using a hot air gun around U110, or the Ohm protection transistors Q203 - Q210, either to replace or re-insert them.
That guy used too much heat, also too long, and did not cover the other components, so creating this PCB delamination.
It's not clearly visible, but all components inside  the area of delamination might have been damaged by the heat, at least all the solder joints are under suspect.
Please make a macro photo exactly vertical above this area, so that every component and solder joint is sharp, and better illuminated, maybe from different sides.

The transistors  have been soldered very badly, and I see solder balls. The transistors might be defective, or there might be a short to GND as well , via Q211 which should be tested in situ.
If you select 4W Ohm, do you measure the appropriate test currents out of + Input and - Input for the different (manually selected) ranges?

You have to de-solder these transistors anyhow, so you can test them individually, later on.

W/o the transistors back in place, you might then solder a short from Q202 to CR202 temporarily, to make further tests on the Ohm mode.
Descriptions how to do that, has been written down elsewhere .. I guess you found that thread already, as you copied the OHM schematic here.

I guess, that U110, that is used for current mode protection, has been replaced, but that mode should work properly, and has nothing to do with your OHM problem. Anyhow, many of the solder joints look very bad. U304 also looks bad.

We will definitely continue analysis, when you have presented further results.

Frank
 
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