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 14, 2024, 08:21:00 am ---The current to look at from the ohms source should be DC from 0.5 µA to 1 mA, depending on the range. Beside the current source part, that is also a chance that a realy is at fault. One could check this by taking the current off CR202 instead of the + input.

No current may also be faulty fuse at the external DMM - so maybe check the continuity.

The already "repaired" area needs to be cleaned anyway - so a touch-up with new flux can be worth a try.

If the resoldering changes things it may be woth to re-measure the voltages (to GND) at Q202, e.g. in the 1 K range, both with the terminals open and shorted. This should give a good idea if the issue is more with the actual current source of the protection.

--- End quote ---

I think my 117 has no ability to measure so small currents.
I need to test it out with 287.

Thanks
The_Spectrum.A_idiot:

--- Quote from: Kleinstein on September 14, 2024, 08:21:00 am ---The current to look at from the ohms source should be DC from 0.5 µA to 1 mA, depending on the range. Beside the current source part, that is also a chance that a realy is at fault. One could check this by taking the current off CR202 instead of the + input.

No current may also be faulty fuse at the external DMM - so maybe check the continuity.

The already "repaired" area needs to be cleaned anyway - so a touch-up with new flux can be worth a try.

If the resoldering changes things it may be woth to re-measure the voltages (to GND) at Q202, e.g. in the 1 K range, both with the terminals open and shorted. This should give a good idea if the issue is more with the actual current source of the protection.

--- End quote ---

okay tested with the 289.
0.57-0.5μADC on any scale ohms.

the areas in question are cleaned and seem ok.

**If the resoldering changes things it may be woth to re-measure the voltages (to GND) at Q202, e.g. in the 1 K range, both with the terminals open and shorted. This should give a good idea if the issue is more with the actual current source of the protection.**

Are there any expected values ? so set manual omhs , at kohms range , short low and hi ( red and black connectors )

I will post my findings

The_Spectrum.A_idiot:

--- Quote from: Kleinstein on September 14, 2024, 08:21:00 am ---The current to look at from the ohms source should be DC from 0.5 µA to 1 mA, depending on the range. Beside the current source part, that is also a chance that a realy is at fault. One could check this by taking the current off CR202 instead of the + input.

No current may also be faulty fuse at the external DMM - so maybe check the continuity.

The already "repaired" area needs to be cleaned anyway - so a touch-up with new flux can be worth a try.

If the resoldering changes things it may be woth to re-measure the voltages (to GND) at Q202, e.g. in the 1 K range, both with the terminals open and shorted. This should give a good idea if the issue is more with the actual current source of the protection.

--- End quote ---

based on this -see attached
-----------////////////---------------
kohms - short
R201 4.993v
R202 6.972v
U201-A 3 - 6.972v
U201-A 2 - 6.972V
+7REF - 6.972V

Q201-1 - 11.099
Q201-2 - 7.110
Q201-3 - 4.4636

CR203-A 1 - 13.594
CR203-A 3 - 18.811

1SRC Q202 - 1 12.415
Q202 - 2 - 1.2771
R290 - Q202 - 3- 18.008
 
CR201-A -3 - 18.008
CR201-A -1 14.006
 
U201-B 6 11.925
U201-B 5 11.098
The_Spectrum.A_idiot:

--- Quote from: Kleinstein on September 14, 2024, 08:21:00 am ---The current to look at from the ohms source should be DC from 0.5 µA to 1 mA, depending on the range. Beside the current source part, that is also a chance that a realy is at fault. One could check this by taking the current off CR202 instead of the + input.

No current may also be faulty fuse at the external DMM - so maybe check the continuity.

The already "repaired" area needs to be cleaned anyway - so a touch-up with new flux can be worth a try.

If the resoldering changes things it may be woth to re-measure the voltages (to GND) at Q202, e.g. in the 1 K range, both with the terminals open and shorted. This should give a good idea if the issue is more with the actual current source of the protection.

--- End quote ---


**If the resoldering changes things it may be woth to re-measure the voltages (to GND) at Q202, e.g. in the 1 K range, both with the terminals open and shorted. This should give a good idea if the issue is more with the actual current source of the protection.
**
open
Q202-1 - 12.416
Q202-2 - 3.4-3.45
Q202-3 - 18.006

short
Q202-1 - 12.416
Q202-2 - 1.2460
Q202-3 - 18.006
Kleinstein:
The full 18 V at the Q202 gate suggsts that the regulation tries to turn the fet off (it is a P channel FET and thus off with a positive voltage).

The voltages at the OP-amp inputs (pins 5 and 6) suggest that the output should go negative and not positive as observed. So chances are high that U201 B is bad.

Still having some voltage at the output side (Q202 pin 2)  of the curren source somewhat points to leakage at Q202 - so this part may also be damaged ( I would at least order or look for a replacement, if ordering an AD706 (or substitute).
This leaves the question if the protection is working OK. This could be  a shorted transistors, or maybe one with too low a break down votlage and also an open Q211. Off cause the reason of the damage could also be from just too much voltage for the transistors.
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