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, 06:14:11 pm ---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.

--- End quote ---

Dear Mr. Kleinstein  Thank you so very much for your input and help,


Started Loading the parts canon.

Q201 - MMBF4392LT1G VS MMBF4392

Q202-MMBF5461 - is unobtanium. I am trying to find something similar.

Q211 -  MMBF4117 VS MMBF4117A

U201 - AD706JRZ the closest candidate VS AD706JR

Q203-210 - MMBT6520LT1G VS MMBT6520L

CR203 - BZX84-C5V1-QR VS BZX84C5V1

CR201 - BZX84C4V7-E3-08  VS BZX84-C4V7


Lets hope I will manage to pull this off :D

Any input on parts selection is always welcome :D
Kleinstein:
For the P-JFET the MMBF176 should be OK - one of the few P-channel fets that are usually available.

The_Spectrum.A_idiot:

--- Quote from: Kleinstein on September 14, 2024, 10:06:12 pm ---For the P-JFET the MMBF176 should be OK - one of the few P-channel fets that are usually available.

--- End quote ---

For the Q202 right?
Kleinstein:
The mmbf176 would be my suggesting for Q202. There is not that much choice if p-channels JFETs anyway. The speed and thus to a large part the capacitance should not matter much. The mmbf176 has roughly a similar threshold and Idss to the MMBF5461. The voltage rating should be sufficient for the use case.
The_Spectrum.A_idiot:

--- Quote from: Kleinstein on September 15, 2024, 08:58:47 am ---The mmbf176 would be my suggesting for Q202. There is not that much choice if p-channels JFETs anyway. The speed and thus to a large part the capacitance should not matter much. The mmbf176 has roughly a similar threshold and Idss to the MMBF5461. The voltage rating should be sufficient for the use case.

--- End quote ---


MMBFJ176 you mean?
https://eu.mouser.com/ProductDetail/512-MMBFJ176

Many thanks
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