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

--- Quote from: chilternview on September 12, 2024, 04:57:57 pm ---
--- Quote from: The_Spectrum.A_idiot on September 12, 2024, 04:55:18 pm ---okay   U552 and Ground show -27V
U551 show 18V ...
this is not right ... right ?

--- End quote ---

Sounds like U552 is shorted. Not good (easy to replace, but may have caused/be caused by other damage).

--- End quote ---

Okay I am stupid , I just noticed that the negative rail uses a different pinout VS the positive rail.

So all is good. I think.

As the master said @EEVBlog "Trap for Young Players .... "
The_Spectrum.A_idiot:

--- Quote from: chilternview on September 12, 2024, 05:13:09 pm ---You could try. Also check F101 and F102, the internal and external fuses for the amps input. It may be someone put a large voltage on that input.

--- End quote ---

F101 and F102 check out ok.
The_Spectrum.A_idiot:

--- Quote from: Kleinstein on September 12, 2024, 04:26:22 pm ---A first thing is to check if the U110 gets too hot and thus the dark discoloration (could have been a one off latch up that the chip survived after turned off).
Another points to check are the supplies, especiall the +-18 V part.

For the ohms part one could check the test current - if reading zero ohms, this could be from no current.
There are a few repaorts about broken ohms current sources, like a bad JFET and OP-amps. The BJT cascode is also a bit suscpect.

--- End quote ---

U110 is not hot at all. While the instrument is powered up.
iMo:
When the Vcc and Vee of the opamps show +/-18V then the 3.2V serial zener diodes (3pin smd) got shorted (around 3-4 pairs there on the pcb). It happens when you short +/15V somewhere..
The 3.2V zeners in series with 18.2V create +/-15V for opamps..
The_Spectrum.A_idiot:
input resistors :
R104-R110
+
R180-182
+
R170-172
Checked for cracked joins , all seems good with multimeter measurement though all of them in line.
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