Agilent 34401A Bubble on PCB ... can it be saved ? (SOLVED)

[The_Spectrum.A_idiot]

Dear All ,

I have bought an Agilent 34401A with the following errors on self test : 621 , 619 , 619 , 617, 615, 613 ,612

after opening it up I see this following abomination.




is there hope ? or I should give up ?

I have another post with some issues with said meter. I will try to get some test procedure done , BUT mine mostly works on DC except continuity that is flaky at best non trustworthy at worst.

I will try to test the rest of the functions somehow ( as I don't have any standards to test it against )

Let em know what you think.

Thank you in advance for your time

[iMo]

It looks like the U110 (MC34081BD) has been smoked (plus minus something around it, double check). The pcb around it has been delaminated, but unless some vias are broken, the pcb may work fine..

[tszaboo]

Water damage. With enough determination you can fix it.

[Kleinstein]

It looks like the OP-amp U110 get a bit hot. That is part of the amps part, as an auxiliary buffer. If that is the only thing really broken it would be relatively easy to replace, as it is not so critical a part.

[The_Spectrum.A_idiot]

Dear All ,

So I checked almost any VIA I can see on the problematic part of the board. All seems good ( suspiciously someone was in here before and all the vias are filled with solder.... ) anyway.

After Tinkering a bit more with the meter.
-I have V DC
-Continuity partial sometimes it works sometimes it does not - Diode mode is also dead  same issue as Continuity
- DC A ( i managed to check up to 24mA and it works )
- Resistance is dead ( any resistor shows 0 ohms ) and at the beginning in the Mohms random.

Is there a guideline how to continue ?

Before I start replacing things since the PCB is not at its best I would like to test all theories before loading the parts canon.

Thanks

[Kleinstein]

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.

[The_Spectrum.A_idiot]

okay   U552 and Ground show -27V
U551 show 18V ...
this is not right ... right ?

[chilternview]

okay   U552 and Ground show -27V
U551 show 18V ...
this is not right ... right ?

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

[The_Spectrum.A_idiot]

is it a good idea to inject -18V after I remove the part ? and see how it goes ?

[chilternview]

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.

[The_Spectrum.A_idiot]

okay   U552 and Ground show -27V
U551 show 18V ...
this is not right ... right ?

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

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]

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.

F101 and F102 check out ok.

[The_Spectrum.A_idiot]

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.

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.

[factory]

To me it looks as if this has been worked on before, some evidence of hand soldering and possibly someone used a lot of heat to desolder the 8 pin IC, i.e. too much heat from something hotair based?

David

[The_Spectrum.A_idiot]

When the Vcc and Vee of the opamps shows +/-18V then the serial zener diodes 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..

OP-amps , do you have a name for them ?
U110? U201 ? U303?

[The_Spectrum.A_idiot]

To me it looks as if this has been worked on before, some evidence of hand soldering and possibly someone used a lot of heat to desolder the 8 pin IC, i.e. too much heat from something hotair based?

David

Looks like it. yes that pin seems to be floating. and not connected anywhere.

[The_Spectrum.A_idiot]

Almost all in the ADC and perhaps in other parts of the system. Look into the Service Manual (easy to download).
Kdyz vsechno selze procti si navod..

I am trying to RT*M. But I am a bit out of my depth as you can see.
I am trying to understand what is allegedly the issue. Nevertheless , I will try to figure it out , thanks !

[iMo]

People here know the 34401 pretty well. So chances are you will get it working..

[coromonadalix]

you do have the schematics ...

http://www.ko4bb.com/getsimple/index.php?id=manuals


type 34401a in the search box   and magic   loll

[chilternview]

The error numbers are saying the ohms function is broken, check Q202-Q211, U201 and CR201/202.

[The_Spectrum.A_idiot]

you do have the schematics ...

http://www.ko4bb.com/getsimple/index.php?id=manuals


type 34401a in the search box   and magic   loll

I got the schematics from keysight directly.... as service manuals and user manual. I can't expect them to be different direct from source.
Allas I will check you the link , Thank you.

[coromonadalix]

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  ....

[Smokey]

The light-through-the-board photo makes it look like a 2 layer board (with no ground plane??).  That should be easily fixable in this case.  Just need to reconnect top to bottom, and don't need to worry about inner layer connections for the vias.  lucky.