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Aligent 34401A LCD characters Shorted

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simba15:
Hi there All,

I feel this is the only place I may find a reasonable answer for my issue.

I have 1 (Out of 50) 34401a that will not display the units correctly

The display seems like characters 10/11 are shorted. Lets say for voltage should be mVDC but it will show VVDC with small m shown under the V.  If I use freq mode then it reads HHZ not HZ.

Any ideas what i could do to resolve this? I don't want to buy just the lcd and have it be the same. I could do a display card swap.

Kleinstein:
The display is VFD, not LCD.  This looks like an odd problem at the display PCB. If lucky this would be something like a solder ball or tin wisker (hard to see)  causing the short at the display driver or near the display itself. Some cleaning with a brush could help.
The error will move with the display PCB, so little need to move it to another unit.
AFAIK there are newer and old display PCBs that are not interchangible.

There are service manuals available, that may show which pins are the suspects ( digit select for digit 10 and 11).

coromonadalix:
50x  34401a  ??  wow

it depends on the serial numbers,   old one ... have the non available vfd driver ic ....   unless you put a cloned vfd, you wont be sure if it is the ic of the vfd itself

and more recent, have other vfd driver chip that can be found or subbed,  and too this cloned vfd can be found ... prices have gone up

you must be very careful if you remove the lcd and put a new one, legs can be much longer and may kick on the front panel and you may damage it while sliding it into place

and yes  check all voltages going on the vfd / front pcb


and  one more advise,  you can not switch old vfd driver board into the more recent one, and the other way, the display firmware  is not the same

if i recall < 7 display fw  is the old vfd ic  and >7 is the newer ...          others members may chime in

you have to find Keysight notes about the SN ...

simba15:

--- Quote from: Kleinstein on September 23, 2024, 03:34:07 pm ---The display is VFD, not LCD.  This looks like an odd problem at the display PCB. If lucky this would be something like a solder ball or tin wisker (hard to see)  causing the short at the display driver or near the display itself. Some cleaning with a brush could help.
The error will move with the display PCB, so little need to move it to another unit.
AFAIK there are newer and old display PCBs that are not interchangible.

There are service manuals available, that may show which pins are the suspects ( digit select for digit 10 and 11).

--- End quote ---

Wow you were bang on!!! A quick brush and it is working perfect now!
Thanks! I will be sure to keep this in the notes for the future.



Yes we have about 50 34401a and 50x 33210a for student use.  Keeping them working for another 20 years will be interesting. ( I wouldn't dare replace them with 34461a).


coromonadalix:
good advice Klein  ill keep this one,   ok  i've slightly gone too far

the 34410a and 34411a  are very good replacement too  for the venerable 34401a, schematics exist too,   after theses ones   ...  nothing

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