Agilent 34401A display board 34401-26522, searching schematic

[BerlinGreyCat]

Hello together from a real newbie here

I‘m looking for a schematic and a component locator for a 34401 display board.
The board is nearly like the board 34401-66512 but, not identical!
All documents i found are only the schematic 66501 or 66512.
On my board are some more components and of cours different component locations for maybe
5 little components. But minimum one of this part was definitive defect! Unfortunately done
by my self  Now I hope, somebody has a schematic for this board?

Thank you and best regard from Berlin
A grey cat

[coromonadalix]

To my knowledge and having 2x 34401a   they had only 2 board variant has you wrote

It depend on the installed firmware(s), i have one of each variant

Did you modify this board with the machined socket pins   your photo is the 34401-66512   board

What are your problems ?  you blew up the board ?    The last prices i've got from keysight for this board was 200$  usd, and they dont have many left ....   

You have small logic ic's   and the two biggest parts is the display cpu and the display driver, the display driver ic is orderable at digikey, mouser  etc ....  , the programmed cpu need to be programmed if you change it, a firmware dump exist,    the firmware version 11 is not available  you have to buy the chip from keysight


You can order  the display on aliexpress or some Ebay resellers,  dont pay more than 40 - 60$ usd  they are copies,  no originals vacuum display exist .. only on the orderable kysight parts.
Bought 2x display on aliexpres, they worked very well in one of my meter.

service manual  here :
http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=HP_Agilent/HP_34401A_6.5_Digit_Multimeter


you have many threads here:  worth reading them
https://www.eevblog.com/forum/testgear/agilent-34401a-revisions/
https://www.eevblog.com/forum/repair/hp-34401a-dmm-with-leaking-segments/
https://www.eevblog.com/forum/testgear/hp34401a-display-with-dots-after-every-digit/

[BerlinGreyCat]

Hello again, thank you for your message!
At first, yes i modified the Board with pins, for better testing 
I have not a theme with the display himself yet. The theme is, there was not any function.
And I am absolute shure, the theme is on the display board, not on the base device.
In the picture you can see a Melf diode ZD6v2. The original component on this place was smoked. The reason for this: i have produce a short circuit with my measuring test pins.

After this I have a short look in the service manual and see, ok, its a Zener and I change it. It doesn‘t works!
Now I looking deeper in the schematic / component location paper and i see, the 6v2 zd has an other place as drawn in the component location. Now i don't know, what a component should be on the place, where now my Melf diode in the picture was.

The board was similar the same as 66501, but there are changes in the datails. So I am searching for the schematic.

At least, sorry for my bad english    Best regards

[coromonadalix]

Hi   First  i have a hard time to read you, i dont know if you translate your language ??  maybe others here will understand you better ?


Second

The things to do is : check all the power supply voltages in the meter

page 107 and 107 of this manual explain them

Third
If you move the flexible gray cable too much, it may get damaged, its fragile,  the white connector on the main board can be stuck open sometime, if you put the gray flexible flat cable it in, sometimes the contacts are not good.


I dont know what are your electronic skills or level,  but a multimeter like this is a complicated instrument to work with

The board photo you show us is a different revision, not a different model,  you see some parts are rotated, and slightly changed place,   yes its a revision change,  but he board model will still be the same part
number. I see in the schematic your diode,  in the page 153  figure 9-6  you see where your diode is ... near ic u604,  and in your photo, its supposed to be the small round thing with a red and blue color.
[/s]
And Hp Agilent  now called Keysight can show us that last board revision in the service manual  ?

If youre not satisfied, you can try to get help from a Keysight customer service in your country ?

In the photo  i have put a red circle around your diode.

[JDubU]

It looks like you have a Front Panel/Display board that is a revision not shown in the commonly available 34401A service manuals.
They do not show the MELF part at all in the two common board versions (34401-66512 and 34401-66502).  The pcb that is most similar to yours is 34461-66512.  The other pcb, 34401-66502, is a completely different design, using a different uController.

On your board, 6.2V zener diode CR603 (part number BZX84C6V2)  is the voltage regulator for the +6V supply that is used by the op amps in the schematic.  It is the black, 3 pin, SOT23 package near the red stripe on the ribbon cable  You should check pin 3 of U607 (the 14 pin surface mount package just below the right side of the grey ribbon cable) for +6V to verify that the CR603 is still working.

Edit:
On your board, it looks like the right end of the MELF part is connected to pin 38(?) of uController U602 but, on the schematic, that pin is not connected to anything.  Have you tried just removing the MELF to see if you can get the display board to do anything? 


Looking at the photo again, it appears that the MELF part might actually be connected to pin 44 of uController U602 (green dot = pin 1?).  That means it would be connected to VCC and, according to the schematic, VCC = -13V.