Electronics > Repair
HP 34401a DMM with leaking segments
<< < (7/45) > >>
qu1ck:
free_electron:

--- Quote from: qu1ck on May 31, 2018, 07:39:38 pm ---@free_electron
If you read my first post you would have seen that I linked to your earlier equivalent statement. And later in the thread I disproved it, at least in my particular case.

--- End quote ---
I still say it is the driver chip.
Leaving segment pins floating capacitively couples them to adjacent ones and they 'ghost'. Thats why the pulldown was able to switch em off.
The high voltage driver is a push pull type.  The display lines are supposed to be driven hard high or hard low. if one of the transistors in the totem pole output dies the display line is left floating during one phase and it capacitively picks up the adjacent lines. These things are mulitplexed so the signals couple if they ar enot hard tied high or low by the driver.

i have repaired at least 6 or 7 by replcing the drivers. i have never seen a bad display , apart from an aged one where the heaters have gone bad. pumping up the heater voltage briefly to burn off the cruft solves that ( temporarily )
floobydust:
Wait- I think we're confusing the 34401a two different VFD display boards.

Front Panel board 34401-66502 (for S/N 3146A59641 and below) used NEC 4-bit MCU uPD7527a which has HV VFD driver built in.
Second Gen board 34401-66512 uses 80C51 MCU with TI SN75518 VFD driver IC, replaced by Microchip (Supertex) HV518PJ

The TI driver has push-pull output stage, while the NEC FIP output stage is PMOS with weak pull-down.
bitseeker:
@qu1ck: For not having much time to work on it, this is really awesome progress!

@floobydust: Yeah, there's been quite an upswing in prices over the past year or two. Pretty interesting. Although they're discontinued, it's not like there aren't many available. I wonder what's been driving the steady rise.
coromonadalix:
ther popularity maybe ?   they seem's to be easily repairable with all the threads and common knowledge about them, once they work, they are reliable ...
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod