Electronics > Repair

HP 34401a DMM with leaking segments

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qu1ck:
wictor
Yep, dimming and flashing is done in front panel but cpu does send a special control character in the message to instruct FP to do it. At least that's the case in 34401a.

Cyberdragon:

--- Quote from: free_electron on June 01, 2018, 04:06:59 pm ---
--- 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 )

--- End quote ---

It's most likely bad blanking timing or something from the driver. As can be seen here, the timing is relatively tight so it wouldn't have to be off by much to cause a problem.

https://www.noritake-elec.com/technology/general-technical-information/vfd-operation

EDIT: I would think a VFD that's ionized badly and leaking would be somewhat blackened.

bitseeker:
@wictor, that's interesting. Is the protocol for the E3631A/E364xA power supply front panels the same as either generation of 34401A? It'd be great to have a configurable front panel replacement for many/all the HP VFD modules.

wictor:

--- Quote from: bitseeker on June 21, 2018, 11:40:05 pm ---@wictor, that's interesting. Is the protocol for the E3631A/E364xA power supply front panels the same as either generation of 34401A? It'd be great to have a configurable front panel replacement for many/all the HP VFD modules.

--- End quote ---
Hi,
I'm confident that it would be possible to have configurable replacement for at least E3631A and E3634A. I have swapped the panels between these models and they work, but of course annunciators are different. Annunciators are sent as bitmask, so you just would need to know used model, and then you could display correct annunciators. I think that front panel in E363xA and 34401 are using same FW, so I'm pretty sure that you could have general replacement for most of that era Hp/agilent models. Some day I'll check the 34401 model for the used protocol, but now I don't have the need or time for that job.
Wictor

qu1ck:
I will publish 34401 protocol.
It is substantially different from 34970a though, that one uses uart for bidirectional data transfers instead of spi like protocol in 34401. Even control frame markers are different, let alone annunciator bitmasks or button press events.

Sent from my moto g(6) using Tapatalk

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