Electronics > Repair

HP 34401a DMM with leaking segments

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floobydust:

--- Quote from: coromonadalix on May 31, 2018, 02:43:36 am ---would be nice to have another type of display adapted  ...

--- End quote ---
It's the dozen annunciators that are a PITA, and sometimes multiple ones are lit.
A new display board that supports the VFD (as these are still available), or supports a graphic LCD/OLED display is about all you can do.

free_electron:
its the driver.
the adjacent pins in the chip leak signals. they ALL go bad sooner or later.
simply replace the driver chip.

qu1ck:
@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.

floobydust:
It's a difficult problem to troubleshoot. Everything is level-shifted, there's six different rails/bias voltages and VFD's are uncommon knowledge. A troubleshooting mistake is too costly for these gems.

The NEC MCU VFD driver stage could age and go leaky from ion migration on the die.
I thought this was the root cause of ghosting segments, but xemax replaced the VFD which fixed the problem  :-//


My understanding is OP disconnected a VFD segment from the driver IC and the segment remained lit, ghosting. He then added a very strong pull-down of 2k7 (to -18V? GND would not go to cutoff) before the segments went out. The driver MCU output stage has internal ~140k ohm pulldowns to -18V, there should be very little current in VFD cutoff. This implicated the VFD display as being leaky.

I do not know of any current-leakage path inside a VDF other than a gassy tube (partial air ingress), or stray capacitance causing ghosting due to mux frequency.

Measuring voltage (to GND) on a disconnected segment or digit control grid will give a -ve voltage reading, as electrons flow from the cathode to the anode or grid then multimeter (+). You can have grid current flow, it is not always high-impedance if near or above the anode potential.

This is the problem troubleshooting- if the VFD+driver IC pin stays too +ve and the segment ghosts, it could be the IC or the VFD it seems.

edit: fixed URL

qu1ck:

--- Quote from: floobydust on May 31, 2018, 09:49:21 pm ---My understanding is OP disconnected a VFD segment from the driver IC and the segment remained lit, ghosting. He then added a very strong pull-down of 2k7 (to -18V? GND would not go to cutoff) before the segments went out. The driver MCU output stage has internal ~140k ohm pulldowns to -18V, there should be very little current in VFD cutoff. This implicated the VFD display as being leaky.

--- End quote ---

Exactly. To reiterate my experiments more clearly:
Segment connected to driver: ghosting.
Segment disconnected completely: ghosting.
Segment pulled to -18v with 100k: ghosting.
Segment pulled to -18v with 2k7: no ghosting.
Waveform observed on the driver output when it's connected to ghosting segment: fuzzy +2v when nearby segment is high instead of expected -18v.
Waveform observed on the driver output when it's not connected to the ghosting segment: proper square +-18v.
Waveforms on signal repeater using 2 bjts between driver and vfd segment: clean square wave on repeater input, output is same fuzzy +2v when it should be low (repeater uses 100k pull down to -18v).

By fuzzy +2v I mean it varies from 0 to 2v on sort of randomly, but it does not look like capacitive charge/discharge exponent so I don't think it's some stray capacitance, but it's hard to tell.

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