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VFD: Uneven brightness on lowest brightness

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gjpmho:
Hi,
Sorry for any redudancies. I see there are posts on similar issues. In this case I don’t have any issues with the display being too dim. It’s plenty bright on the brightest setting. When I turn the brightness down to the lowest setting I see a gradient effect apparently with the anode and cathode?  Some of the labels remain lit at lowest brightness. I’ve replaced electrolytic capacitors, and I’m thinking about replacing the linear voltage regulator, also inexpensive. I’ve spot checked voltages at 19V, but I’m not seeing the reverse polarity on the other side of the display. This VFD is 20 years old. Is this just normal for wear and tear? Make and model: Futaba 9-bt-79gk. I’ve attached a photo of the display issue and datasheets of the ICs on the display control board. HT16512 appears to be driving the VFD. My guess is there’s an issue with driving the VFD, and/or it is just going out. Any guidance would appreciated in troubleshooting and repairing.

pqass:

--- Quote ---Some of the labels remain lit at lowest brightness.

--- End quote ---
Are you expecting all segments/labels to be off at the lowest brightness setting?
Or, are you just expecting the middle part of the display to not be complete off?
I would think the latter.

I'd look at how the filament (Ef) voltage is being generated and biased (voltage Ek).
Read this guide on VFDs especially sections 5.2 and 6.2

FYI: in the guide link above all voltages are positive (Ek, ec, eb) but the H16512 puts a twist on this just so that it can use a single P-FET on its PMOS outputs.  The VFD filament (center tap) is made negative with respect to ground Ek above -Vee. It's a bit odd when seeing it for the first time. I suspect there's something wrong with the cap or zener or DC-DC or however Ek and Ef (AC filament voltage) are being generated.

I don't think changing a fixed voltage regulator will fix your issue.
If it maintains +5V with respect to ground then it's fine.

gjpmho:
Thanks for your response. As long as it’s uniform. I’m not sure if the display should be dim or off at lowest brightness. I’ll read guide and check powers supply voltages, unfortunately I don’t have the schematics. Attached is a photo of the board. The linear voltage supply is steady at 5Volts.

Le_Bassiste:
just a shot in the dark: uneven dimming can be caused by the filament being supplied with DC rather than AC.

gjpmho:
Thank you, I might have a schematic shortly. Awaiting a follow up from the manufacturer,🤞🏻

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