VFD: Uneven brightness on lowest brightness

[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]

Some of the labels remain lit at lowest brightness.

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,🤞🏻

[gjpmho]

Creek sent me the schematics . I've attached the voltage readings on the power supply circuit. I found some potential issues: 3.4VAC from the transformer - this appears to power the display (F1 and F2), On the 20VDC power supply I found 30mVAC. Could 30mVAC cause an issue? Is 3.4VAC close enough to the 3.2VAC in the design? Not picking up VDC on the 3.4VAC power. The zener diodes and schottky rectifiers would be easy and inexpensive to replace.

I'll do more work on 5.2 AC Filament Drive (50 or 60Hz) and 6.2 Filament Bias Voltages (Ek)
It looks like I'm seeing Luminance Slant as described. Vee is 20VDC. I need to read more on how VAC and VDC work in the display.

The filament bias voltage (Ek) is a voltage applied to the filament center-tap in order to cut off background illumination
Which pin on HT16512 corresponds to Ek? or how do I find the filament center-tap?

[pqass]

3.4Vrms is close enough to 3.2Vrms; not usually an issue.
30mVrms superimposed on the -20VDC isn't an issue either.
VFDs are fairly robust. Just don't grossly over-voltage the filament or segment/grid pins from recommended values.
AC is preferred between filament pins although you can drive them with DC but then you'll get a noticeable gradient especially on long displays.
The ZD2 zener appears to be doing its job seeing that you measure -20VDC. So do the diodes given the low 30mVrms; possibly due to leakage so unavoidable yet inconsequential (3 orders of magnitude less).

What's interesting [to me] is that there is NO Ek bias at all. By design!
The filament winding, instead of having a 3rd center-tap wire, derives it from a simple R7+R8 divider.
Usually, you'll have another 2-4V zener between center of R7+R8 and -Vee.
This would make the "center-tap" of the filament a zener drop HIGHER than -Vee.
    ie. -Vee(-20VDC)---2-4Vzener---filamentCT------grid/segment---PFET---Vdd(+5VDC)
And therefore, the ends of the display where the AC swing is the highest (1/2*3.4Vrms) is still kept completely above -Vee.
Q: Why keep the filament (at the ends) above -Vee?
A: Because when a grid or segment is turned-off by an HT16512 output an internal resistor pulls it to -Vee which will be BELOW any filament potential guaranteeing it goes completely dark.
So if you are seeing lit segments/grids at the ends, ergo, the filament ends must be going below -Vee!
Yeah, but it'll only be 1/2*3.4Vrms below -Vee which isn't really enough to turn-on a grid/segment.
Q: Could there be something wrong with the HT16512 output where the PFET isn't shutting-off completely?
A: No because it's only the ends that are lit but the VFD is multiplexed and a bad output would exhibit the same problem equally across all digits.
Q: But given that there is no Ek bias by design, was this always the case and you just noticed it now?
A: Not likely since seeing only lit ends is definitely a sign of something odd.
Q: Is turning to the lowest brightness level the way to go completely lights-out? 
A: Probably since some people are very particular about seeing any lights in a dark room and this is a likely function provided.

So, I think I can conclude that something changed to raise the lowest brightness from just below noticeable to definitely noticeable.
Q1: Could your mains be at the high-end of what the design expects?  ie. 125VAC+ vs 120VAC nominal which translates into a higher filament winding voltage.
Q2: Has the combined R7+R8 resistance gotten higher such that more current is directed to the filament?

I can't say for sure that it's just the higher filament voltage plus no Ek bias being enough to turn on the ends (maybe with slightly leaking PFETs contributing too).
But I can confirm that one of my VFDs does begin to light-up at +5V between filament and grid/segment.  So maybe.

You could mitigate by adding some resistance to R4 and R3 (from 1R to 5R) and see what happens. 
It should lower the overall brightness with the lowest setting going completely dark.

[gjpmho]

Thanks for the explanation! Much appreciated. I forgot to include the other page of schematics, attached. Please let me know if the update changes anything. Or is it by design? I have found some google images of this tuner that don’t show the gradient, but that could be more favorable lighting in the photo. At least it’s good to know the display is not going out, would still like to see if more uniform brightness (and fully off at lowest brightness level) is possible. The tuner was recently purchased on eBay. BTW very happy with the sound.

Measured 123VAC on mains.

[pqass]

I couldn't find a datasheet for the VFD (9-BT-079GK). Not surprised.  The updated schematic doesn't really change my last assessment.

123VAC isn't high at all.  But what does the label on the back/bottom of the unit say; 115VAC?   
Still not really high to cause a higher than usual filament voltage.
1/2*3.4Vrms means +/-4.6Vpeak above and below -Vee.

I don't think the display is going out but may be over-driven a bit.  Try experimenting with some resistance for R3 and R4 (about 1R to 5R; typically, the cold filament is ~5-10R) even if you have to tack axial resistors between pads temporarily.  Add enough R3+R4 resistance to target 2.8Vrms to 3.0Vrms across the filament.

Alternatively, you could correct the design and add a 2-4Vzener between R7+R8 center-point and -Vee.  That would likely mean removing the R7+R8 resistors and "air-wiring" two axial 220R resistors+zener (anode to -Vee).   Alternative to the zener, you can use 4 signal diodes (eg. 1N914, 1N4148) in-series (cathode to -Vee direction).   This will move the filament potential (at all points) above -Vee so that an off output means off. Even-though it lowers the filament-to-segment/grid difference by 4V shouldn't make a noticeable difference (from 25V to 21V).

[gjpmho]

Thank you! I’ll follow up in a couple weeks after vacation.