EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: lothian on December 14, 2022, 05:13:52 pm
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Hopefully I've chosen the correct sub-forum for this post...
I am t'shooting a dim vacuum fluorescent display (VFD) on an appliance control panel. See attached image—the two displays' intensities are obviously different. The top display shows normal brightness for both displays.
There is no 'display intensity' setting for either display. The intensity of the lower display is not intermittent; meaning, temperature/humidity differences, thumping the control panel, etc. do not effect the brightness—the display has settled at its current intensity.
The control panel cover conceals two distinct circuit boards that drive a respective appliance; no interconnect exists between them. Each circuit board appears to use two different display technologies. Notice red cathode lines visible in the lower display (which is why I presume this is a VFD), while none are visible in the top display (I'm not certain what manner of display unit is used here).
My initial presumption is that some upstream component has failed and is causing a voltage drop to the display unit proper—like perhaps the circuit board needs recapped..? I have no idea.
Given the electronics nature of this problem, I figure it prudent to present my quandary in this forum rather than post my issue in an appliance repair forum—which would likely suggest replacing the expensive control panel. I searched within eevblog.com for "VFD Dimming"; existing posts don't seem applicable to my situation.
I'd like to repair the circuit board rather than replace it since the thing functions normally and the display, though annoying, is still readable.
Suggestions, recommendations?
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Did it get dim all of a sudden, of has it been gradually fading over time (years)?
You can check the filament and plate voltages of the VFD to make sure they are within spec, and that all of the power supplies are good with respect to voltage and ripple. If everything seems good, it's probably just a worn out VFD if the appliance is several years old
Top display looks like a segmented LED display, which has mostly replaced VFD in appliances over the last decade.
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That first thing.
The bottom display has remained at the same intensity (relative to the top display) for months.
My hunch is the problem precedes the display rather than the thing simply failing from age. As to the former, a post within this forum suggests scoping for irregular waveform at the full-wave rectifier; so I'll give that a lookie-look. Regarding the latter, a post elsewhere mentions that aged cathode-VFDs typically fail by progressively dimming until out. Sooo... while the bottom display is definitely old—17 years and counting of constant ON—it doesn't exhibit the symptom of a failing cathode-VFD.
I've been scratching my head trying to identify the tech used in the top display which is obviously different than the afflicted bottom display, so thanks for your insight.
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The AM indicator at bottom right of the top display makes me doubt that display is LED.
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The AM indicator at bottom right of the top display makes me doubt that display is LED.
I have seen masked LED before, especially on cheap devices, even 7-segment displays (LED bonded directly to PCB, segmented plastic carrier on top, with a mask glued on top.)
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A common problem causing appliance VFD displays to go dim is the electrolytic capacitors on the control board aging and causing low voltage for the display.
Many such repairs like this one here: https://www.badcaps.net/forum/showthread.php?t=7902 (https://www.badcaps.net/forum/showthread.php?t=7902)
I would post there to get details on the capacitors to replace.
The VFD display tube does age as well, these things are on 24/7 and accumulate many 1,000's of hours.
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These are often powered by capacitive droppers, check that the dropper cap has not decreased in value significantly.
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This GE WB27T10566 oven control board for the JT965, and all others use a power transformer with voltage-doubler to make the VFD voltage.
There is no capacitive-dropper, not enough power for the display and all the relays. This is a big appliance.
To power the VFD filaments, some (small displays) use capacitive droppers but typically a dedicated filament winding. Have to see closeup pics of the board to figure that out.
The voltage doubler-caps were failing in many (other OEM built) oven control boards due to placing them right next to the power transformer (=warm) and mainly because the ripple current was too high. So OEM mods are to upsize the cap and add a series resistor- for the boards having short lifetime issues like the JennAir.
OP, it looks like only 5 caps to replace, the board has a "-VDISP" net but measuring it in the oven would be risky and scary. I would replace the caps before doing rejuvenation on the VFD.