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 here. rather than post my issue in an appliance repair forum—which would likely suggest replacing the expensive control panel. I'd like to repair the circuit board rather than replace it since the thing functions normally and the display is still readable.

Suggestions, recommendations?

 

My guess is that the display itself is worn out. How old is the appliance with the dim display and how hours a day is the display operated?

 

Suggestions, recommendations?

See if you can find a schematic that gives part numbers for the displays. If you can find a replacement, you can try replacing the display.

If the displays aren't available, board replacement is likely the only other option.

Each circuit board appears to use two different display technologies.

That seems like a dumb thing to do.

 

Displays wear out?

Both displays have been burning steady ON for 17 years and counting.

 

I vote -- Recap.
If that doesn't work just buy a new Microwave-Oven.

At ~17-Years, it's very likely that no replacement parts will be available,
and, if by some remote chance, some parts are still available,
the cost of those parts would probably equal the cost of a new Oven.
.
.
.

 

Displays wear out?

They can. Even LEDs wear out; though it takes many decades in properly designed applications.

Both displays have been burning steady ON for 17 years and counting.

You may have many more years on the good one. My oven/microwave combo is still working after 26 years.

 

I'll try recap'g with Rubycon caps.

A post in another forum suggests scoping for irregular waveform at the full-wave rectifier; so I'll give that a lookie-look if the recap thing doesn't pan out.

 

Good luck, I hope you find an easy-to-fix problem in the power supply.

 

Is the display dirty, or you could check if the heater wires are getting enough voltage.

 

From what I've read elsewhere, cathode-fed VFDs fail by progressively dimming until out. I mentioned at the start of my post this is not my experience.

The top and bottom displays use different tech—the bottom display is obviously VFD while I am unable to identify the nature of the top display.

 

I'll try recap'g with Rubycon caps.

A post in another forum suggests scoping for irregular waveform at the full-wave rectifier; so I'll give that a lookie-look if the recap thing doesn't pan out.

That might do it. I did our old Bose Lifestyle 20 awhile back with low ESR electrolytic caps and it restored the 25 year old VFD. After the rectifier on the Bose, the V+ voltage was supplied by a cap in series with the display. Several other voltages on the display board were derived the same way IIRC. It's worth a shot; caps are cheap. That said, VFDs do have a useful visual lifetime determined by degradation of the phosphors on the segments over time.

The top display looks like a green LED.

Good luck!