What could be wrong with this LCD? - Page 1
Forum section: electronics / general

author: Began He
author_rank: Newbie
timestamp: November 17, 2012, 06:23:03 pm
content: Hey, first time poster here. I hope this is the right place for this thread, it was a tossup between here or the Technical Stuff forum.So I was visiting my grandparents earlier today and they're having some appliance woes. Their stove, a Kenmore, which is probably old enough to drink in certain countries, has an LCD display (or perhaps VFD?) that works sometimes and not others. It randomly fades in and out like someone is messing with the alpha slider in photoshop.Here it is working:Here it is, about a minute later, not working at all (as one might expect):Afterwards, it wobbled somewhere at around half brightness before coming back.I know quite a bit about electronics - I'm studying to be an EE - but aside from some hunches I don't know for sure what's wrong. I don't want to take it apart without a pretty good idea, since it will leave them without a stove. My first hunch was that the LCD/VFD is simply wearing out, but I guess if that were true, it would just stop working and stay that way. This one comes back to life as if nothing ever went wrong. And even when the display is dead, the buttons and controls still work perfectly - including the little piezo buzzer that beeps whenever you push a button. So I doubt it's a microcontroller issue.My current hunches are dying capacitors or power supply. I'm hoping it's the former since it's $1 of parts, a soldering iron and an afternoon. If it's the latter, I'm hoping it's just a regulator on the board I can replace, otherwise I'm not too comfortable messing with the electricals of someone's stove.Oh, and Sears wants $800 to fix it. lol.So, does anyone have any ideas what could be busted? I have no idea what the model number of this stove is, so I don't have any technical docs, if they even exist. I'd appreciate any help. Anything to avoid them having to eateight hundred dollars. If that's the only option, then I told them just to put it towards a new stove.

author: Sionyn
author_rank: Frequent Contributor
timestamp: November 17, 2012, 06:44:26 pm
content: its not a lcd its a segmented displayno pictures of of boards ?

author: SeanB
author_rank: Super Contributor
timestamp: November 17, 2012, 06:45:29 pm
content: I will guess dying caps for sure, along with dry joints on the heater connections either at the power transformer or at the display itself. Easily fixable, and it is old enough that dry joints will start to appear, especially as it lives in a hot location.Pull the board out ( and take a few good photos of where exactly each wire goes in each connector and label them with a tag as well, as you will find a number of spade connectors with the same colour on the wire, they are not interchangeable) and open it. Change the caps that are bulged or that look sick, and resolder the pins of the VFD where they go in the board, as well as the transformer pins. Check the windings are still soldered to the pins, you find an intermittent connection there often as well where the winding wire is soldered to the pin.If it still does this then either the winding in the transformer is broken and intermittent or the heater itself is broken.

author: Began He
author_rank: Newbie
timestamp: November 17, 2012, 07:06:15 pm
content: Quote from: SeanB on November 17, 2012, 06:45:29 pmThanks for the tip. Do you mean "header" when you say "heater"? I didn't really understand that part, since the stove otherwise works fine; the coil controls are on their own mechanical knobs.Quote from: Sionyn on November 17, 2012, 06:44:26 pmQuote from: Began He

author: free_electron
author_rank: Super Contributor
timestamp: November 17, 2012, 07:11:26 pm
content: Quote from: Sionyn on November 17, 2012, 06:44:26 pmlike lcd's cant be segmented ...its a FVD : vacuum fluorescent display.works in the priciple of electron emission hittong a phosophor coated surface. a acceleration grid extracts electrons from a heated filament. by applying an approprita positive voltage on a segment the electrons will strike and it will light up. a shield grid allows to pinch off a complete digit so they can do mulitplexing.these things can fail in two ways electronically : the filament voltage collapses and there is no electron emission.the segment drive voltage collapses and there is no attraction.judging form the fact that in one picture it is very bright while dark in the other i think you are dealing with either a bad contact or a collapsing voltage.the filament is driven with AC while the grid and the segments are DC driven

author: SeanB
author_rank: Super Contributor
timestamp: November 17, 2012, 07:13:15 pm
content: Heater, as this is a Vacuum Flourescent Display, and the outer pins of the display are connected internally to a thin set of wires that are heated up to emit electrons to drive the display. Careful of the display, it is very fragile and glass, and be very wary of the area where you see a glass pip as it is all to easy to break it - dead forever then.There will be 2 or more secondary windings on the transformer, one is around 4VAC that is for the heater, and it will either have a centre tap or will have one end connected to a negative voltage rail of around 30V.

author: Began He
author_rank: Newbie
timestamp: November 17, 2012, 07:29:19 pm
content: Very much appreciated guys. Guess it's time to rip it open and take a look.

author: ablacon64
author_rank: Frequent Contributor
timestamp: November 18, 2012, 05:35:27 pm
content: Here, a mini system with that same problem a while ago turned out to be the fuse.