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LCD repair?

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NutmegCT

NutmegCT

Great Pumpkin
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I just got a digital volt/ohm meter - but it needs help. Turn it on, screen remains blank, except for a random line or dot on the right side when I press on the screen itself.

Sounds like a bad connection, right?

But take a look at the innards. First photo shows top of circuit board with the line of contacts for the screen. Above that is the flexible "connector" that goes between contacts and screen. Above that is the screen (face down). I assume it's an LCD screen. Board contacts touch the flexible connector, which touches the bottom of the screen.


IMG_0303.jpg


There's no ribbon cable, or solid connection between circuit board and screen. How the heck do the signals get to the screen to control the display? I've cleaned off the board contacts, the edges of that rubbery connector, and the screen itself, with alcohol. No change.

Assembly is: display goes in window of case, then flexible connector at bottom of display, then circuit board is flipped over and placed so the line of contacts touches the flexible connector. Seems pretty inaccurate - but that's the way it was when I opened the case.

Second photo shows same layout, but display screen is flipped to show the face which should display the characters.

IMG_0304.jpg


LCD panels don't usually fail - it's more likely to be a connection issue.

How on earth does that system work, if there's no ribbon cable?

And of course ... does anyone have a suggestion on getting it to work?

Thanks.

Tom
 
Have you checked the battery of the meter?
 
Yep - tried 3 different 9 volt batteries. Same result.

I'm just puzzled that the connection between the circuit board and the display is that simple, flexible "rubber" block. Like a black layer with a pink layer on either side of it. Weird.
 
Tom, have you googled it? The display screen of SWMBO's camera broke recently and I was able to replace it with a $10 ebay replacement and a youtube video.
 
yep - plenty of googling. Can't find any example of a display panel "connecting" to a circuit board with what appears to be a rubber "spacer".

I'm not even sure the display is really an LCD, as LCDs are usually connected with a ribbon cable. So I'm not going to worry about replacing the display until I know if the display is the problem. Still seems like a connection issue, not a display issue.

oy
 
Photo showing LCD in place, with flexible "connector" in place below it.

IMG_0305.jpg


Photo showing LCD, connector and circuit board (flipped over) in place:

IMG_0306.jpg


Doesn't seem logical to me that the signals from that line of circuit board contacts could somehow get through the flexible connector and control the LCD display. Weird.
 
Tom, what is the name/model number?
 
The black layer in the rubber connector has carbon in it and acts as the conductor. lcd's use very little current so they can use this kind of "connector". Make sure that the copper contacts are in exactly the right spots. If they look OK, run it over with the Benz on the way to buy a new one.
 
It's a DT-830B Digital Multimeter. Funny thing about the connection: the rubber connector just fits into a slot between the circuit board and the LCD. You can't check if the board copper contacts are in exactly the right spot, as they're facing the rubber connector.

The board is held by four tiny screws, so it can't move around. The LCD snaps into place. But how those copper contacts can be lined up correctly to the barely visible contacts on the LCD itself, is beyond me. You can't move the LCD once it's in place; you can't move the board once it's in place. So you "grin and bear it", hoping the two are aligned.

George - thanks for pointing out how that carbon/rubber connector functions. I'm assuming the "signal" just passes right through it, going between board and LCD contacts.

I'd sure like to know how to "align" the copper contacts - but as you can't see them, and can't move either the board or the LCD, it seems impossible.

Thanks!
Tom
 
I googled it and the general consensus is that it is cheap stuff made from a country far from here that makes cheap stuff - and that it is easier to replace. However, they do mention a fuse.
 
The fuse is just for the ampmeter function. The copper contacts I mean are for the rotary switch in the center of the meter. Even if the rubber connector was off a little, the display would be jumbled, not dead.
 
Definitely a cheap meter - but I enjoy the challenge.

George - sorry, I thought you meant the copper contacts for the LCD. I did clean all the contacts (for LCD as well as rotary dial switch) with alcohol. No change.

You mention "even if the rubber connector was off a little". The rubber connector is just a pass through, right? It's a "solid" carbon rubber block (I think), so seems it shouldn't matter if it's not in the "perfect" place. And as it can slide left/right in the slot, with no way to permanently set it, seems there's no way to put it in a specific space.

Note - if I push at various spots where the rubber touches the LCD, the LCD will give random lines and dots. Always on the right hand side. Just doesn't seem that 95% of the LCD would be dead. Wouldn't it more likely be 100% dead or 100% alive, if the connection was correct?

Weird x2!
Thanks.
Tom
 
The carbon is "slotted" not solid but you are correct in that its placment is not very fussy. and yes LCDs normally are good or bad(and rarely bad).It seems like the "chip" running the meter is dead. I also enjoy the "challenge" but in this case I would enjoy hitting it with a BFH more.
 
That rubber connector is called a "zebra strip" connector and were the bain of my company 40 years ago. We made common electrical connectors which were much more expensive and those pretty much ran us out of business (company is long gone now). The most probably issue that you have is that the carbon has wiped from one point to several others and is messing you up. Only way that I know of to clean it is to use a pocket knife held perpendicular to the surface to scrape the surface clean and it may put you back in business but maybe not. It needs to be CLEAN with no wipe-over (both sides)
BillM
 
Bill - could you give me more detail on how to "clean" that connector? You mean scraping the knife edge back and forth across the edge of the connector? the two long edges, that touch the board and the LCD?

Also, don't understand "clean - no wipe over".

Thanks.
Tom
 
Yes Tom-
scraping the edges that touch the boards. The problem is that the carbon is a bit powdery and can smear which will "short" across a couple of contacts. You must prevent that "wash over" of the dust.
Does that make sense?
Bill
 
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