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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 9th Jun 2019, 9:52 pm   #1
Retrotechie
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Default Sharp EL displays LJ640U80

Hello, does anyone have any experience repairing these or any schematics, which seem to be unobtainium.
A common problem is leaking nichicon caps but despite replacing them, this display is still no go.
The 12v rail is being dragged down but both on board fuses remain ok, which is strange.

I wasn’t sure whether to post this in the computer section or tv & video, so please move if necessary.
These displays were used a lot in industrial control and early computers.

Thanks,
James
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Old 10th Jun 2019, 12:06 am   #2
SiriusHardware
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Default Re: Sharp EL displays LJ640U80

Is it possible that the display is not actually drawing excess current, but the 12V supply to it is collapsing when it has a load because it can no longer supply any significant current?

If you have a resistance meter, what resistance do you see between the +12V in terminal of the display and 0V/GND of the display when it is disconnected from the circuit which powers it?

Try running something else, maybe just a resistor with the right value, on the 12V supply - presumably 12VDC? - which normally feeds the display. (disconnect the display). See if it holds up at 12V when it is supplying a load.
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Old 10th Jun 2019, 10:18 pm   #3
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Default Re: Sharp EL displays LJ640U80

Thanks, good advice, I will do some further investigation at the weekend.
My hunch is that it is the display driver board, as these have been problematic in the past.
I have some junk displays in the loft, which I must dig out as they may yield some useful parts.
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Old 11th Jun 2019, 8:31 am   #4
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Default Re: Sharp EL displays LJ640U80

Did you say it's an EL display?: I've had problems with internal tracking and subsequent shorting of the driver ICs, writing-off the panel. Not specifically Sharp but it seems the high voltage nature of the technology is prone to problems like this.
The last one I saw taking heavy current was off a digital micrometer readout on a lathe. Someone tried putting a larger power supply on it and it caught fire.
I always hoped someone might come up with a little PIC or similar to do a magic conversion for an LCD replacement, I haven't been confident enough to try it myself. In the last example I ordered a replacement EL display from Planar which ran to around 1200 but there you go.
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Old 11th Jun 2019, 6:02 pm   #5
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Default Re: Sharp EL displays LJ640U80

Yeh, they are essentially a plasma display or electro luminescent (EL).
Also made by planar (USA) and others. Used on industrial video equipment such as digital mixers and VTRs.
Of course the display itself could be faulty, dragging it all down or the driver ICs.
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Old 14th Jun 2019, 12:14 am   #6
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Default Re: Sharp EL displays LJ640U80

I’m getting about 5ohms between the 12v rail and ground on the display (with its power & comms plug disconnected).

I have never been able to get hold of a schematic for these displays, which is frustrating.
Will do some further checks. I suspect the leaking caps have caused damage elsewhere.
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