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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
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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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Old 24th Oct 2019, 11:12 pm   #7
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Default Re: Sharp EL displays LJ640U80

I was working on some of these today, part of a Panasonic VTR. The faulty ones all had leaking caps. I changed the five in a row (47mf) and the two near the connector (100mf). As long as the board hasn't been corroded by the electrolyte - all should be well. The duff caps will drag the 5V rail and sometimes blow the 500mA fuse.
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Old 14th Feb 2020, 3:26 pm   #8
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Default Re: Sharp EL displays LJ640U80

Hi Phil, yes, the dreaded Ďnichiconí capacitors leaking, which can damage the tracks and I think in cases of severe damage, short out some of the FETs.
The whole board is coated in a lacquer, which can make soldering a little tricky & unpleasant smell (fume extraction advised).

I think in other other cases, the fault can be caused by the EL panel itself, especially if the seal has gone or one of the mux ICs on the flexible Pcb.
There was a company in the states that offered a repair service but it starts to get costly.

Iíve been unable to source any schematics and used displays seem to fetch high prices, as they used a lot in industrial control (and those horrendous Panasonic VTRs), and sometimes, thereís no modern compatible equivalent that can be sourced/fitted.

Iím in the process of making up some test jigs, so hopefully will be able to fix some more of these.
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