[sdiy] Why do LCDs die?

MTG grant at musictechnologiesgroup.com
Mon Dec 31 17:07:56 CET 2018


It's just a standard EL circuit as far as I know. Maybe there is some 
natural artifact of the EL "foil" that as it ages it also changes 
resistance or something. The self-adjusting does not appear to work in 
favor of readability. :)

Lets start at the issue. LCD not readable. Person changes old LCD with 
new LED backlit version and removes the EL circuit. New LCD shows 
garbage. I probably didn't make that last bit clear.

So, the gate doesn't have anything to do with brightness. When you pop a 
new LCD in, the timing (from the Z80 memory mapped LCD circuit) causes 
the new LCD to mess up in a variety of ways.  Changing the gate clears 
it up. Imagine grandiose hand-waving. I don't have any other notes handy 
but a photo of a repair showing before and after and that's what the fix 
was. If someone worked out the Z80 and logic timing on E, RS, R/W* and 
data I'm sure you'd see some parameter is pretty iffy regarding setup or 
hold time.

Aside: the new LCD will probably be of type HD44780A00 (Hitachi 500ns 
controller) versus the old HD447800 (which is the legacy 1us 
controller). You'd think that would make things better, not worse.

Anyway the timing issue is solved by replacing said gate(s). You have to 
unsolder the original gate. There may be other fixes that accomplish the 
same thing but I don't know for sure.


On 12/30/2018 11:03 PM, Tony Kalomiris wrote:
>>From the MRC service manual:
> 
> "A DC to AC converter, T101, converts +5VDC to approximately 100 VAC which
> supplies power to the fluorescent lighting on the LCD display via J110 and
> is self-adjusting as the LCD ages."
> 
> That last part surprised me, self-adjusting backlight? It's not shown on
> the schematic but that 14 pin connector, J110, looks familiar (Hitachi
> 44780).
> I don't see how the chip select, CS6, from that NAND gate, could make a
> difference in brightness, but hey if you say so, who am I to question why.
> 
> Tony
> 
> On 18-12-30 11:17 PM, "MTG" <grant at musictechnologiesgroup.com> wrote:
> 
>> Anyway the MRC .. U118 is a 74HCTLS on the ones
>> I fixed. I had to replace it with a 74LS00. Then it was fine. Don't ask
>> me how I came to that conclusion but maybe with a schematic in front of
>> me I could say better.
> 
> 
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