I haven’t seen this before - a non-backlit Portable where the whole display dims perfectly evenly to black in about 5-10 minutes, then recovers from black to normal when powered off for about the same length of time.
In that respect (the recovery), it seems the same as Tunnel Vision, except without the circular effect, blotchy corners, or any inversion or burn-in going on. I have seen Tunnel Vision on another Portable and on numerous 170/180 and there’s usually a ‘speckled’ effect where the pixels darken - this doesn’t have that. All the pixels darken smoothly by the same amount.
I thought I had found the cause when I discovered the contrast control in the Portable control panel had no effect during the initial five minutes of correct display, but then I tried the LCD on another Portable and had the same darkening even after the contrast control worked normally.
And conversely, the good LCD from the other Portable works fine on this Portable (despite the contrast not being adjustable - so that’s obviously a separate problem on the logic board, probably a broken trace between PMGR chip and a transistor).
I tried replacing the two electrolytic caps on the back of the affected display panel; no change. Next, I’ll replace the tantalums as well (I note they have silkscreen markings indicating space reserved for radial electrolytic caps being mounted instead of the tantalums). There are also three variable resistors, I tried tweaking those but couldn’t observe any change (wonder what they do - timing of the data?) I hoped perhaps they might adjust out a DC offset that I guess is causing the darkening. I’m suspecting perhaps a partial short in a tantalum is pulling something to ground slowly.
In an ideal, logical world, the fault in the display panel’s drive circuit will have directly caused the failure of the contrast control adjustment, e.g. a shorted tantalum has overloaded the contrast-signal drive transistor, but of course we know that 35-year-old hardware doesn’t have to obey logic
multiple failures from separate causes are perfectly possible.
I don’t want to separate the good LCD from its PCB so as to swap with this other display, for fear of introducing Zebra Strip problems that neither display has at present. But I suppose that would be the only way to really prove that the LCD isn’t faulty, and that the problem is in the drive circuits.
I just wondered if anyone had seen this before, I spent several hours searching (and of course, re-reading through many Tunnel Vision threads with baking etc.), and so thought I’d post to start something for the next person to find in a few years
In that respect (the recovery), it seems the same as Tunnel Vision, except without the circular effect, blotchy corners, or any inversion or burn-in going on. I have seen Tunnel Vision on another Portable and on numerous 170/180 and there’s usually a ‘speckled’ effect where the pixels darken - this doesn’t have that. All the pixels darken smoothly by the same amount.
I thought I had found the cause when I discovered the contrast control in the Portable control panel had no effect during the initial five minutes of correct display, but then I tried the LCD on another Portable and had the same darkening even after the contrast control worked normally.
And conversely, the good LCD from the other Portable works fine on this Portable (despite the contrast not being adjustable - so that’s obviously a separate problem on the logic board, probably a broken trace between PMGR chip and a transistor).
I tried replacing the two electrolytic caps on the back of the affected display panel; no change. Next, I’ll replace the tantalums as well (I note they have silkscreen markings indicating space reserved for radial electrolytic caps being mounted instead of the tantalums). There are also three variable resistors, I tried tweaking those but couldn’t observe any change (wonder what they do - timing of the data?) I hoped perhaps they might adjust out a DC offset that I guess is causing the darkening. I’m suspecting perhaps a partial short in a tantalum is pulling something to ground slowly.
In an ideal, logical world, the fault in the display panel’s drive circuit will have directly caused the failure of the contrast control adjustment, e.g. a shorted tantalum has overloaded the contrast-signal drive transistor, but of course we know that 35-year-old hardware doesn’t have to obey logic
multiple failures from separate causes are perfectly possible.I don’t want to separate the good LCD from its PCB so as to swap with this other display, for fear of introducing Zebra Strip problems that neither display has at present. But I suppose that would be the only way to really prove that the LCD isn’t faulty, and that the problem is in the drive circuits.
I just wondered if anyone had seen this before, I spent several hours searching (and of course, re-reading through many Tunnel Vision threads with baking etc.), and so thought I’d post to start something for the next person to find in a few years
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