Yesterday when I turned the E4421 the LCD screen was not working properly, showing random black and white lines
I found this forum post also showing an LCD fix, but my problem seemed a bit different.
https://www.eevblog.com/forum/repair/e4421b-restauration-project/I managed to fix it changing all ceramic capacitors of the LCD board. It seemed that more than one were shorting the bus intermittent.
Capacitors were 1206 3.9u, 0805 100n across the horizontal COF drivers.
Two capacitors near the connector seem to be connected to data lines, and if the value is not correct, the image will not show.
The COF drivers seemed to be very well attached to the screen.
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The unit is back into operation!