How did you test the contrast pot? You'll want to turn the machine off, and use a multimeter to test the resistance between the wiper and each of the terminals (they're each connected to R10 and R77) on the pot... and of course, make sure you're taking the measurements from the ends of the resistors that connect to the pot... or take the board out, flip it over, and measure it on the pot's solder joints 🙂 According to the 8201 service manual, you should be seeing 0 - 50kohm on either terminal when the dial is turned to each extreme.
Is the screen displaying non-garbled text? If so, then it's not a broken trace - that means voltage, data, and chip select lines are all making it to the LCD. The fact that the contrast pot doesn't do anything and the 'pulsing' indicates to me a failed/dirty pot - especially if you've already verified that Q2 is good. In that case, unscrew the pot's dial cover, spray some deoxit into the pot, and work it in several times... see if that solves the problem. //Ken ________________________________ From: M100 <[email protected]> on behalf of David Plass <[email protected]> Sent: Tuesday, February 6, 2024 9:46 PM To: [email protected] <[email protected]> Subject: [M100] NEC LCD woes My new-to-me NEC 8201 is showing weird LCD behavior First, the contrast dial does nothing. Second, the screen "pulses" a little bit. So, I took it apart and found that: *. The contrast pot is fine *. VDD (+5V) and VEE (-5V) are correct when measured on the LCD connector *. According to the schematic, R10 (100K) is supposed to go between VEE and the pot. The connection to the pot is there, but on the other side of R10 it measures +4.3V. (positive, not negative) *. Similarly, according to the schematic R77 (10k) is supposed to go between VDD and the other side of the pot. The connection to the pot is there, but on the other side of R77 the voltage is also around +4.3V *. The voltage at VR1 (the contrast voltage) kind of oscillates between +4.1 and +4.3V, regardless of the setting of the pot. This explains the "pulsing" I was seeing on the screen. The only things I could think of are: 1. A broken trace (or two?). I couldn't find any broken traces but also couldn't find the connections between R10 and VEE or R77 and VDD as shown in the schematic. I may have not been testing correctly, since there *is* a voltage on each of them with power on.... 2. Failure of Q2, the transistor between the pot and VR1. So, I removed it, but it tested OK. So... thoughts/suggestions?
