Update to this old thread: I did in fact discover a broken trace from VEE to R10, and there was definitely no connection from VDD to R77.
So I added some bodge wires, and it's all working now. On Wed, Feb 7, 2024 at 7:01 PM David Plass <[email protected]> wrote: > > > On Tue, Feb 6, 2024 at 10:58 PM Ken St. Cyr <[email protected]> wrote: > >> 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. >> > > That's exactly what I did and I saw the correct measurements. > > >> >> 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. >> > > Sorry, I should have said, it's not a data issue, it's *only* the > contrast. See above, it's working, and I can see the resistance go from 0 > to 50K. The thing that really bothers me is the voltage on the upside of > R10 isn't VEE. It's hm, a diode drop under 5v... > > >> >> //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? >> >
