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? >
