Sprite_tm

2025-12-03 05:25:40

Sprite_tm

sprite_tm@social.spritesmods.com

So, last update on the little CRT. I kinda didn't do it on the fly as it's all kinda... unfulfilling, even if the CRT now works succesfully.

Last time I posted, I was having the issue that the horizontal sync didn't want to lock in; the trimmer for the horizontal lock settings seemed to not have any place where it would lock on, and a range that was way too wide. I traced it back to around the horizontal sync chip. I initially said it wasn't anything in the splash zone of the capacitor glue, but desoldered the resistors there anyway, and replaced the one that was 1K: I happen to have 1K through-hole resistors and I seemed to measure an off value for that resistor once (and only once). Faffed around a bit more as that didn't seem to fix it, measured voltages etc, until I touched the horizontal lock potentiometer again... now it did have the proper range and I managed to make it lock in.

Super-unfulfilling as I have no idea what changed it: could be the 1K, could be me resoldering joints, could be some sort of short in some component burning through or a cap reforming, no clue.

2025-12-03 05:31:10

Sprite_tm

sprite_tm@social.spritesmods.com

After that, the monitor worked great, and I played around with it a bunch... until I noticed that the color somehow changed without me noticing: the red channel was gone, and somehow green now also controlled the red (making everything green be yellow instead). Me faultfinding again... First of all, reflowed all the usual spots, to no avail. VGA pre-amp seemed OK (and the fault stayed with red if I swapped the red and blue pre-amp outputs), so I reverse engineered the cascode after that that drives the actual CRT. Couldn't find an obvious fault there, so I decided to swap around drive transistors, with the idea being that if I swapped a transistor and the issue swapped as well, that must've been the part. And after swapping the main drive transistors... the problems just went away? I needed to re-calibrate the outputs a fair bit, so perhaps the red transistors just went so much out of spec that the added error made the thing stopped working and swapping them around generated a combo that did work OK? No clue; I've marked down the types of transistors for if this thing ever goes wonky again.

2025-12-03 05:36:59

Sprite_tm

sprite_tm@social.spritesmods.com

Anyway, the thing is together and works pretty well. No sign of burn-in, color accuracy seems OK (although I could probably calibrate it a bit better), the tube doesn't seem to be too tired. CRT seems to be happy to do 1024x786, although the image is a tad fuzzy at that resolution. I'm probably gonna use it for DOS games anyway, so that doesn't really matter that much.