CRT flashing dimly - strange issue - Page 1
Forum section: electronics / repair

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 03, 2016, 05:17:46 pm
content: Hey! So I'm trying to reuse an old CRT+ driver board from a logic analyzer, and I just hooked up the tube to power (no input) to see if it lit.Besides a high pitch noise and a bit of initial crackling (dust at the HV anode?), it lightsdimly, in some areas of the screen, and flashes, a bit like a CFL bulb not lighting properly.I've never heard of this sort of error, is it just internal arcing in the tube?Any info is appreciatedThanks in advance!-Christoffer

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2016, 06:38:59 pm
content: As in, the display module from a piece of gear? Usually those expect VSYNC and HSYNC signals, but they should have free-running oscillators inside which start up normally, even if not drawing a proper raster. Which certainly explains the high voltage and all that (yes, some crackling static sound is typical of high voltage starting up), but diffuse flashes (often with a sparking sound, louder than static) probably means something bad like internal arcing.Has it seen bad conditions, moisture...??Tim

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 03, 2016, 07:10:05 pm
content: I don't know what kind of ordeals it's been through, just got it for free the other day.There's no loud sounds associated with the diffuse flickering.Yes, it takes Vsync, Hsync, and video, as well as 15V. I assumed i could just leave V, H and video unconnected to see if the tube lit at all, but it might be an issue with that. Dunno.--Christoffer

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 03, 2016, 07:14:49 pm
content: Just noticed, i didn't have the chassis of the tube (and that spring loaded braid strap) grounded. When connecting that to ground (as it would be in a metal chassis, the intensity on the screen increases a bit, but still not an even light on the screen.--Christoffer

author: poot36
author_rank: Frequent Contributor
timestamp: March 05, 2016, 04:33:45 am
content: Do you have a service manual for the logic analyzer or even better the CRT module? It may be a Sony CRT. The problem with most test gear CRTs is that they run at non standard resolutions so it may be difficult in getting it working.

author: Agent24
author_rank: Regular Contributor
timestamp: March 05, 2016, 10:12:20 pm
content: Can you post photos of what it looks like on screen?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 05, 2016, 10:20:32 pm
content: I do have the instrument's service manual, but it doesn't say much about the CRT module.The CRT itself is an NEC c822p4 with a custom driver board.I'll post some pictures tomorrow, but I've begun to suspect you can't run a CRT without any input connected. Can you? Normal monitors would just show a dim, uniform light depending on the contrast, but there might be some terminating/driving circuitry I've omitted.--Christoffer

author: Agent24
author_rank: Regular Contributor
timestamp: March 05, 2016, 10:32:32 pm
content: That dim uniform light on a monitor with no input is the raster - assuming a raster-scan display.A vector display (like that used in a traditional CRO) will not have a raster.I can't find much useful info on a NEC c822p4. Maybe someone else can.Can you give more details about the instrument it came out of?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 05, 2016, 11:06:45 pm
content: Hadn't thought about that. Sounds right.It's a Gould/ Biomation K-100D logic analyzer from the mid-1970sThus it's meant to display a lot of text info and menus. Don't know if one'd use a raster display for that or not.--Christoffer