Checking an old CRT tv - Page 1
Forum section: electronics / repair

author: _c32_
author_rank: Newbie
timestamp: July 15, 2018, 03:12:51 pm
content: Hello,sorry if this post belongs to another category but i'm wondering what the SCRN marked pot does in a crt screen . it is on a small board connected to the neck of the crtpictures:https://imgur.com/a/Vj7Lven

author: Nusa
author_rank: Super Contributor
timestamp: July 15, 2018, 03:54:40 pm
content: Short for Screen or Master Screen. It adjusts the G2 voltage. Adjusting it changes the brightness of the tube. Too high you'll see lines on the screen. Too low, your colors will smear.

author: james_s
author_rank: Super Contributor
timestamp: July 15, 2018, 03:57:47 pm
content: If the TV is not working right, don't adjust anything, figure out what the fault is. On many occasions I saw someone try to tweak every adjustment they could find after something broke, turning what was previously an easily repairable TV into scrap.

author: drussell
author_rank: Super Contributor
timestamp: July 15, 2018, 04:07:02 pm
content: Thescreencontrol on the flyback adjusts the voltage applied to the screen grid (usually G2) in the electron gun of the CRT. It is typically on the order of about 400 volts and is derived via a divider from the HV in the flyback. The focus control is also generally there as well with typical focus voltages around perhaps 300 volts.When you add a screen grid to a tube (which reduces the grid-anode capacitance, among other things) you need to bias the screen up from the cathode potential in order for the later anodes (modern CRTs often have multiple accelerator anodes, sometimes at different potentials) to be able to pull the electrons from the cathode, otherwise they would just fall back into the space charge around the cathode.If you look at the gun assembly of a modern CRT (like after the 1940-50s and color CRTs) it will usually go something like:Cathode(s) - G1 (control) - G2 (screen) - first accelerator - G3 / focus - 2nd accelerator - anode (the HV at the glass screen at the front with the phosphor)Using the service information for your set, you generally follow a procedure like setting a specific bias level on your gun(s) cathodes or control grid using the gun balance controls or the brightness on a B&W or something, then bring up the screen until just before you see the raster appear, then adjust the biases using some procedure, but this varies between set manufacturer, design, era, etc.What TV make and model is this that you are working on? That would make it much easier to give better specific information.Edit: I see you posted a picture. On this model the screen and focus controls are on the neck board rather than the flyback transformer itself, obviously, but have the same function regardless of where the control is physically located.

author: drussell
author_rank: Super Contributor
timestamp: July 15, 2018, 04:17:45 pm
content: Quote from: james_s on July 15, 2018, 03:57:47 pmIndeed. Don't go around randomly twiddling adjustments hoping to fix something.Perhaps you could tell us what TV this is and what the problem(s) are so we can better assist you.

author: _c32_
author_rank: Newbie
timestamp: July 15, 2018, 05:21:32 pm
content: Thanks for all the replies!This tv works fine, but some years ago I had made a mod where a switch would either connect the vertical to the normal circuit on the tv or on some banana plugs so I could then kinda use it as oscilloscope because I didn't have one (I was like 13) and I tweaked those pots while I was inside (possible to see what would happen, I don't remember). The mod wasn't very useful because I couldn't make an amplifier and it turns out my arduino's I/O weren't able to handle the resistance. Now, some years later I'm trying to get a v9958 working with a CXA1645 and I wanted to use this CRT tv. The reason I'm asking here is because I don't know if I changed something like the filament voltage and I'm causing harm to the CRT.Is it safe to turn the pot and see which side lowers the voltage?Looking at the potentiometer on the board, the output goes through the 2 resistors to a pin on the neck and the pin from one side says 190V and the other goes with a cable somewhere near the flyback.The TV model is KV-14M1D (sony)i don't have any test equipmnet with me btw not even a multimeatersome more pics:https://imgur.com/a/EvgVzzI (some may still be uploading)

author: james_s
author_rank: Super Contributor
timestamp: July 15, 2018, 06:37:34 pm
content: Yes it's safe to adjust the Screen control, it sets the master brightness.Those little Sony Trinitron sets are somewhat desireable and relatively valuable as far as these things go. You're not going to get rich selling it but I've seen them fetch $50-$100 so it's worth keeping it in good shape.

author: _c32_
author_rank: Newbie
timestamp: July 15, 2018, 07:25:19 pm
content: i knowi turned the pot all the way to lower the brighness and the picture looks much much better !thanks everyone

author: lordvader88
author_rank: Frequent Contributor
timestamp: July 15, 2018, 10:49:44 pm
content: People give away working CRT TVs all the time here. Wish I had broken ones to try fixing and to learn a bunch. Never worked on a TV yet, I just used to pull parts off them.

author: james_s
author_rank: Super Contributor
timestamp: July 16, 2018, 01:10:28 am
content: It's not hard to find CRT TVs for free now, but they are going to dry up *real* soon here. That's how it was with early computers, it used to be easy to score stuff like Apple II's for a few dollars in thrift stores or find them on the curb, then practically overnight it seemed they were just gone.Some of the more special stuff though like those small Sony sets have always been relatively rare, they were quite a bit more expensive than competing portable sets, I don't think most people who wanted a small TV for their RV or boat tended to care as much about the picture quality.