Advice Admiral 12X12 20Z1 low contrast w/ lines

[Electronic M]

Was the weakness emission, cutoff or both?

[Yamamaya42]

I have the same CRT in my FADA, but it's hooked in differently, the cathode on mine is grounded, and video fed into G1.

The voltage readings for each pin on yours is listed on the SAMS, also you need to be sure you have a good video to the cathode, not sure what it will be for yours, but mine was about 39v PtP, my guess yours should be about the same, but inverted, since its going to the cathode, as opposed G1 on mine.

You also need to find a way to check that you have good HV, if it is low, it may be causing you problems, and/or a misaligned / weak ion trap.

But however, if all checks nominal, you most likely have a very tired / dead CRT, and must find another, and most likely end up paying way too much to some fool on Ebay, like.

https://www.ebay.com/itm/Vintage-Syl...-/174589841825

[rca21d645]

Originally Posted by Kevin Kuehn

Well what kind of DC voltage range do you read from the wiper leg of the brightness pot when you rotate it from one end to the other? One would expect approximately 150v at one extreme and 0v at the other. Referenced from chassis ground.

I just checked this morning, it goes from 0V to about 165V.

[rca21d645]

Originally Posted by Electronic M

Was the weakness emission, cutoff or both?

It had weak emissions but the cutoff seemed fine

[rca21d645]

Also after doing the modification to remove the retrace lines it did make them dimmer but they're still there even at the lowest brightness setting.

[rca21d645]

Originally Posted by Yamamaya42

I have the same CRT in my FADA, but it's hooked in differently, the cathode on mine is grounded, and video fed into G1.

The voltage readings for each pin on yours is listed on the SAMS, also you need to be sure you have a good video to the cathode, not sure what it will be for yours, but mine was about 39v PtP, my guess yours should be about the same, but inverted, since its going to the cathode, as opposed G1 on mine.

You also need to find a way to check that you have good HV, if it is low, it may be causing you problems, and/or a misaligned / weak ion trap.

But however, if all checks nominal, you most likely have a very tired / dead CRT, and must find another, and most likely end up paying way too much to some fool on Ebay, like.

https://www.ebay.com/itm/Vintage-Syl...-/174589841825

Is the 12LP4A a common tube, would it be possible to find someone willing to sell one on here?

[Yamamaya42]

Originally Posted by rca21d645

Is the 12LP4A a common tube, would it be possible to find someone willing to sell one on here?

All you can do is ask in the classified area, and hope, and you do not have to limit yourself to a 12LP4A exactly, if you find a 12LP4, ( same CRT, but without the gray face ) you could use that, or some other compatible one, for example a 12KP4 will work, just leave off the ion trap, I'm sure there are others that can work as well.

Also, when I was bringing up my FADA, I did not want to risk hurting my 12LP4, so I used a 8YP4 Test CRT that I got from ETM, not exactly the right one to use, but it worked for me with some minor mods!

you can see it all here :p
http://videokarma.org/showthread.php?t=272325

[rca21d645]

Originally Posted by Yamamaya42

All you can do is ask in the classified area, and hope, and you do not have to limit yourself to a 12LP4A exactly, if you find a 12LP4, ( same CRT, but without the gray face ) you could use that, or some other compatible one, for example a 12KP4 will work, just leave off the ion trap, I'm sure there are others that can work as well.

Also, when I was bringing up my FADA, I did not want to risk hurting my 12LP4, so I used a 8YP4 Test CRT that I got from ETM, not exactly the right one to use, but it worked for me with some minor mods!

you can see it all here :p
http://videokarma.org/showthread.php?t=272325

Thanks for the advice, I'll try the classifieds.

[rca21d645]

Here's what it looks like with the retrace supression modification. The lines aren't really visible so long as the brightness is turned all the way down, however if I turn the brightness up at all they appear. I also noticed that the longer I leave the set on, the more visible the lines become. I'm guessing this might be an issue with the high voltage?

[Yamamaya42]

from another source...

"A CRT with weak emission can produce a picture that's dim overall, and if you crank up the contrast and brightness in an attempt to improve it, the picture can get a silvery or almost-photo-negative appearance. Kind of hard to describe, but easy to recognize if you have seen it before."

This very much describes what was seen on yours, where it's hard to tell if you have good HV just by looking at yours, there does not appear to be any blooming, a good sign of it being low, but the only real way to be sure is to use a HV probe.

ETF has a sub guide.
https://www.earlytelevision.org/pdf/...guide_1959.pdf
any in the group 7 if I remember, can be used.

[Kevin Kuehn]

Originally Posted by Yamamaya42

from another source...

"A CRT with weak emission can produce a picture that's dim overall, and if you crank up the contrast and brightness in an attempt to improve it, the picture can get a silvery or almost-photo-negative appearance. Kind of hard to describe, but easy to recognize if you have seen it before."

This very much describes what was seen on yours, where it's hard to tell if you have good HV just by looking at yours, there does not appear to be any blooming, a good sign of it being low, but the only real way to be sure is to use a HV probe.

I'm pretty sure it is a very weak CRT. I'm not sure why the brightness control can't cut the picture off completely. Just for kicks I fired my 12X12 chassis up with a known very low emissions 12KP4 and get the same weak silvery appearing picture, although I do have complete control over the brightness all the way to dark. Measuring the DC bias at the video output could possibly shed more light on that issue. Half the fun of this hobby is figuring out why things are doing what they do.

[Eric H]

I see some neck shadow on the screen, that would indicate some mis-adjustment of the Yoke, Focus coil or Ion trap. The Ion Trap in particular can cause a dim picture.

[Kevin Kuehn]

Originally Posted by Eric H

I see some neck shadow on the screen, that would indicate some mis-adjustment of the Yoke, Focus coil or Ion trap. The Ion Trap in particular can cause a dim picture.

Ion trap is for sure very sensitive. And if you don't get it right you risk depositing vaporized metal on the back side of the phosphor, which eventually leads to an area that shields the phosphor so it can't light up properly. Many folks mistake it for Ion burn, but it is not Ion burn because it's only visible when the CRT is operational.

[rca21d645]

I checked the horizontal output tube and the HV tube and both measured bad on emissions. I was able to swap the 6BG6 with another that tested ok, but I don't currently have another 1X2 tube to swap for the HV. I also changed a resistor on the HV which measured as 446k in the spot where a 1 meg resistor should be, so I put in a 1.1 meg resistor that I had on hand. The picture looks significantly better but now the audio is worse, with a fairly loud hum drowning out the audio, much more than it did originally. Video of it working here: https://drive.google.com/file/d/1rF1...ew?usp=sharing Photos are with the brightness turned all the way down.

[rca21d645]

Originally Posted by Eric H

I see some neck shadow on the screen, that would indicate some mis-adjustment of the Yoke, Focus coil or Ion trap. The Ion Trap in particular can cause a dim picture.

I tried adjusting the ion trap after reading this, as well as the focus coil and I think I got the picture as good as I can.