Repairing old 1951 sentinel 424

[Patnukem]

Ok I went though it once more and found a loose screw on the flyback also heavily insulated everything under the HV rect with electrical tape and replaced a resistor under there. still no changed so I tried another 1x2b still no change. I am left with thinking it must be a small gap in the doorknob causing arcing inside to get to ground, or one of the other tubes in the HV cage, I will change all the tubes out in the HV and see if it makes a difference. I tried the paper cone trick to try and isolate the sound but it is not easy enough in the HV cage. I did check with the lights out and could not see any arcing.

UPDATE: changed out all the HV tubes I am left with the conclusion it is the HV filter "doorknob" cap. the sound is definitely originating from the top in the HV cage.

[Patnukem]

I will be replacing the doorknob capacitor (as soon as I find a reasonably priced one) and the Horz. centering rheostat. Been looking for a 60ohm rheostat but a 75ohm one would be way cheaper would this be a huge deal in the tv? I do not think it would but thought i would ask just to be sure. I just want to get rid of that extra resistor I have in there to keep the resistance in range.

[Reece]

Easier to use a length of plastic or rubber hose to listen for a localized noise.

[wa2ise]

Originally Posted by Patnukem

I will be replacing the... and the Horz. centering rheostat. Been looking for a 60ohm rheostat but a 75ohm one would be way cheaper would this be a huge deal in the tv?

It should work, as long as the wattage of the rheostat is the same or higher than the original. The control will be a bit twitcher than the old one, as the amount of change vs angle of rotation will be a little higher. Or connect a 300 ohm resistor in parallel, wiper to the end that is connected to the existing circuit, to make the 75 ohm look like 60 ohms. That will make it adjust like the real 60 ohm one would.

I assume that the rheostat is to be connected one end only, and the other connection is the wiper. the far end of it not connected to anything. And the parallel resistor connects to the wiper to the end that also has the connection, and nothing connected to the far end.

[Patnukem]

Thanks, I ended up getting qty of 5 new 75Ω 2watt for the price of one.

Anyway after looking for a doorknob I found a Hi-Q one but do not know if it will mount properly any idea?

they also have on like the one in my set but it tested at only 400pf and not the labeled 500pf on the side.

so I am torn to get the tested hi-q one 500mmf/20kv or the sprague 500mmf/20kv that was tested around 400mmf?

EDIT: Both with shipping was only 17 bucks so I will just do that.

[Patnukem]

The new Doorknob capacitor seems to have fixed the issue. I do have a new problem now I installed the new 75Ω pot. and now the picture seems to wide even with Horz. adjustments and the picture is too short and if stretched out vertically it seems to only multiply the image if I get it short enough for a single image it is heavily folded over vertically.

-note-
now the vertical has collapsed all the way so I know for sure that is where the trouble is, as this happened i could hear a tiny sound near the vertical circuit. before this there was a small spark in the HV cage I think by the HV damper tube I changed out some of the tubes and this seem to fix that issue.

[Patnukem]

well Hope there is not a ton of damage....

Measuring the B+ they all seemed very high spikes up into the 600v+ area and they seem to come back down to 399V for the 345v and 335v only when the Vert. Size is turned all the way down (counter clockwise). Any Idea what would cause such a thing to happen?

I also noticed the 75Ω rheostat is connected on both ends but one end connects to the wipers middle pin, so I should take wa2ise's advice and find a 300Ω 2watt resistor tomorrow.

[Patnukem]

Finding the B+ spike was caused by the added resistance in the new horizontal Centering 75Ω rheostat, I added 3 100Ω resistors and the b+ seems to be back to where it was before still a little high but around 399 instead of 350. anyway I took the Vertical Centering rheostat off to check that it was in range and took it apart and cleaned it very well just in case. it seems to be almost still right on at 21Ω tapped at 10.7. Turning to the screen I now seem to have a collapsed image vertically. and most of the image is gathered on the bottom of the 3 inch or so tall image with no controls helping it.

Hope I can finish this tv soon I am getting another one and it will be my 2nd pre transistor tv and my first round screen!

[Patnukem]

Found the issue it was a bad 4.7meg resistor I had subbed in to check for a bad one and forgot to take it out and put the new one back in.

The image is nice now but a little dark I noticed the focus assembly was a little to far forward and moved it back also noticed when adjusting the ion trap that touching a magnet (screwdriver) by where the end of one of the "blades" that come out from the ion trap magnet made the image a whole lot brighter could I have a weak ion trap or could it be backwards? I know I had messed with it long long ago before I knew much about tv's so who knows what I did to it. would it hurt the tv in another magnet was temporarily installed? it seems to about double the brightness of the tube!

[bandersen]

Have you tried rotating / sliding the magnet ? Maybe it's just not in the sweet spot. I'd try flipping it around next. I've never encountered a weak magnet, but I suppose anything is possible. No, I don't think you could hurt the TV if you tried another one.

Good luck. It was a real PITA to get the focus and trap properly adjusted properly on mine.

[Patnukem]

i have rotated it and slid it till it was in the brightest spot, the focus where it was (all the way to the front) with the adjustment screw setting the focus back as far as it would go in the holder but seemed like it needed to go back a hair more so i moved it back.

I will try to flip it maybe the polarity is just off and when the 2nd magnet is added it corrects it.

I am so happy the clicking sound went away in the HV, it runs nearly quiet now (with the volume down of course )

[Patnukem]

guess it was just the focus being a bit off.

anyway I can not seem to get the horizontal size down (it does not help that half the horizontal controls are wedged between the HV and crt support) I will try to get some pictures/video of it today.

[Patnukem]

Of course when I start figuring one thing out something else happens. It still needs a couple of small adjustments but it looks fine for now I did run it a long time today and noticed the Flyback seems to be getting to hot and could see a little bit of bubbling on the corona dope I put on. Is this from the excess B+ and or from running it so long today while I made adjustments I would like If it would be ok to use for 30 minutes at a time a few times a month. anyway here is a photo and video the horizontal has a bit of a roll over on the left (the drive was helping but now its turned up all the way) and I probably could tweak the vertical to adjust the shape a bit but It is a million times better then before!



video
http://www.flickr.com/photos/patnuke...n/photostream/

EDIT:
I fiddled with the controls a bit more and was satisfied enough to put it back in the cabinet! I think the Flyback overheating was caused by it being on so long with the slightly higher b+ and it being in the upper 80's today. The garage I was working in was probably 90 or so! I had not heard or seen any more little bubbles or signs of the Flyback overheating since toward the end of that long adjustment session!



Video
http://www.flickr.com/photos/patnukem/4654457417/