Ive got two rcas with focus problems. One is a ctc31 from 67 and the other is a ctc38 from 1969. My problem here is that the focus just doesnt get sharp enough. Its not way off or too blurry but it can be better. For example, small lettering like on car commercials is barely legible because of poor focus. These chassis used the coil as opposed to zeniths potentiometers for focus adjustment. The focus circuits are pretty simple, what exactly goes wrong on the rca? Ive seen some other rcas in the past like this. Simple tube change of the 1v2 or 2av2 doesnt do it, niether will hv tube changing and hv voltage adjustment. Just as a note, The picture DOES NOT bloom when the brightness is advanced which is a good sign. Im wondering if there is some resistor or something that is known to go off value in these circuits that you guys might know of right off the bat. Id rather go this way than to tear into the chassis trying to find the problem.
Id like to mention, that in my ctc31, the flyback does get warm, sometimes it feels hot, but the set still works. Whats the story on hot flybacks? Junk?
Thanks-Doug

Are you saying that the focus control is at one extreme and the picture still isn't in focus, or that the picture isn't sharp enough even when the focus is set to its optimum position? If the control is at one extreme, you might want to try measuring the focus voltage with a HV probe to see if it should be higher, or lower. That might make it easier to figure out what could be wrong with the circuit. If you can adjust the control to its optimum setting, but it just isn't very sharp, I would suspect a weak CRT. Definitely check the HV is you can because if it's incorrect it will affect the focus. Most CRTs require that the focus voltage be somewhere around 20% of the anode voltage (eg. 5kV).

do you hit a piont where it gets as good as its gonna then gets worse if you keep turning?
or do you hit the stop before it gets sharp?

Yes, the control is all the way at one extreme, The slug moves all the way up and down the coil. Cant really get that sharp pic to come in though.

I most likely have a hv problem. I will go make some measurements of the focus. Since Im at the end of the range, its definitly voltage problem. Lets say however that the focus gets "as good as its going to get" and the control is balanced (somewhere in the center of its travel) and gets worse when I turn in either direction. This would then mean Ive got CRT issues. How could that be corrected?

ok, just made some HV measures. This is on a ctc38, there is no HV adjustment on this set, its all off eff. coil. Anode voltage is 20 KV, specs: 25KV +/- 2 KV. So thats incorrect, however, focus reads 5 KV. No specs for focus, but almost all of them are 5 KV. I am scared to adjust the efficiancy coil as they always break apart on rca sets when they get old. Basically, my eff. coil is the only thing I can adjust to correct hv problems. The tubes were tested about a year ago and they were all ok. This problem did exist then too, so Im going to rule out tubes. If I had a huge stock of eff. coils I would be in great shape. My advice: Dont adjust an rca eff. coil unless you must! Even in the zeniths! The pastic spool heats up from the current in the coil and deteriorates those too. Ive had all of them crack to peices!

Thanks
Doug

The late model CTC 38's use precision resistors in the 6BK4 circuit rather than a pot, to prevent people from fooling with the HV control. Have you measured the voltage drop across the 1000 ohm resistor in the cathode circuit? (note, 400 volts on both ends of the resistor). Should be about 0.9 volts (900 ma cathode current in the 6BK4). Sometimes the resistors in the circuit will go off value resulting in too much 6BK4 current, low HV, and a poor picture.

Well,

I got into the set today, basically all components in the HV area tested within tolerance. I did hook up appropriate test equipment and adjusted cathode current on the h. output and monitored voltage drop on the 6BK4. Basically, everything aligned well, cathode current is at 220ma. Focus voltage about 5KV and the HV is just above 20KV. Picture quality did improve, but still not good enough for my liking. My suspect is the crt. It seems like its going dead on me. I have the screen bias fully advanced to get the green line to show up. I need to "zap" it to try to get some life into it again. I am having a heak of a time getting good red purity. Ive got the adjusting components properly set on the neck of the crt but I cant get the purple and greens to go away on a red raster, even after demagnatizing. I tryed new purity rings that made no difference. A weak crt doesnt cause this does it?

Have you determined whether the focus voltage is too low, or too high? You might be able to get an acceptable picture by making a temporary modification to the focus circuit to extend the range of the focus control in one direction. This should be really easy if the focus voltage needs to be lower than it is.

Poor solder joints on the focus control caused some problems. After resoldering correctly, the focus now balances. (no longer fully in or out) Focus is right at 5KV. The problem is most certianly the crt. It doesnt get to bright. The focus is blurry untill the set warms up, then gets better. So basically, the crt is shot. I need to rejuvinate it. This should make a big difference.

try running the heaters at 9-10v for a while before you hit it .
sometimes this will help esp if the set was idle for a long time.

Doug,

Regarding the poor red purity. Have you done the 'slide the yoke' forward purity adjustment, full size raster with the green and blue guns cut off?

Regarding 'zapping'. Don't do it if you can avoid it. Sometimes zapping makes it irreversibly worse. I've had good luck bringing sleeping cathodes back to life in color tubes with elevated filament voltage at 7-8 volts for overnight. The 21" round tubes usually fully recover this way after measuring almost dead from sitting 30 years. I guess you have to burn the cobwebs outa there. :)

Also, don't forget to examine the focus pin on the CRT and the associated contact in the gun socket. The corona around here can cause ozone that is very corrosive. This can lead to poor contact. I have a Zenith with 21FJP22 with a bad case of pin/socket corrosion...and a focus problem.

Rob

Sounds good rob and kc8. Thanks for the tips!

always use the least risky approach when working with a weak tube.
i dont try to rejuve till all other ways have been tried like running the heaters a bit high.and i only shoot it if it is useless as is or i have another jug to go in.