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#1
10-27-2009, 04:55 PM
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Picture Tube Swap?
I think the very first post I ever made to the forum was about this issue, and now I think I may have a means to solve it:
I have an RCA 21" set that uses a 21amp4a crt. That tube is now REALLY shot, after a rejuvenation that kept it chugging along for a bit, it must now run on a brightener to deliver any image at all. I have a 21CBP4 sitting in the shop now, and it tests like new. Is it possible to swap these out? The only difference between the two crts lies in their methods of focus-the set uses magnetic focus, while the 21CBP4 uses electrostatic focus. I was told that one could run a jumper from the G2 pin to the focus pin and it should work in place of the 21amp4a. If that is true, shouldn't I remove the magnetic focus assembly as well, and I shouldn't need the ion trap on the new tube, either, right? I just happened to have this set on the bench right now for a vertical problem, so I figured I'd replace the crt (if this is really possible) while I was at it. 
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#2
10-27-2009, 06:07 PM
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Should work. If Focus is a little soft, you might try connecting focus electrode to a voltage less than g2 (spec for 21cbp4 is 123V, 21cbp4a is 225v when g2=300V) but I don't think it is very critical. Focus coil is no longer needed, of course, and the tube does not use an ion trap magnet.
jr Last edited by jr_tech; 10-27-2009 at 06:11 PM. Reason: add both tube types
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#3
10-27-2009, 07:04 PM
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Thanks Jr. Tech- Just wanted to check before blasting ahead.
It only took about ten minutes to swap out the tubes. The original 21AMP4 must be the one it was built with-a 1954 date code, and it was purchased in November of '55. I photographed the cathodes of the tubes, as there is something I have noticed in my collecting. It seems to be that the cathode/g1 of well used crts darken as they age. In the case of my old one, look how it has turned almost black from use, while the 21CBP4's (has a date code of '59) cathode/g1 is the same youthful silveryness of the rest of the tubes' elements. As it is, nothing is hooked up to the new crt's focus pin #6. It looks fine without it. I removed all of the magnetic focusing hardware, and of course, the ion trap. It's nice finally having a bright, useful image on this set! Glad to have it back as a daily driver again! 100_1864.jpg 100_1863.jpg 100_1861.jpg Last edited by leadlike; 10-27-2009 at 07:09 PM.
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