View Full Version : 21ct55 question


dtvmcdonald
02-25-2017, 02:16 PM
With the sickness of my 15gp22 red gun I'm considering looking for
a 21ct55, preferable with an all-glass tube already installed.

But these sets are notorious for HV problems. I recently found one
of my old Spellman HV power supplies, a 30 kV at 2.5mA (75 watts)
model, and it works well (its regulated).

Would the horizontal system in a 21ct55 still work properly if the
two HV rectifiers and regulator were simply removed from the set
and the focus voltage regulated with Zeners as I do in my CT-100?
Or would the resultant reduced load bother the yoke drive too much
to adjust?

Doug McDonald

Electronic M
02-25-2017, 04:33 PM
I'd only do that mod if I had one and thought the HV winding was failing...It likely will work. IIRC others have done similar mods by driving a trippler from a SS set off the H out tube plate.

old_tv_nut
02-25-2017, 06:17 PM
It should work (although I agree don't do it unless you need to). Only thing to watch for, I think, would be the B+ going up due to less loading, but you could fix that with an appropriate added power resistor to load it down again.

Edit: if you do use separate HV and sweeps, there is a danger of sweep failing and HV staying up, which I think could burn a nice mark into your expensive or unobtainable CRT.

Electronic M
02-25-2017, 07:06 PM
It should work (although I agree don't do it unless you need to). Only thing to watch for, I think, would be the B+ going up due to less loading, but you could fix that with an appropriate added power resistor to load it down again.

Edit: if you do use separate HV and sweeps, there is a danger of sweep failing and HV staying up, which I think could burn a nice mark into your expensive or unobtainable CRT.

Good points. One thing to consider on the last one is that separating the HV from the horiz does not create the risk of burning a line in, it merely doubles the existent risk, which stems from independent vertical sweep failure.

miniman82
02-25-2017, 07:29 PM
Define 'notorious'? Both my 2B chassis are fine, I assume you're referring to tomcomm? He did that to himself, his troubles are well documented. It's simple, set the horizontal section up the way it's supposed to be set up and you won't have problems.

benman94
02-25-2017, 08:24 PM
Nick,
There are at least two different 21-CT-55 type flybacks; RCA introduced the second to fix issues with the first burning up. The first has a tendency to burn up, the second seems to be much more reliable. Ed told me that the LB-962 sets used a third variation, but I haven't been able to confirm this. The flyback in the Hoffman is identical to the "second" 21-CT-55 flyback.

old_tv_nut
02-25-2017, 09:28 PM
Good points. One thing to consider on the last one is that separating the HV from the horiz does not create the risk of burning a line in, it merely doubles the existent risk, which stems from independent vertical sweep failure.

That depends on whether the vertical is as likely to fail as the horizontal. If the Horz is really much more failure prone than the Vert, independent HV makes a burn much more likely.