Newbie's First Questions RCA TS 721 Restoration

[Chris K]

OK I saw in the introduction thread that what I described above is the focus coil. I have attached a picture from a later model restored on Phil's Old Radios website. What's in the frame behind the yoke is what I described. I surmise what's on the ground behind the chassis is the ion trap and it's electromagnetic. My 721 does not have that. It just has a metal band and I don't think it's magnetic in any way. I just looked at Bob Anderson's restoration on this site and his also has the electromagnet. Oy veh!!! Can an ion trap be just a passive metal band with symmetrical projections coming off of it?

[Chris K]

Boy...my thread has a lot of posts eh? All mine!!!

So attached is a radiomuseum picture that shows my setup. If you look closely, right up against the back of the focus coil is what I think is the passive ion trap. It looks as if this set does not have the electromagnetic version.

[Electronic M]

Originally Posted by Chris K

OK I saw in the introduction thread that what I described above is the focus coil. I have attached a picture from a later model restored on Phil's Old Radios website. What's in the frame behind the yoke is what I described. I surmise what's on the ground behind the chassis is the ion trap and it's electromagnetic. My 721 does not have that. It just has a metal band and I don't think it's magnetic in any way. I just looked at Bob Anderson's restoration on this site and his also has the electromagnet. Oy veh!!! Can an ion trap be just a passive metal band with symmetrical projections coming off of it?

Yes behind that set is the EM ion trap (brown cylinders) and the CRT socket (black cylinder).

There were several styles of PM ion trap. The oldest 2 common ones (double ion trap style) were a black cylinder with 2 different sized magnet rings (same one in shangos Philco 48-1001 series), and a rectangle magnet with 2 bands that wrap around the neck (the one closest to the screen was typically blue the other black). There were also various single ion traps usually with a rectangular magnet and a single around the neck band.

[Chris K]

Hi Tom...Thanks to you and a response from shango as to what it was and how to move it...First Light! Thank you so much for the education and guidance. On to more difficult things like sound and an actual picture. Stay tuned!

Chris

[old_coot88]

Yep, always adjust the trap for brightest screen consistent with no neck shadow.

[Electronic M]

Great news. Time to hook up a test pattern generator and see how much adjustment/troubleshooting it still needs.

[Chris K]

Yep...I just bought one off ebay. They are pretty cheap!

[Notimetolooz]

I was away doing other things and missed a lot of the conversation.

This is what a permanent magnet single ion trap looks like from the 1950s.
[ATTACH]

Some early CRTs needed double ion traps.

The design of the electron gun in the CRT determines what type and whether an ion trap is needed. In a nutshell the guns are designed to send the beam out in a way that it does not reach the screen. Either the beam hits the side of the CRT neck or some internal structure in the gun. That because the beam contains ions that are heavy and would damage the screen over time. The ion trap acts on the electrons and redirects them to hit the screen.