Newbie's First Questions RCA TS 721 Restoration

[Chris K]

Hi Everyone. I'm so happy to be here and a little intimidated! I posted a thread in the Introduction section describing my background and experience with this hobby. While not quite a novice, I am still pretty low on the learning curve. I've restored a dozen or so tube radios and moved into TV restoration about a year ago and have restored 3 so far.

I had an opportunity to purchase a single family owned RCA TS 721 in virgin condition. The cabinet is in excellent shape and the owners still had the RCA matching 721 roller stand and a magnifier stand for the 10 inch tube but, sadly, a couple of years ago the magnifying lens fell and broke. I'm not sure the owners knew what they had. I got it for around $130.

Tested the CRT on a B&K 465 and got good emissions with no shorts. I ohmed out the power transformer, flyback and VOT and they were all fine. All of the electrolytic cans were problematic...open sections and I replaced all of them with Rubycon caps before initial power up on a dim bulb series. Went through 75, 100, 150, 200, 300 and 500 watt bulbs with no issues so I went bareback! High voltage is fine at around 9kv. Brightness control ohmed out perfect with negligible resistance at the full brightness position. CRT socket voltages all checked out fine and in spec. No light from the tube. Filament is glowing ok, and the high voltage anode is connected fine. No sound yet but that's going to be addressed down the line. I checked and rechecked the installation of the electrolytics and their point to point wire runs (the Riders actually has a wiring diagram...made my eyes hurt!) and as far as I can see, I didn't mess that up. Any ideas? Did I miss something basic? I thought getting high voltage on a CRT that tested fine with a VOT chirping away at 30hz was all you needed for "first light". I'm stumped!!!

[Notimetolooz]

If the CRT tested good, you have good HV and the CRT socket voltages are OK then there is only one other thing that I can think of, the ion trap.
Could you have moved it?
If it is the electromagnet type, is it passing the correct current?

Any sound from the speaker?

[old_coot88]

Ion trap was my first thought.

[Chris K]

Hi there...thank you so much for responding! So, I'm at work and can't check this but, there's a ion trap magnetic coil that is supposed to have 17.5v DC running through it. I'll check the voltage and continuity tonight. I think I ohmed that out as well. It has 2 coils, 33 ohms and 3 ohms. Since it's electrified, do you think it's an electromagnet and not a passive magnet that in a set this old, would have lost its magnetic qualities?

[Chris K]

Nothing from the speaker

[Chris K]

I attached the page of the schematic that shows the ion trap as electromagnetic...just so you can take a look and confirm.

[Electronic M]

Originally Posted by Chris K

I attached the page of the schematic that shows the ion trap as electromagnetic...just so you can take a look and confirm.

Not enough resolution in that schematic to read, but RCA and others did use electromagnetic ion traps for the first couple of years after the war.
If there's wires going to the ion trap and you're not mistaking it for the focus coil or yoke, then it's electromagnetic. Some chassis switched to permanent magnet ion traps mid production and those sets will show an electromagnet trap on the schematic but not have one*.
Regardless of whether a trap is electro or permanent magnet it needs to be positioned correctly. Standard adjustment procedure is, with the set running, to spin the trap 360 on the neck while moving it fore and aft and watch (in a mirror if necessary) the screen for light as you do so. It's not uncommon for a permanent magnet trap to be on backwards, sometimes you need to remove it and flip it so that the ring opening that faced the screen faces away from the screen.
The electromagnetic ones often have an intensity adjustment on top of the mechanical positioning to worry about. I wish I had more experience with electromagnetic traps, but my Philco 48-1000 and RCA 8TS30 (630 chassis) are both later production PM trap sets.

*Schematics are rarely gospel on how a set is actually wired. Most schematics only capture one revision, or production run of many, and production and revision changes aren't always documented, or if documented often are hard to find documentation for. Also Sam's photofacts while great for readability and standardization of drawings are prone to typos. It's not a big deal when you're used to it and are able to distinguish factory workmanship from someone's 50 year old sketchy hack repair/modification, but it can easily trip up a novice who hasn't heard about it before.

[Chris K]

Sorry about the resolution as I just did a quick screen grab. It is the ion magnet and not the yoke or focus coil. It has very limited adjustment. I will take some photos later and post. It is sitting inside a chassis mounted H frame. It looks like a squared off doughnut, about an inch thick and the CRT neck goes through the hole in the middle. It is positioned behind the yoke. It is attached in the frame in 3 places. At the top there's a threaded post on the trap that goes through the horizontal member of the H frame. There's a wing nut holding it in place and a slot in the frame that allows about 1/2" to the left or the right of center. The other 2 threaded posts are on both sides of the trap sticking out horizontally, almost like a center axle. The posts go through the vertical members of the H frame and allow basically the trap to swing in a very small arc. Full swing can move the bottom of the trap closer to the bulb part of the tube while the top part pivots to the rear. Vice versa, when the top is swung forward, the bottom swings toward the rear. But the hole in the center for the neck of the tube is not much larger than the diameter of the neck so the limits of the trap "swing" are very small. I don't know if the metallic ring that slips over the neck behind the trap works with it or has a separate function. It can be slid closer to the trap or back toward the CRT socket. It's just a band of metal with some projections off of it front and rear and there's a slip ring that goes on it, I suppose to put tension on it once it's in the correct place. I watched a shango video where he resurrected a 1948 Crosley 12 inch TV and he had the same situation I have...high voltage...a correctly energized CRT pin socket and he moved a dissimilar ion trap around and got a picturebut he didn't show or talk about what he did and how he did it. Arughhhhhhhh!

[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!