Newbie's First Questions RCA TS 721 Restoration

[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.

[old_coot88]

The bent gun shoots both electrons and ions off to one side. Electrons have very little mass compared to ions, so the "trap" is actually a beam-straightener for electrons. The the ions, being much heavier, remain trapped while the electron beam is re-directed down the axis of the tube. It's analogous to winnowing wheat in the wind; the grain as it falls is much heavier than the chaff, so the grain is "trapped" while the chaff gets blown away.

[Chris K]

So here's a picture of what the trap on my 721 looks like and where I positioned it for the brightest picture...and an additional photo of it on my bench.

[Notimetolooz]

Yes, that is a early type of permanent magnet ion trap. I've seen pictures. Sometimes those are hard to remove, there was a thread that involved that problem.

Are you aware of the "Search" feature on the forum? It may help you avoid the mistakes others might have made. There are several threads involving the 721.

Zenith liked to call ion traps "beam benders" which is more descriptive.

[Chris K]

Thanks! Pattern generator on its way and it looks like it's arriving today. Nothing special but the price was right ($10) and there are pictures of it working.

[Notimetolooz]

That seems good enough.
Some generators put out other patterns that are more useful when looking at the signals with a scope.
If you can, try the generator with another working TV, some flat screens can receive the analog channels. I don't know if the kinks at the intersections are due to the generator or that TV. Flat screen sets do not have this type of distortion.

[Chris K]

Yes I'll check. I just want to get the picture close and then I'll trust my eye to adjust it with normal viewing of something.

[Yamamaya42]

Originally Posted by Chris K

Thanks! Pattern generator on its way and it looks like it's arriving today. Nothing special but the price was right ($10) and there are pictures of it working.

I have the same unit, my dad built it long ago, it does an... adequate job, not one of the best ones by far! It has it's odd quirks.
I need to replace the filter caps in mine because it has started to develop a sort of hula effect to it's output! :O

[Chris K]

I'm not worthy yet of anything closer to the best!

[old_tv_nut]

Originally Posted by Notimetolooz

... I don't know if the kinks at the intersections are due to the generator or that TV. Flat screen sets do not have this type of distortion.

The kinks are common in simple generators, probably due to non-ideal modulation depth/fine tuning of the receiver. Just ignore them.

[Chris K]

Looks like I've got a long night ahead!

[Yamamaya42]

Originally Posted by Notimetolooz

That seems good enough.
Some generators put out other patterns that are more useful when looking at the signals with a scope.
If you can, try the generator with another working TV, some flat screens can receive the analog channels. I don't know if the kinks at the intersections are due to the generator or that TV. Flat screen sets do not have this type of distortion.

odd

i do not see that on mine
http://suzaku.live-evil.org/0127211936(1).jpg
But then again, these were kits and hand built
who knows what strange things they may do!

[Chris K]

That's supposed to be a cross hatch pattern on the 721. I tried the pattern on a 1962 Zenith I restored and it was perfect. Bob Andersen had a horizontal frequency/lock issue on his 721 on initial start up and was able to dial it in adjusting the frequency.slug and coil and messing with the lock trimmer. I'll need to try that out soon. The pattern on my screen is similar to what he got and was able to adjust to normal.

[Notimetolooz]

Originally Posted by Chris K

Yes I'll check. I just want to get the picture close and then I'll trust my eye to adjust it with normal viewing of something.

I think you missed the point. A generator is supposed to put out a perfect reference pattern. A generator can be used to do a final checkout of the TV, you can see problems that you might not notice with a normal TV show.
If you never try the generator on other TVs or hopefully a very well operating TV you can never know whether the fault is in the generator or TV.

[Chris K]

Got it. Thank you. Another question specific to this TV. The tuner does not appear to have any channel detents when I rotate the VHF tuning knob. There is very slight resistance at around half of the channel numbers but essentially, the tuner is behaving like a smooth FM dial tuner. I was able to rough and fine tune in channel 3 and get it stable without physical drift in the tuner but before I go opening the tuner up to see what's going on, I thought it would be best to put this out there to check if the condition is a broken tuner or that it's supposed to be this way.

TIA
Chris