Poor focus on an Admiral 20Y1

[Kamakiri]

Been working on this Admiral 20Y1, and I'm stumped.

I can get the focus somewhat close by cranking the control clockwise, but I can't get it locked in. The control is smooth and linear....in other words, it works fine but just doesn't have the range to lock in the focus.

To begin with, I've done a complete recap of the set.

I have replaced and tested every resistor in the focus circuit. I have tested and then subbed the focus coil with one off a parts chassis, to no effect. I have subbed the ion trap. I have tested all the tubes of course, and subbed the 6SN7 and the 5U4 just for grins, and no change. There are no wiring errors that I can spot, and I've been working on this focus issue for at least 6 hours and have been through the schematic with a fine tooth comb.

Link to the Sams:

http://www.earlytelevision.org/pdf/A...Sams_100-1.pdf

AGC, brightness seem to be operating fine, sound is good, but the horizontal and vertical lock aren't very stable either.

I'm stumped as to where to look next??

[Kevin Kuehn]

In a later Riders there was a whole page of revisions to these chassis, and it seems they changed the value of the focus control in later versions. But it's been a while since I read about it. Doesn't the Sams also have a couple notes about optional part values in the parts list? I'll see if I can dig up the Riders info.

[edit] Would probably be wise to pm Mr(Admiral)BandersenTV.

[dtvmcdonald]

Look at that focus control circuit. Its as bizarre as I have ever seen in one of these old TVs!

It simply changes where some current goes off to a modest amount of drain, e.g. the audio output screen. Not a lot of range. In the process it changes the voltage on the CRT cathode and grid in sync.


Which end of the control are you at?

[Eric H]

I had a focus problem on a Admiral a while back, I don't remember what fixed it, but I seem to recall a bad coil. Have you ohmed the coil out? Also I seem to remember the polarity of the coil is important, not likely it got reversed but possible.

Is the best focus at one end of the control or does it pass through the best focus somewhere in the middle without actually focusing?
If it's at one end it would indicate it's either not getting enough current or getting too much.

Have you tried another CRT?

[tom.j.fla]

Hi I agree with comment in post #3, strange, very strange. One thing that was mentioned in the Admiral notes was the need to have the yoke and focus bracket pushed foreword as far as possible, rubber yoke collar pressed against the bell of the picture tube. BTW does your set have 4 adjustable screws on the focus coil or the lever that goes out the back of the set. If you don't have the Rider info from book 4, the ETF has the 48 pages for the 20X1,Y1 chassis. All the best,Tom

[Kamakiri]

Originally Posted by Kevin Kuehn

In a later Riders there was a whole page of revisions to these chassis, and it seems they changed the value of the focus control in later versions. But it's been a while since I read about it. Doesn't the Sams also have a couple notes about optional part values in the parts list? I'll see if I can dig up the Riders info.

[edit] Would probably be wise to pm Mr(Admiral)BandersenTV.

This is an early version for the schematic, but y'know that's a great idea. I'm gonna jumper in another resistor value and see what happens

[Kamakiri]

Originally Posted by dtvmcdonald

Look at that focus control circuit. Its as bizarre as I have ever seen in one of these old TVs!

It simply changes where some current goes off to a modest amount of drain, e.g. the audio output screen. Not a lot of range. In the process it changes the voltage on the CRT cathode and grid in sync.


Which end of the control are you at?

I'm at the full clockwise...

[Kamakiri]

Originally Posted by Eric H

I had a focus problem on a Admiral a while back, I don't remember what fixed it, but I seem to recall a bad coil. Have you ohmed the coil out? Also I seem to remember the polarity of the coil is important, not likely it got reversed but possible.

Is the best focus at one end of the control or does it pass through the best focus somewhere in the middle without actually focusing?
If it's at one end it would indicate it's either not getting enough current or getting too much.

Have you tried another CRT?

It's a 12LP4, and the only set that I own that has a 12LP4. I could probably pull the 12UP4 out of another set to try it, but that's a lot of work on that set

[bandersen]

Usually these sets allow you to physically move the focus coil assembly back and forth to find the sweet spot by loosening up some screws. Maybe you could give that a try.

[Kamakiri]

Nope, on this chassis the coil is in a fixed spot. You can move it around, but not back and forth. I even took the screws out of the focus coil and tried moving it back and forth on the picture tube neck to no avail.

[Eric H]

Do you have an extra 10BP4 laying around that you could try? I think it would be close enough to work.

Also, how's the B+ close to spec?

[Kevin Kuehn]

Originally Posted by dtvmcdonald

Look at that focus control circuit. Its as bizarre as I have ever seen in one of these old TVs!

It simply changes where some current goes off to a modest amount of drain, e.g. the audio output screen. Not a lot of range. In the process it changes the voltage on the CRT cathode and grid in sync.


Which end of the control are you at?

Also the plate current, which flows through the primary of the output transformer. Sams schematic shows the speaker plug disconnected, so it looks like only the screen draws current. The audio output cathode current(screen and plate combined) also sources the IF's, video, and sync circuit. So really all those circuits combined are pulling current though that focus control. As crazy as it looks, the design was an efficient use of power.

[Kamakiri]

This is absolutely crazy.

I added another 750 ohm resistor with little effect. All of a sudden, BAM, picture blinks into focus, and the beam is off center. I center the beam and much to my surprise, start watching TV, adjusting the now inadequate width and too much height.

All of a sudden, BINK....it goes out of focus again, loses horizontal sync, and all of a sudden the picture beam shifts down and to the right. I tap around, nothing, and then BINK, comes back.

Picture tube? Yoke?

I replaced the focus magnet with one off an identical chassis, even though it tested fine....but that had no effect. With everything I replaced, the ONLY thing I can think of is an intermittent short or something in the electron gun of the picture tube changing the angle of the beam and bringing it out of focus.....or a bad yoke.

[Kevin Kuehn]

Is your sound working OK during all this, because if it's not, that tube is sourcing power to a lot of other circuits. Make sure the speaker plug is making good contact, as all that current needs to flow through the primary of the output transformer. Make sure the audio output tube is making good contact it it's socket.

[Kamakiri]

Sound never changes. After letting it run for about an hour, the issue's stabilized. Having some issues with DC restoration also, now.

I think I'll run it for another hour or so and see if this set decides what it wants to do....