Videokarma.org

Go Back   Videokarma.org TV - Video - Vintage Television & Radio Forums > Early Color Television

We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #46  
Old 11-18-2013, 03:39 AM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
VideoKarma Member
 
Join Date: Mar 2010
Location: San Jose, CA
Posts: 612
I haven't had too much time to work on the set further this weekend, but I do have a vague semblance of a picture on the screen tonight.



As you can see, the red isn't very strong, but it is functioning. From the picture here, one might get the impression there is no red at all.

I'm not out of the woods yet by any means on the flyback. What I find is that with any significant brightness on the screen, the HV collapses down to 16 kV, and even lower at full brightness. So I'm definitely short a bunch of turns on the flyback. Focus is also very unstable, and I had to modify the range of the focus control to get even close to focusing properly.

I'll work on some other aspects of the set for a while, and then return to the HV problem. I may try adding more turns to the flyback, or try the voltage tripler module I have in another set (and run it off the primary of the flyback as was suggested back on page 1).

I have the vertical dynamic convergence transformer out of circuit now so I can apply the focus voltage. New transformer has been ordered from our fine colleague John Folsom.
Attached Images
File Type: jpg CT-100 picture on screen.jpg (34.3 KB, 311 views)

Last edited by Tom Albrecht; 11-18-2013 at 03:48 AM.
Reply With Quote
  #47  
Old 11-18-2013, 06:42 AM
wiseguy wiseguy is offline
VideoKarma Member
 
Join Date: Jun 2002
Location: Madison,WI
Posts: 372
CT-100 Focus control

I am not sure if anyone is aware of this but there is a Series of Tektronix tube type scopes from the 50's era that used an almost identical Focus control assy ( I can't exactly remember if it were focus but for sure a control in the HV area) it was a VERY large Scope, as a matter a fact I sold it to someone a couple of years ago for their CT-100 and it worked fine, the actual value was a small amount off but very close to the ct-100, the physical size and everything was identical, I had removed this myself from the older tube type scope for parts, the only part number on the control itself was 01080100., I remember the HV section of the scope had solder in small tube rectifiers, but its been a few years so I do not know the Model number.
I know the Focus and conv controls are impossible to find for the CT100
just passing this on from my experience on the CT-100

Last edited by wiseguy; 11-18-2013 at 03:53 PM.
Reply With Quote
  #48  
Old 11-18-2013, 11:53 AM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
VideoKarma Member
 
Join Date: Mar 2010
Location: San Jose, CA
Posts: 612
Fortunately my focus and convergence potentiometers are OK, but I imagine others who are not so lucky will appreciate knowing there is a possible source for replacement pots.
Reply With Quote
  #49  
Old 11-19-2013, 03:21 AM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
VideoKarma Member
 
Join Date: Mar 2010
Location: San Jose, CA
Posts: 612
Gentlemen,

I have a pretty nice black and white picture on the set now (with sufficient red gun contribution as well). Color circuitry not working at all, but we'll get to that later.

What I'm having trouble with right now is focus stability.

I modified the HV regulator circuit to regulate well at a lower voltage for the time being, since my HV supply is a bit low. That allows the HV to be stable and well regulated at around 15 kV.

However, the focus is still not stable at all, and varies wildly (from nice and sharp to so badly out of focus you can't see anything). Probing the focus voltage, I see that it is moving around quite a bit (easily a factor of 2 with scene changes). Convergence transformer is out of circuit, so it is not the problem.

That got me wondering about whether the focus control was in fact correct. I had noticed when I got the set, that a previous tech had added extra resistors both above and below the focus control, which I thought peculiar. Measuring the resistance of the focus control I find it is only 1 M instead of the 5 M called for in both the Sams and RCA schematics.

Then I checked the DC convergence control and find it is only 10 M instead of the 15 M called for in the schematics.

Can you take a look at the attached pictures (first three) and let me know whether the controls look original? Or were they both replaced at some point by a tech that took the time to get shafts of the right length with no evidence of cutting, etc.? Maybe he couldn't get the right values.

Much to my surprise I found in my junk box a NOS 15 M "high voltage" control, IRC type HV-15 (last two pictures). Always wondered what that would ever be good for. I may swap that in for the convergence control (although the 10 M in there seems to work fairly well).

What I'm really curious about is why my focus is all over the place. With the 1 M control in there, the load on the focus rectifier is a bit lower resistance than it should be -- 11 M versus 13 M (I have added 2 M above the focus control to get a usable range, so only 2 M missing overall from the chain). That doesn't really seem like enough to really make such a big difference. Obviously the focus range adjustment is much smaller than it is supposed to be, but that wouldn't appear to explain the instability.

Would having a low second anode voltage somehow cause the CRT itself to draw more current on the the focus electrodes and overload my focus supply, in a manner that appears to be scene dependent?

It also seems to like to focus with a fairly low focus voltage -- more like 1-2 kV instead of the 3-4 kV in the schematic.

The 1X2 tests good, and I've also swapped it for an NOS tube, so I don't think that's the problem.

Not quite sure what's happening here.
Attached Images
File Type: jpg Existing HV controls 1.jpg (81.5 KB, 78 views)
File Type: jpg Existing HV controls 2.jpg (87.4 KB, 74 views)
File Type: jpg Existing HV controls 3.jpg (89.2 KB, 74 views)
File Type: jpg 15M high voltage control NOS IRC with box.jpg (105.1 KB, 76 views)
File Type: jpg 15M high voltage control NOS IRC.jpg (91.7 KB, 71 views)

Last edited by Tom Albrecht; 11-19-2013 at 03:38 AM.
Reply With Quote
  #50  
Old 11-19-2013, 11:27 AM
Phil Nelson's Avatar
Phil Nelson Phil Nelson is offline
VideoKarma Member
 
Join Date: Jan 2006
Posts: 3,957
Those look (superficially) the same as the pots in my CT-100. I found a couple of old photos that may be helpful.

http://antiqueradio.org/art/temp/RCA...VCagePots1.jpg
http://antiqueradio.org/art/temp/RCA...VCagePots2.jpg

Whether mine are original, who's to say, although I have no reason to think that they are replacements.

Nice progress on this set, by the way. You work about 100 times faster than I do.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html
Reply With Quote
Audiokarma
  #51  
Old 11-19-2013, 01:45 PM
John Folsom's Avatar
John Folsom John Folsom is offline
VideoKarma Member
 
Join Date: May 2004
Location: Melbourne Florida
Posts: 932
Tom, have you replaced the focus filter capacitor? The one on the wiper of the focus pot.
__________________
John Folsom
Reply With Quote
  #52  
Old 11-19-2013, 02:35 PM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
VideoKarma Member
 
Join Date: Mar 2010
Location: San Jose, CA
Posts: 612
Yes, I replaced that with .01 uF 6000 VDC capacitor from ASC.
Reply With Quote
  #53  
Old 11-19-2013, 03:13 PM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 1,938
Re. the erratic focus, is there any chance there could be an open condition in the CRT socket, either on the lead going to the focus anode or the conv. anode?
Reply With Quote
  #54  
Old 11-19-2013, 03:18 PM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
VideoKarma Member
 
Join Date: Mar 2010
Location: San Jose, CA
Posts: 612
The convergence pin showed a lot of white corrosion, which looked like evidence of poor contact in the past. So I shined that one up. I also tightened all the socket pins, since I have intermittent contact on the filament. Much better, but still fails to have proper contact on the filament once in a while when I first put the socket back on the CRT after working on the chassis. The focus pin doesn't look bad, but that in itself doesn't really mean anything. The convergence pin seems to be working, since the convergence control has a big effect on convergence, and the convergence is stable once set.

I do wonder a bit whether the focus pin is making poor contact. The way the socket is designed, it's a little tricky to get a wire in there to wrap around the CRT pin as an independent method to see if the voltage is really getting to the pin. Sounds like I should probably put some effort into that. Or, if you have any particular suggestion how to go after that possible problem, let me know.
Reply With Quote
  #55  
Old 11-19-2013, 03:38 PM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 1,938
I would use an ohmeter on RX1 to verify solid continuity up thru the focus lead to the socket lug. If good, then maybe re-flow the CRT pin (is it pin 6?).
Reply With Quote
Audiokarma
  #56  
Old 11-20-2013, 12:53 AM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
VideoKarma Member
 
Join Date: Mar 2010
Location: San Jose, CA
Posts: 612
I've learned quite a bit more about what's going on with the unstable focus, and I suspect all of this is related to the flyback being different than the set was designed around (fewer turns on HV winding).

I checked out the connections from the focus supply through the potentiometer and all the way to the CRT pin 6. All looks OK.

Observing the behavior, it was clear that the focus voltage was varying a lot with picture brightness. A little more observation revealed that the focus rectifier filament was rather dim since I had modified the HV regulator to bring the voltage down to 15 kV (which I had done hoping to improve HV stability with too little voltage coming from the flyback).

Right away I was able to observe that by increasing the HV, the focus rectifier would glow much more brightly. In other words, loading down the flyback by regulating the HV down also knocked down the focus filament current a lot. With too little current, the focus supply had too little current to remain stable with changing current draw by the CRT focus electrode.

On the other hand, if the HV regulator was set too high, so that it was basically allowing the HV to be unregulated, there was an equally interesting effect. Now, the filament of the focus rectifier would increase and decrease in brightness along with changes in the scene brightness. In other words, a varying load on the HV also caused focus instability.

It looks like the regulation has to be just right -- low enough to keep a constant load on the flyback with changing scene brightness, but not so low that the flyback is overloaded and the focus rectifier filament runs with too low current.

At the moment, I can't really find a sweet spot where the focus is totally stable, but it is far better than before. It's also a question of how much brightness and contrast this set is really designed to deliver. With relatively low contrast, I could probably get things stable.

Or quite possibly, with a proper flyback, everything would work much better. I'll give some thought to either adding more turns to the HV winding (which will be a bit of a project, requiring much more disassembly and modification of the flyback than I did before), or seeing if a voltage tripler in place of using the HV winding can work.

So far, this is all very educational as far as the HV system of the CT-100 goes!
Reply With Quote
  #57  
Old 11-20-2013, 01:14 AM
Tim's Avatar
Tim Tim is offline
VideoKarma Member
 
Join Date: Mar 2004
Location: New Jersey
Posts: 507
Tom:

Very interesting analysis. One thing I discovered when I restored my set is the HV/Focus is a marginal design even with an original flyback. High brightness/contrast scenes can regularly cause the regulation to fail and focus to go out. At first I thought it was just my set but this was then confirmed by other owners to be normal. In order to provide a stable picture, the contrast and brightness need to be set lower than we are used to in modern sets. I do find it hard to believe that this is the way these sets were designed but it seems so.
__________________
Tim
Reply With Quote
  #58  
Old 11-20-2013, 02:14 PM
Tom Albrecht's Avatar
Tom Albrecht Tom Albrecht is offline
VideoKarma Member
 
Join Date: Mar 2010
Location: San Jose, CA
Posts: 612
It would be interesting to hear what everyone who has a working CT-100 is seeing for focus stability, brightness, and contrast. Mine is almost acceptable now that I've backed off the contrast, and set the HV regulation to around 18 kV.

What it cannot handle well at all right now is a white screen. Quite a few commercials these days include a white screen with some text and a small picture. When a scene like that comes along, I get total focus collapse, with the screen practically going blank in a cloud of fog. Almost all other "normal" scenes are OK.

I'll probably leave it like this for a while and take some time to work on the color circuitry, and come back to the flyback / HV issues later.
Reply With Quote
  #59  
Old 11-20-2013, 03:02 PM
Steve D.'s Avatar
Steve D. Steve D. is offline
VideoKarma Member
 
Join Date: Sep 2002
Location: Hollywood Hills, Ca.
Posts: 1,754
Tim is correct. "blooming" is common on CT-100's when large bright/white images appear on the screen. This due to the design of the HV regulation circuits. Keeping the brightness/contrast at their minimums while maintaining a viewable picture helps. The focus on my CT-100 does tend to jump in & out but is unrelated to the HV design. I believe I have a bad focus pot. Current back problems prevent me from pulling the chassis to confirm.

-Steve D.
__________________
Please visit my CT-100, CTC-5, vintage color tv site:
http://www.wtv-zone.com/Stevetek/
Reply With Quote
  #60  
Old 11-20-2013, 03:18 PM
old_coot88 old_coot88 is offline
VideoKarma Member
 
Join Date: Sep 2009
Posts: 1,938
Maybe the marginal 'stiffness' of the HV/focus supply is just part of the charm and challenge of using the set. Maybe sorta akin to using an early radio.
Reply With Quote
Audiokarma
Reply


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -5. The time now is 09:04 AM.



Powered by vBulletin® Version 3.8.4
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
ęCopyright 2012 VideoKarma.org, All rights reserved.