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  #61  
Old 11-20-2013, 07:51 PM
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I haven't noticed any of the symptoms being discussed, though I'm lucky to have a tube with perfect emissions on all three guns so it's pretty easy to get a bright image without taxing the HV supply.




Again, it's important to set up the horizontal and HV regulation circuits correctly. When doing a set, I always test the HV unloaded with the regulator disconnected to see where it tops out. My CTC-2 makes about 22.5kv, so I know it will easily make a stable 20kv with the regulator tube doing its thing. Other sets are not as good, my Wingate struggles to make 22.5 with no image on the screen and the regulator cap off. My CTC-4 on the other hand is absolutely leathal, I'm not sure where it topped out at since it overloaded my probe but WAG is around 38-40kv? They can be all over the map.

Tom: if the filaments are being affected too much, you might consider taking the filament windings off the flyback and moving them to a step down transformer. Take a 120v primary transformer from something small like a clock radio, and remove the secondary windings. Replace those with a few turns of HV wire and run that to the rectifiers, the current you save won't have to come from the flyback anymore and may just give it the oomph it needs to make respectable pictures again. You could test the theory by temping in some 1.5v batteries to power the rectifiers, if it doesn't help at least you didn't spend any money. Just a thought.
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  #62  
Old 11-20-2013, 09:47 PM
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Good to hear that some CT-100s can produce a bright stable picture. Do you think the CRT is the difference? Normally I might expect a high emission tube to have greater danger of loading down the HV than a lower emission tube. Or maybe a tube with more usage somehow "wastes" electrons on something other than lighting up the screen?
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  #63  
Old 11-21-2013, 04:12 AM
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Since the set is reasonably functional as a black and white TV at the moment, I'm looking into why there is no color. It looks like this will be a fairly labor-intensive bring-up.

The first problem I noticed is that the chroma reference oscillator does not run properly. It fires up for a few milliseconds, and then goes quiet for about 1/2 second. This cycle repeats continuously. I found that this behavior is occurring because as soon as the reference oscillator produces a signal, it causes the chroma sync phase detector to output a strongly negative signal, which has the effect of shutting down the oscillator (I'm not sure a negative signal to the reactance tube should cause the oscillator to shut down, but it does). Adjusting the tuning of the phase detector transformer has no effect. If I short the output of the phase detector to ground, the reference oscillator runs continuously. I haven't found any bad components under the chassis in the surrounding circuitry (except the 100 meg resistor in the color killer circuit, which I replaced), so I will check the coils and transformers next. Everything is recapped, including most micas at this point.

[new info] - When I short the phase detector output to ground so the reference oscillator runs freely, it turns out that its frequency is 2.47 MHz, not 3.58 MHz. Furthermore, it keeps right on running at 2.47 MHz if you pull out the crystal, and when the crystal is replaced with another 3.58 MHz crystal. So perhaps the tuned circuits do indeed need some attention. Will check that this evening. Maybe the phase detector is fine, simply doing what it does when the reference oscillator frequency is way below target.

Another problem I notice is that if I look at the output of the first video amp on the scope, the chroma sync burst on the back porch of the horizontal sync pulse is not visible. I gather this is probably a video IF alignment issue, causing insufficient bandwidth to pass 3.58 MHz? I suspect that's a big problem, so will do an IF alignment soon. Adjusting the fine tuning doesn't seem to provide any setting where the chroma sync burst comes through, at least not without a general massive increase in noise as I tune beyond the point where the black and white video looks reasonable. I imagine if I can't see the chroma sync burst, there won't be any chroma signal getting through either, so color is not going to happen even if the reference oscillator is brought to life.

Last edited by Tom Albrecht; 11-21-2013 at 11:30 AM.
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  #64  
Old 11-22-2013, 03:01 AM
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Got the chroma reference oscillator fixed. It's an oscillator design that is new to me, and is quite sensitive to the value of a "choke" in the control signal from the phase detector back to the oscillator to adjust its frequency. Although Sams calls it a "choke," that is quite a misnomer -- it's actually really an inductor with a specific value affecting some RF response on the control line. Put in a coil much more closely matched to the original and got it running OK.

Still no color, however, so I ran some checks on the video alignment, and found things are not even close. This explains the lack of chroma sync burst on the detected video and so-so sharpness on the black and white picture (apart from focus issues). It appears that virtually every slug on this set has been fiddled with in the past. It's got traps in the middle of the passband, the wrong center frequency (by more than 2 MHz!), a generally peaked (rather than flat) and narrow passband shape, and worst of all, one of the little IF coils in the tuner pushed inside the tuner and damaged (core cracked and stuck). I'm repairing that coil -- nice that that top of the tuner comes off so easily for service.

Alignment looks like it will be very time consuming... and may uncover some additional problems.

Convergence transformer arrived today, so that is also ready for attention. Lots to do.
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  #65  
Old 11-22-2013, 07:39 PM
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Are you aware of the chronic problem these sets have with video peaking coil failures?
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  #66  
Old 11-22-2013, 07:55 PM
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I restored a 21CT55 CTC2B chassis and that inductor in mine had also failed. Like Electronic M says, the peaking coils are common trouble areas. Check all of your inductors and suspect all that are coated in a white cement like substance. It seems to corrode everything it touches.

The CTC2 and CTC2B are similar in a lot of areas. When I aligned mine I found it helpful to inject a color bar pattern at the 1st video amp (pulling the last video IF tube) and get the video and color problems fixed before getting a final good RF/IF alignment.

Dave
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  #67  
Old 11-22-2013, 09:01 PM
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I came across lots of good info online about the failure of coils in these sets. I tested all those first thing when I started and replaced five that were open. I thought this particular one in the chroma reference oscillator circuit wouldn't be particularly critical in value, since it appeared to be an RF choke intended to pass only low frequencies. That turned out to be an incorrect understanding of how that oscillator is designed, so I guess I created my own problem in the process of replacing that coil. Once it was replaced with one close to the original value, the chroma reference oscillator started working.

The circuit is basically a phase locked loop, and usually the control voltage output by the phase detector would be filtered to remove all trace of the RF. Such is not the case here -- apparently a significant RF component appears on the control line and needs to be there for the oscillator to function properly. An unusual design to me.

I'll keep an eye on the remaining white coated coils in this set, since they are obviously subject to failure sooner or later as well.

Dave, thanks for the tip on feeding a composite video signal into the video amp; that sounds like a good "divide and conquer" approach. Since I can't get a nice video signal through the IF chain just yet, that might be a morale booster to get some color on the screen earlier in the process!

Last edited by Tom Albrecht; 11-22-2013 at 09:05 PM.
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  #68  
Old 11-23-2013, 03:34 AM
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After a full evening's work on IF alignment, the video IF response looks better. Not an exact match to what Sams thinks would be ideal (rolloff slope on high end of passband is too steep), but hopefully OK?

The first photo is the bench setup. Second is the connection to the tuner I used for injecting the IF sweep for quite a few of the alignment steps. This has very short leads (less than 1" beyond the shield). The third picture is the IF response from injection point at the diode in the tuner through to the video detector. Although I don't show my markers here (they are not very photogenic on the scope), I have the scope carefully calibrated, with the center being exactly 44 MHz, and each division exactly 1 MHz. The null point on the low end is 41.25 MHz, and the top end goes to zero just under 47 MHz.

Alignment of this set is quite an undertaking compared to a typical black and white set, which I can usually align in a pretty short time. And there's still plenty more to do in the color circuitry.
Attached Images
File Type: jpg Video IF alignment setup.jpg (92.3 KB, 85 views)
File Type: jpg Signal injection into tuner for IF alignment.jpg (69.6 KB, 79 views)
File Type: jpg Video IF after alignment.jpg (60.8 KB, 73 views)
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  #69  
Old 11-23-2013, 01:09 PM
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dtvmcdonald dtvmcdonald is offline
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Wow! I've never seen a response that excellent. No wonder those
sets give great pictures. Is a 21CT55 as good?
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  #70  
Old 11-23-2013, 08:30 PM
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Yes, they have the same IF and are both a royal bi!tch to align if you aren't careful.
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  #71  
Old 11-24-2013, 01:02 AM
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Well, today there were several steps forward, and one big step backwards.

Was still having trouble getting the chroma reference oscillator to lock properly, and then figured out that I had a grid emission problem on the 6AN8 in the oscillator. A voltage check found the grid of the reactance tube much too positive given what the circuitry around it was doing. After swapping that tube out (it looks fine on a tester), I was able to lock the chroma oscillator. At that point, I had lots of wild colors on the screen, and got busy with alignment of the chroma bandpass filter and various other tuned circuits in the color demodulators. Then worked on purity, which was only so-so around the edges (maybe I need a degausser?). Finally installed the new convergence transformer. While setting up to do convergence adjustments, I adjusted the width, height, linearity, to get a nice looking crosshatch on the screen. So far so good, and then...

I heard a gentle pop, and the picture went dark. Very quickly took a look at the flyback, and low and behold, the tape holding the 3A3 anode lead in place had actually caught fire! Obviously an arc occurred (maybe triggered by my changing the width setting?). Quickly blew out the flame, and then pulled out the flyback to see what condition it was in.

Quite surprisingly, all windings good, and no shorts. Good ringdown. But it would obviously arc again without repairs. I unwound a few layers from the outside diameter, and am coating with corona dope and varnish while I contemplate what to do next.

There were significant problems running things with reduced HV because of the turns I previously removed from the flyback, so simply getting back to where I was earlier today probably isn't a real solution.

If I add more turns to this flyback, I wonder if it will arc again? Doesn't look promising.

Maybe tomorrow I'll try a voltage tripler running of the primary of the flyback. If I get enough voltage that way, I can remove the HV winding entirely. If I don't, maybe I'll unwind most of the HV winding, leaving just enough to run the tripler?

I wish there were someone out there with a spare flyback or a junker chassis or set. That would be a much more straightforward solution. But there might not be any spares out there that people are willing to part with.
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  #72  
Old 11-24-2013, 09:55 AM
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Another solution, one I would use, would be to disable the HV system
entirely and find somewhere an appropriate Spellman HV-in-a-box supply and
use that until the correct working flyback turned up. And the HV regulation
would be spot on as these thing are really well regulated. I used to have several
... not on inventory! ... but they got thrown out by the disposal Nazis around here.
You can't hide a Spellman supply, just set in down on the floor of the cabinet.

This might cause a focus voltage headache however, and would of course
reduce the load on the whole horizontal system a lot and that would have to
be taken care of.
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  #73  
Old 11-24-2013, 04:53 PM
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Maybe something like a HV board from an arcade XY monitor would be an idea for sets with unobtainium flybacks and parts.

They've been building replacement boards and flybacks for these on and off for a while, and they have the focus divider built in. 19KV is 19KV no?
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  #74  
Old 12-03-2013, 12:45 AM
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Had to take a few days off from this project. Too many radios/TVs/hi-fis came in and the shop was filling up. Got those done, so back to this set.

With the current condition of my flyback, I decided to follow up on this great idea that was suggested way back on page 1:

Quote:
Originally Posted by Electronic M View Post
On newer roundys (like CTC-16 era) I've heard of folks having successfully removed the fly's HV winding and getting their HV from a solid state HV trippler the input of which being connected to the HO Tube's top cap....This may also be a solution for your CT-100.

...

Good luck fixing it.
Gave it a try this evening. Pulled the TCG523 tripler that has been serving very nicely in my Philco 48-2500 projection TV, and wired it up to the CT-100 chassis. I've got the HV winding of the flyback coated with a relatively thick coat of insulating red varnish, with the end of the HV winding buried in the varnish. That seems to be working OK to keep the HV winding from arcing right now. If this ends up being a usable solution, I'll coat it even better, or possibly remove it altogether.

The first picture shows the tripler on the bench temporarily wired up to the chassis. The input of the tripler is connected to the top of the primary -- the same place the 6BG6 plate is connected. The ground connection of the tripler is connected to the chassis, and the HV lead is resting against my HV probe.

Fired it up, and got 26-27 kV at the tripler output (see second picture). Putting the focus rectifier in place to provide at least a little load for the flyback lowers it to around 24 kV, which seems like a really lucky break. Just about right. The focus rectifier is wired as it originally was, off its own tap on the flyback, and powered by its own filament winding. The 3A3 HV rectifier is removed, and its filament winding not connected to anything.

Connected the tripler output to the HV regulator circuit in the set by touching the HV lead from the tripler to the metal housing of the 3A3 socket. The HV regulation works fine. If I set it to 20 kV, I get a mild red glow from the inside of the 6BD4 regulator (similar to what I had before at lower voltages when I was using the reduced-output flyback). Third picture shows the setup and the HV probe reading. I can adjust the HV from 16 kV to 22 kV with the CT-100's HV adjustment pot.

This looks like it may be a viable solution for dealing with my bad flyback. Will try it with the CRT next.

By the way, don't pay too much attention to the way I've got the replacement convergence transformer hanging off the back of the chassis. I will pot it in the original transformer housing once a few other issues are out of the way here.
Attached Images
File Type: jpg Tripler without regulation - first test.jpg (74.9 KB, 98 views)
File Type: jpg Tripler without regulation - HV probe closeup.jpg (48.4 KB, 65 views)
File Type: jpg Tripler with regulation.jpg (77.5 KB, 102 views)
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  #75  
Old 12-03-2013, 01:45 AM
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Well, here's what I see on the CRT when using the tripler. Looks like there is plenty more work to be done on the color demodulator and video driver circuitry. Convergence is also pretty bad, so some serious work needed there as well. Anyway, this seems to be a step forward. My education in color TV continues...
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File Type: jpg CRT image with tripler.jpg (19.1 KB, 122 views)
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