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Old 01-06-2017, 05:16 PM
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CTC-5 21AXP22 convergence

When my CTC-5 had a glass 21FJP22 subbed in, I was able to get excellent convergence. When I finally got a 21AX to put in it, I found the dynamic convergence to be much poorer. I recently went back to trying to improve it, and here's what I'm finding:

I have to move the yoke almost all the way forward to get full screen purity. Maybe this is normal, but with the yoke in this position, it appears there is not enough dynamic convergence action to converge red and green at the edges, either vertically or horizontally. Pulling the yoke backwards improves the convergence, but of course then purity occurs only in the center of the screen. I have experimented with the position of the convergence coils, since you cannot see the pole pieces beneath the dag in the neck, but at the point of strongest action there is still not enough.

The DC adjustments have enough range, and generally want to be near center range, which is as it should be.

I am beginning to wonder if the gun is in the right position, since this is a rebuilt tube.

I also need to recheck whether the resistance values of the DC convergence controls that I hacked in (because the original tapped controls were burned) could affect the dynamic current. Not sure yet if they are in series with the dynamic voltages or not. (Apparently burning was a common problem, because the pulled parts some kind people here sent me were burned too, so I had to give up on originals.)

Looking at the top and bottom convergence, I wondered if a weak vertical output could produce low convergence amplitude, but both V and H are bad, so I'm discounting that.

One thing I did learn: the purity magnets have short springs on the base, which apparently act as small pole pieces as well as mechanical mounts, and it helps to keep them properly oriented while the magnetic rings are rotated. One of the rings binds to the base, and if you're not careful, they rotate together, making it more difficult to get uniform purity.

Any thoughts of what I may be missing here would be appreciated!
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Old 01-06-2017, 09:24 PM
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Hi Wayne,

I had similar problems with my CTC5. I initially found the CTC5 tough and had difficulty getting good convergence consistent with good purity. Ultimately through perseverence I got excellent convergence. Here is some of the things I found.

It is very important to first fully retract the edge purity permanent magnets. With the mask off, the magnet screws are adjusted clockwise to submerge the magnets into the cups. Then thoroughly degauss the tube. Next you need to aim getting good purity in the center of the tube and ignore poor purity within a couple of inches of the edge of the tube. The edge magnets will fix the edge purity.

The convergence cloverleaf should be located as depicted in the manual. Due to the symmetry of the convergence cloverleaf coils, after when disassembling the coils to clean them, I inadvertently reassembled one coil reversed. This complicated my first attempts to get convergence.

To help diagnose the convergence problems, I reasoned I should aim for good static convergence and address dynamic convergence later. I reckoned that the dynamic convergence circuitry is mainly to makeup for manufacturing tolerances and fair convergence should be achieveable with minimal dc or dynamic convergence control. Once I was able to establish static convergence with the three DC controls (I found one was open), the application of dynamic convergence did improve the edge convergence (once I rotated the coil on the cloverleaf.)

Of course the CTC5 convergence circuitry is early which means there is more interaction between the static and dynamic adjustments on the CTC5. The CTC5 I found tougher to converge than later sets but once you get the feel of the adjustment and pay heed to the RCA notes, you should be able bring the convergence in or at least identify better where the problem is.

Good luck.

Terry

Quote:
Originally Posted by old_tv_nut View Post
When my CTC-5 had a glass 21FJP22 subbed in, I was able to get excellent convergence. When I finally got a 21AX to put in it, I found the dynamic convergence to be much poorer. I recently went back to trying to improve it, and here's what I'm finding:

I have to move the yoke almost all the way forward to get full screen purity. Maybe this is normal, but with the yoke in this position, it appears there is not enough dynamic convergence action to converge red and green at the edges, either vertically or horizontally. Pulling the yoke backwards improves the convergence, but of course then purity occurs only in the center of the screen. I have experimented with the position of the convergence coils, since you cannot see the pole pieces beneath the dag in the neck, but at the point of strongest action there is still not enough.

The DC adjustments have enough range, and generally want to be near center range, which is as it should be.

I am beginning to wonder if the gun is in the right position, since this is a rebuilt tube.

I also need to recheck whether the resistance values of the DC convergence controls that I hacked in (because the original tapped controls were burned) could affect the dynamic current. Not sure yet if they are in series with the dynamic voltages or not. (Apparently burning was a common problem, because the pulled parts some kind people here sent me were burned too, so I had to give up on originals.)

Looking at the top and bottom convergence, I wondered if a weak vertical output could produce low convergence amplitude, but both V and H are bad, so I'm discounting that.

One thing I did learn: the purity magnets have short springs on the base, which apparently act as small pole pieces as well as mechanical mounts, and it helps to keep them properly oriented while the magnetic rings are rotated. One of the rings binds to the base, and if you're not careful, they rotate together, making it more difficult to get uniform purity.

Any thoughts of what I may be missing here would be appreciated!
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Old 01-07-2017, 05:15 PM
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I went back to look at the RCA manual. As I said, I can't see the convergence pole pieces in the tube, but I noticed the diagram showed the convergence coils spaced back from the yoke shroud, whereas I had them pushed forward quite a bit. The manual also says the purity ring should be 1/4 inch from the convergence coils, and the illustration seemed to show the convergence coils, purity ring and blue lateral equispaced from each other. Since I can see the blue lateral pole piece, I started there, and spaced all the parts about 1/4 inch apart.

Voila! Convergence can now be obtained.

I also went through the retract edge magnets and degaussing routine, and then purity adjustment. I discovered that with the neck components this close, they all interact - making large purity adjustments can screw up the convergence.

The photos show the result. Overall white purity is now not so good (even with edge magnets optimized), but I compared the RCA procedures (which say red spot in the middle to start) and SAMS procedures (which say put the red spot off center towards 8 o'clock). I will try following the SAM's procedure tomorrow, and see if it makes it any better.
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Old 01-08-2017, 09:13 PM
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Well, I tried the SAMS purity procedure, and got essentially the same result, in the process verifying that, yes, everything on the tube neck interacts with everything else on the tube neck. So, after a new purity starting point, I had to redo convergence and go back and forth a few times to finally not get the white screen uniformity much better.

When I first turned the set on today, I got reminded I should have checked that all the tubes were firmly seated after shipping (dummy!), because the high voltage died two seconds after the picture lit up. Fortunately nothing had popped and reseating tubes fixed it.

Tomorrow I'm going to play with the edge magnets some more to see what happens at the extremes of adjustment.
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Old 01-16-2017, 03:46 PM
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Well, today I redid the purity and convergence from scratch again, starting with the edge magnets fully retracted (as before). The purity is now 100% better than before (still some white field non-uniformity), and the convergence has got a bit worse in the NE and SW corners. All in all, the color picture now looks much better.

I've got a another observation: off-channel, the snow and audio noise is very weak, and the AGC is cranked to max gain without getting any tearing or distortion. Is this normal for the Super chassis, or should I go looking for weak tuner/IF tubes? If I could get more signal out of the detector, it would help the generally weak drive that this chassis has. Have not put a scope on it yet to see what's actually there.
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Old 01-17-2017, 11:43 PM
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The Super and Deluxe share the same tuner, video IF and luminance amplifier. If the agc is cranked to maximum and overload is not reached and you confirm a good level RF input, gain must be lost somewhere.

My CTC5 had low gain which simply was a bad 3rd VIF tube.

Even though my set had a quiet previous life, I found the video IF alignment off. A touch up further increased gain.

I now find the agc control set about mid-point. Cranking it up further sends it to overload.
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Old 01-18-2017, 09:32 AM
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Thanks. Will check tuner and IF next.
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Old 01-18-2017, 09:11 PM
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I don't have subs for the 1st and 2nd IF, tried the 3rd IF and RF amp, and made no difference. I will try an external AGC supply next to see if I can make it increase gain to the overload point. Maybe some parts values have gone wonky in the AGC. I am also ordering some tubes for the tuner, IF, and AGC.
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Old 01-18-2017, 10:46 PM
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Quote:
Originally Posted by old_tv_nut View Post
I will try an external AGC supply next to see if I can make it increase gain to the overload point. Maybe some parts values have gone wonky in the AGC. .
Hmmm. Yes it could be an agc fault. Does the agc control adjustment make any difference? Cranking down you should observe less gain or lower picture contrast. You at the point when the video clips, you will begin to see the video bend and beyond this rapidly go into overload I guess because of the missing sync affecting the keying action.

I think a separate external bias supply to route out an agc fault is a good idea and best to try first.
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Old 01-18-2017, 10:52 PM
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The AGC control works, but there's apparently not enough gain at max, and you can turn it down to where there is no picture at all.
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Old 01-20-2017, 03:05 PM
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Substituting an external AGC bias does the trick. When the gain is increased to a point just below overload (which is what the AGC pot should do), there is now enough drive such that turning up the contrast to max and increasing the brightness can drive the high voltage out of regulation.

Now to find what's wrong, which may be a pain requiring pulling the chassis for access. I'll see what resistances I can check in-circuit and while it's still in the cabinet. Checking voltages makes no sense because it's a feedback loop and they normally vary with signal level; in fact, RCA and SAMS disagree, probably depending on exact test conditions.
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Old 01-20-2017, 06:12 PM
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A 47k resistor in series with the AGC control looked like the only thing that could reduce the top end of the range - sure enough, it checks as 60k in circuit, so is definitely high. Bridging with 100k restores normal range of the control, so now I will see if I can clip it out and tack in a good 47k without pulling the chassis.
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Old 01-21-2017, 11:52 AM
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Tacked in a new 47k. It doesn't look neat, but works. Another resistor nearby had been similarly tacked in before I got the set, so I'll consider it to be a typical sort of repair.
I suspect there are many resistors out of spec, but I am not ready for a complete shotgun restoration until I am forced to, which may never happen.

One thing I forgot to mention: the sound was weak as well as the picture. With the detector level up to normal, the sound volume is now increased to a normal range.

I reviewed my ratio detector theory. The explanations of the ratio detector always say it rejects amplitude modulation. However, what they usually don't explain is that this only occurs when the FM carrier is quiescent at center frequency. The amplitude of the detected audio is also proportional to the signal level into the ratio detector. This Super chassis, with only one audio IF stage, does a poor job of amplifying the subcarrier to a point where it limits, so the audio level can be affected by the detector output level. This is especially true because the IF audio trap alignment is a compromise between eliminating audio interference in the picture and having enough 4.5 MHz subcarrier left to detect.

At this point, the set seems to be a fair example of the performance of this lowest-form-of life chassis.
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Old 01-24-2017, 08:33 PM
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Have you checked the tubes on a transconductance checker? My set was relatively low hours yet I found a number of the tubes weak in the signal path and needing replacement. The good thing is that most of the replacement tubes are cheap on ebay.

The biggest drawback of the CTC5 in my opinion is the low high voltage coupled with the relatively dim (compared to tubes a few years later) 21AXP22A.

The low high voltage appears to be due to an inherently bad design. My set barely makes the design regulated 19kV: it would dip below the regulated output when the brightness was advanced. An immediate help for this was to substitute a solid state 3A3 HV rectifier so it at least now maintains regualtion at 19kV. If I get the time, I would like to explore further the Horizontal Output/ Flyback to see if I can a bit more HV.

The 21AXP22A in my set is original and shows good emission and life. But the picture is just dim. I compare it with the CTC11: the CTC11 picture is bright and brilliant. No comparison.

I suggest that the earlier less efficient phosphors in the tube coupled with the low high voltage is an inherent problem with the CTC5. Perhaps I should express my gratitude for the dim picture on the CTC5 as it probably explains why my set survived until now.
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Old 01-24-2017, 10:31 PM
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Quote:
Originally Posted by Penthode View Post
Have you checked the tubes on a transconductance checker?

...

The biggest drawback of the CTC5 in my opinion is the low high voltage coupled with the relatively dim (compared to tubes a few years later) 21AXP22A.

...

If I get the time, I would like to explore further the Horizontal Output/ Flyback to see if I can a bit more HV.

....

I suggest that the earlier less efficient phosphors in the tube coupled with the low high voltage is an inherent problem with the CTC5. Perhaps I should express my gratitude for the dim picture on the CTC5 as it probably explains why my set survived until now.
Haven't checked the tubes on a transconductance checker. I got the replacements, and subbing them mad no improvement. One of the first IF subs knocked the alignment way off for some reason.

The Super chassis has lots of drawbacks including the low high voltage.

Since I don't know how much safety factor is in the flyback design, I will not be attempting to raise the high voltage.

The less efficient phosphors are a detriment to brightness, but the greens and cyans have a purity that cannot be obtained in any later tubes (which have the more efficient but more yellowish sulfide green). Some of this colorfulness is lost due to the lower brightness. This is a well known effect in color reproduction, dubbed the "Hunt effect" due to its description by R.W.G. Hunt. Simply put, to look equally "colorful", a dim picture must have more color saturation than a bright picture.
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