Today's Mystery Guest (RCA CTC-4)

[Zenith6S321]

Originally Posted by Phil Nelson

That reminds me, I need to fix the color bars on my VA62. It makes beautiful B/W patterns, but the colors roll.
Phil Nelson

You might try turning the VA62 color bar frequency adjusting capacitor screw circled in this picture. I had to adjust it on mine to get the right colors in the bars. Remove the four case bottom screws, the four rear case screws and remove the bottom cover and case cover. Remove the two screws on the top of the control panel inside the case. The top will swing backward on a hinge like shown in the picture. Its worth a try...

Dave

[Phil Nelson]

Ah, I remember that convertible-top chassis. When I first got the VA62A it had a bad ripple, which I fixed by replacing a couple of p-s electrolytics.

That is one heckuva twitchy adjuster, or else there's something else going on that makes this oscillator circuit unstable. After much careful tweaking with my longest diddle stick, I was able to get the rolling to snap into regular color bars. When I put the case back on, colors went back to rolling.

The ripple has also begun to resurface. I have gotten a good in-circuit cap checker since I bought this thing. While it's on the workbench, I may as well check the other electrolytics and see what else has gone bad in the last few years.

Phil Nelson

[DaveWM]

any updates Phil, last time looked like you were getting close.

[Phil Nelson]

Still grinding along with the resistor hunt. The audio's great and I can tune in a halfway-decent picture with difficulty. I can make bright and correct NTSC color bars, but things fall apart when viewing real program content. I suspect I'll soon hit the wall and conclude that the TV will never look good without a full RF & IF alignment, something I'm not equipped to do.

Phil Nelson

[DaveWM]

seem like you can confrim that if you feed it a ntsc signal to the video amp (if correct and strong enough anyway).

I have done alignments on color sets with a BK415 did not make things any worse so I guess I did it right.

problems I generally had was not getting a wide enough band pass so had issues getting the traps set (traps often fell well outside of the curve, so could not trap anything. Don't know if the built in markers would work with a CTC4 either. I would prob go with a variable marker and a freq counter for that set.

[old_coot88]

FWIW, I've seen a few cases of lousy color that looked like bad IF alignment, and it was nothing but the mixer plate coil in the tuner needed a slight tweaking by eyeball. These were in later sets than the CTC-4, though.
Also if there's a shielded coax from the tuner to the IF strip, be sure it's grounded securely at both ends.

[Phil Nelson]

Originally Posted by DaveWM

seem like you can confrim that if you feed it a ntsc signal to the video amp (if correct and strong enough anyway).

Yes, I have tried injecting video at various spots on and around the video amp grid and cathode. Injecting at the grid seems to work best, but you never get that magical result where the video is suddenly much clearer than when coming through the RF & IF sections.

I suspect my injected signal is not strong enough. The 1st video amp calls for +2.7v at the grid and +4.1v at the cathode, both of which are connected to the video detector output through different paths. This part of the schematic shows the paths between the video detector and the video amp and bandpass amp:

http://antiqueradio.org/art/RCACTC-4SchematicPart2.jpg

I tried disconnecting the detector diode, or grounding the grid of the 3rd IF tube, to make sure there is not a competing signal coming through the IF. That doesn't seem to matter, as long as you don't have a strong signal at the antenna. (You can't just pull the 3rd IF tube, since half of it operates as the 1st sync amp.)

Learning to do a full alignment is on my "someday" to-do list, but I will need to collect the right equipment and then practice on a simpler B/W set that already has good alignment.

Perhaps the CTC-4 is similar to the CT-100/CTC-2B, where injecting video requires that you add a little video preamp. This article describes the preamp that I built following Pete Deksnis's design:

http://antiqueradio.org/VideoAdapter...Television.htm

Possibly something similar would work on the CTC-4, but note that the CT-100 has only one path between the video detector and the 1st video amp.

Phil Nelson

[miniman82]

Note that those voltages on the grid/cathode of the 1st video tube are it's DC operating voltages, not the level of video present at those terminals. I strongly suspect it's alignment time for this chassis.

[old_coot88]

Phil,
I know it's a long shot (well, so were the suspicions about the v.det diode), but see if you can locate the mixer plate adjustment. It may be a slug or a trimmer. Just tweak it slightly in both directions while watching a normal color program. Can't hurt nothin'.

In fact when we used to get tuners back from the rebuilder, there was a reminder tag stating to optimize the mixer plate coil if necessary.

[Username1]

Originally Posted by old_coot88

Phil,
I know it's a long shot (well, so were the suspicions about the v.det diode), but see if you can locate the mixer plate adjustment. It may be a slug or a trimmer. Just tweak it slightly in both directions while watching a normal color program. Can't hurt nothin'.

In fact when we used to get tuners back from the rebuilder, there was a reminder tag stating to optimize the mixer plate coil if necessary.

I agree with this as a first place to check..... If you have a tuner subber use it,
or since you have the ctc 11 sitting there with nothing to do...... Take off the
rca plug from the tuner into the if strip, parallel it to feed both sets.... That way
if running 2 IF loads diminishes the signal you will see it on the 11 also at the
same time..... And it keeps the agc intact for the tuner.....

Double check the long feed line by running it to the 11 first to be sure it is not
getting any loss or noise etc.....

Remember to check grounds..... No spark shows.....

.

[Pete Deksnis]

Originally Posted by Phil Nelson

Perhaps the CTC-4 is similar to the CT-100/CTC-2B, where injecting video requires that you add a little video preamp. This article describes the preamp that I built following Pete Deksnis's design:

http://antiqueradio.org/VideoAdapter...Television.htm

Phil Nelson

Phil,

The CTC4 is actually very different from from a CTC2: the '2' has seven stages of i.f. with five on the main chassis and two on the tuner (it's a cascode stage).

The '4' has three i.f.stages. Where the '2' i.f. is flat across the luma-chroma domain, the '4' rolls-off BIG time after 3 MHz.

The '4' first video amp compensates for the tilted bandpass characteristic by boosting the chroma domain BIG time.

So, unless video that has been altered to match what comes from a '4' i.f. strip (and its detector) is injected into the '4' video stage, I suspect what you see is probably understandable.

Pete

[Phil Nelson]

Originally Posted by old_coot88

see if you can locate the mixer plate adjustment. It may be a slug or a trimmer.

The schematic shows trimmers and adjustable coils connected to pin3 of the 6X8 oscillator/mixer:



Some (maybe all) of those three trimmers may be accessible through holes in the tuner cage. I haven't found a diagram that shows all of that clearly. The Sams schematic does not show the adjustable coils (L42, L56)at all, and my photos of the tuner taken while cleaning don't show the internals clearly enough to guess whether they exist in my set.

Originally Posted by Username1

Take off the rca plug from the tuner into the if strip, parallel it to feed both sets

I was ready to run out and try this, when I remembered that the CTC-11 cable unplugs, but the CTC-4 cable is soldered on. I'm debating whether I want to unsolder it yet another time. It has already come off twice -- once for cleaning the tuner and again to lubricate the fine tuner when it got sticky.

Phil Nelson

[old_coot88]

Phil,
There is no mixer plate coil as there is in later tuners. Please scratch that whole idea. I should not have brought it up. Please don't mess with any of the trimmers or slugs, as they are strictly oscillator related.

[Username1]

While I have not worked on a ctc4, I have looked back at some of my books, and
they do mention color troubles with the bars not looking right on just some colors
like what you have..... So as mentioned before the tuner, and agc could be possible
sources of this trouble...... Subbing the tuner in some way quickly crosses that off,
checking on the agc specific to that set's setup procedure for agc may quickly
eliminate that.... These should definitely be looked at before any alignment is
attempted, because it would actually then end up being a de-alignment, and you
would be adding to your troubles...

Check non alignment trouble sources first....

.

[dtvmcdonald]

When I look at you scope trace from the video amp cathode, it looks
awfully correct. RF-IF alignment may be causing problems at the color edges of
the bars, but across their center they should look quite good, both color and intensity.
Especially the magenta bar.

I would be scoping the video and color demodulator circuits very carefully.
Because the circuits are so very idiosyncratic, you many eventually need
to get scope traces from a similar generator on a working CTC-4. When my CT-100
had similar troubles, especially the magenta bar, scope traces in the color section
were clearly wrong. But ... the Ct-100 color section is a piece of caketo understand,
compared to yours.

In any event, scope everywhere after the video detector and post the traces,
so everybody can look at them and ponder them.