Today's Mystery Guest (RCA CTC-4)

[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.

[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