ctc-15/Silvertone alignment problem

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Last winter I attempted to align the Silvertone IF and chroma.

A little background, the set worked, but had a few nagging little things that suggested it needed alignment. The sound was a little distorted, but sound alignment didnt help, the chroma had gotten kind of weak, and there was always a tiny bit of sound beat in the picture. Very small, but the 4.5mc trap wouldnt fix it.

While the picture tube was out for rebuild, I went through the chassis with a fine tooth comb. More to the point, I changed 3 or 4 out of spec resistors in the IF strip. All of them have been checked. I changed all the tubes in the tuner/if/sound/video/chroma chain except the 1st IF and the matrix tubes. They were all gassy (!). I replaced them with NOS tubes that tested good. After all this, I figured there was little doubt it needed an alignment.

I'm using a b&k 415 for the sweep and marker generator. For anyone unfamilliar with this, it has the sweep, markers, modulator, and three bias supplies all in the same box. Everything you should need except a DVM and an oscilloscope.

I started out using the alignment method in Carl Babcoke's book "RCA Color TV service manual" covering ctc-12 to ctc-40. The method in the book is supposed to address some of the shortcomings of the factory method. I built all the probes and matching pads in the book, but they are just clones of the factory RCA stuff.

I aligned this set several times using Babcoke's method, but kept running into a problem. I had the same results using the SAMS method, and the factory RCA method, too. Ill get back to that in a minute.

As many of you probably know, the IF response should fall off rather drastically at the frequency corresponding to 3.08mc, 3.58mc should be at about 50%, 4.08mc should be quite low. The chroma channel has a response that is the exact opposite, so that these two response curves cancel each other out. The result is a flat response from 3.08mc to 4.08mc at the x and y demodulator grids, at least in theory.

I have done this so many times now I could do it in my sleep. The basic procedure goes something like this: Align the IF strip using sweep. Inject the sweep at the mixer grid test point, and pick up the signal with a 180 ohm loading detector probe at the plate of the first IF. Align the tuner link. Disconnect the loading detector and pickup the signal at TP301 with a direct probe with a 47k resistor in series. Align the rest of the IF strip. All traps were nulled with modulated markers. All this with the AGC line grounded and the tuner biased off (-15v on the tuner AGC pin). It's easy to come up with a textbook perfect IF curve.

Next I pre align the chroma takeoff coil and chroma bandpass transformers. video sweep is injected at tp301. I wont go into too much detail here, because this all goes without issue. At the demod grids you get exactly what you would expect, a rising curve to counteract the falling response of the IF.

Now we get to the problem: We have a perfect IF curve if we inject an IF sweep and pick up the signal at TP301 with a direct probe and a 47k resistor in series. But now we switch to switch to video sweep modulation. A carrier is injected at the IF frequency at the mixer grid test point. This carrier is modulated with a video sweep. The idea here is that the video detector will demodulate the signal, leaving a video sweep at the output of the video detector. This sweep will eventually be used to sweep the chroma circuits. In this way, the whole system response can be checked all the way from the mixer grid to the x & z demodulator grids.

Heres where it all goes sideways: Babcoke reccommends checking the response at TP301 before proceeding. You do this by using a non-loading video detector probe at tp301. The video detector in the set demodulates the signal, leaving the sweep. The detector probe demodulates the sweep. He says the response curve will not be exactly the same as with plain sweep, but should still fall in a straight line from 3.08mc to 4.08mc.

It does not do this. Not even close. The response is ruler-flat from 3.08mc to 4.08mc, then drops like a rock. When this is checked at the x & z demod grids, you have a huge peak at 4.08mc, as you would expect because the response coming off of the IF is flat where it should be falling.

And now the crux of the problem: When you align the IF using sweep, the video detector in the set acts as a detector probe. Everything after the video detector doesnt matter much, because the frequency of the demodulated sweep rising and falling is very low. When we inject a video carrier modulated with sweep, there is a video frequency sweep coming off of the video detector, and suddenly the frequency response of all the stuff after the video detector matters, and that stuff is peaking up 4.08mc enough to make the falling curve completely flat.

There isnt much stuff there. Basically there is a 4.5mc trap and a peaking coil, then you are at TP301, and the signal is hosed by the time it gets to TP301. You can deliberately mistune the 4.5mc trap and make it a little better, but still it is not good. I'm now wondering if the peaking coil could be the wrong value.

I should mention that if you follow the factory procedure *to the letter* it is possible to get response curves that meet spec. It is obvious that you are severely detuning the chroma coils to make this happen.

Have any of you guys ever encountered this? What did you do about it?
Should I find an inductance bridge and measure the peaking coil?

I expect to have this back up on the bench later this week. I'd love to hear any ideas.

Thanks,

John