A little Surprising, CT100 Fans

[miniman82]

Mmkay, back to the CT-100.

Today marks the third round of alignment attempts with this set, but my tenacity has finally paid off. This chassis has the most complicated IF section of any I've ever come across, having 7 total amplifier stages versus the typical 3 or 4 of a more 'evolved' set. Most people say it only has 6, but they are missing the true 1st stage which is built into the tuner. If we're only talking chassis IF stages, then 6 is technically correct.

Anyhow, you'll recall that I messed up the alignment when I first got it. Before I got the 415 my thought was to just run it as a monitor by installing a composite preamplifier, since I didn't think I'd ever be skilled enough to align a set this complex, but that darn 415 is a TV service persons wonder tool. First thing I tried to do was just see if I could look at the response curve, and make adjustments to all the slugs to make it right. Big mistake. I ended up farther off than when I started. After all normal attempts failed, including following the instructions in Sams, I resorted to the B&K manual.

In the B&K manual, it makes reference to a function called 'modulated markers'. This is used when one would like to tune a single coil or trap, such as the sound trap and so forth. What I found was that I could tune every coil in the strip one by one by using the individual markers, peaking and trapping as needed. Here's how it works: you inject the signal from the 415 at the osc injection point on the tuner, to bypass fine tuning and go directly into the strip. Then you activate one marker at a time, and basically target coils till they peak or detune as required. Example- the sound trap needs to have minimum passage at 41.25, so you turn on that marker and adjust till it nulls the signal as much as possible (the signal is a 400hz modulation that shows on the scope as a sine wave). On others you want to see maximum response, such as 42.67 and so forth. Once I had gone through all the coils twice and was satisfied I had done it correctly, I went back and looked at the overall curve and it was much improved! It was good enough that I could get back to tweaking the overall response in the normal fashion, and eventually ended up at a point where I thought it might work pretty well. The curve now closely matched what I saw in other sets, so I stuck the chassis back into the cabinet. I was again disappointed.

Now I'm beginning to think that the composite input option is looking pretty good, but I noticed that the AGC pot had almost no effect on signal strength. I had on screen what looked like classic signs of AGC trouble, such as washed out picture and terrible sync horizontally. I had initially written this off as being related to the bad IF alignment, but since I was now convinced I had the curve correct, I had to start thinking of other faults. First thought is a bad AGC pot, but dismissed that since it actually did something (not much though). Then it dawns on me- I have a tube tester, let's check the tube!

Sure as sh!t, the triode section of the 6AN8 which does duty as the AGC amp was completely dead! Wouldn't even budge the needle off the bad stop, I double checked against a new one. So in the new tube went, and viola the set sprang to life! I was now able to get respectable color bars through the antenna terminals, so I decided to hook up cable and here's what I saw:




Now I'm getting stoked! It's actually receiving signals and displaying color pretty well at this point, so the next steps are lowering horizontal drive just a tad, fixing the focus range so it's sharp, and a complete convergence setup. Right now blue is a little high in the top of the screen, but the rest seems more or less in line. I think I'll follow the procedure for convergence in Sams first, hopefully it will work and I'll be able to show some really nice pics at next update!