How to troubleshoot IF strip

[old_tv_nut]

Originally Posted by miniman82

Thanks to CTC17, I have some direction now.


Here's what I did tonight:

First trace is the output from the 415 right into the scope, with markers shown by the text. I don't know why it shows up reverse of what the Sams sheets show, but it is what it is.




Next in order to check overall alignment, Sams has you inject at the mixer crystal through an 'input head' which consists of a 68-ohm resistor to ground followed by a series 1500pf cap into the crystal. I don't have that, so I made the connection directly with the 75-ohm leads and the 415 function switch on IF. Doesn't look like it's supposed to: the double humped part isn't flat topped at all, and the markers on the right hand side are not where they are supposed to be at all.



Next I set the 415 function switch to channel 4, and attached to the antenna terminals. It seems like something in the areas of 41~42Mhz isn't peaking at all, or some other strange shift in overall response has occurred. Opinions?

I found that curve with the mixer injection is extremely sensitive to loading on the CTC-5 - need to have very short leads and the correct value of coupling capacitor, even then, without a photograph of the factory recommended coupling network, your mileage may vary.

The channel 4 result looks good to me EXCEPT that the fine tuning is off. The 45.75 video carrier should be halfway down the slope on the left, and then the color frequencies will move up on the curve more on the right, and the 41.25 sound will be at the bottom of the right skirt.

[miniman82]

OK, you guys ever hear of Occam's Razor? Simple solutions are better than complex ones, but that was definitely NOT the case here!


The set had not 1, not 2, but 3 different problems so no wonder it's been kicking my ass all over town for the last few months.

As indicated by the title of this thread, I had been leaning towards a video IF problem. I therefore swapped in the tuner from the CT-100 today, to see if the problem got better. In a strange twist, it got much worse! It completely lost sync, nothing I did made it any better. Only thing I could think of was a weak or shorted sync seperator tube, and sure enough it got much better with a new tube. I put the original tuner back in and it worked pretty well, but the faint noise pattern was still creeping up the screen. That turned out to be a bad balun, I removed it and now had clean video when making a direct connection with the pattern generator.

That's the first 2 problems, the last is a slight 3.58 dot crawl. I am confident I can cure that by adjusting the trap in the luma path, so I'm gonna call this one fixed for now. These 3 different problems had me chasing my tail for a long time, no doubt they would have confused a lot of people. I'm just glad it's over now, so I can go back to enjoying this nice set.




As an aside I had also taken current readings from the different circuits during the course of this sharade, bouncing them off the CTC-2 to see if there was excessive current drain- there was none, in fact the CT-100 draws more! But the hot ballast still makes me nervous, so I'm going to build a duct to direct a stream of cooling air at it. Probably being paranoid, but cooler is always better.

[ChrisW6ATV]

Very nice color on that set!