Today's Mystery Guest (RCA CTC-4)

[old_coot88]

Phil,
How does the old diode actually test in forward/reverse resistance measurement? Just curious.
Suspect maybe high reverse leakage(?).

[Phil Nelson]

The old diode looks open (infinite resistance both ways), using either the ohmmeter or the diode check function on my meter. Don't ask me how it was able to pass any signal at all in that condition, but perhaps I torched it during removal, and it worked in some half-baked way before.

Maybe someone can help me make sense of the demodulator adjustment procedure in the RCA manual, which I posted here:

http://antiqueradio.org/art/temp/RCA...dProcedure.jpg

Question 1. It seems like the writer got some colors jumbled:

-- Step 1 tells you to short the blue and green control grids to ground through 100K resistors.
-- Step 3 tells you to remove the 100K resistor from the red grid -- but no resistor was ever placed on that grid -- and then place it the green grid -- which already has a 100K resistor from Step 1.
-- Step 5 tells you to remove the resistors from the red and green grids.

Huh??

One way to make sense of the instructions might be to amend Step 3 to say, "Remove 100K resistor from BLUE control grid and place it between RED control grid and ground." That would leave you with resistors on the red and green grids, which allows Step 5 to make sense.

Question 2. Unfortunately, I don't have a pattern generator that can make the same color bars as their example, so I'm not sure what is meant by an instruction like, "Adjust L125 until the sixth color bar (blue) blends with the background." What color is the "background" of that pattern generator? Black? White? Other?

Adjusting L125 and T115 (top) per those instructions changes the bar hues, but the question is how to do this with my newer pattern generators that produce bars with the usual sequence of white-yellow-cyan-green-magenta-red-blue-black.

An alternate phasing adjustment procedure with oscilloscope is described on page 49-51 of the service clinic manual, but the same question applies -- what waveforms to look for when using the different (i.e., modern) color bar pattern. Perhaps if I stare at Fig. 96 on page 49 long enough, I can draw my own diagram and model waveforms . . . .

Phil Nelson

[ChrisW6ATV]

Originally Posted by Phil Nelson

Unfortunately, I don't have a pattern generator that can make the same color bars as their example, so I'm not sure what is meant by an instruction like, "Adjust L125 until the sixth color bar (blue) blends with the background." What color is the "background" of that pattern generator? Black? White? Other?

Phil-

I thought you had a B&K 1077 Analyzer. If you do, I am pretty sure it has the "gated rainbow" color bar function in it, which was the standard color-TV test pattern for years before the NTSC color-bar generators became affordable. You would run it either without a slide in the scanner or maybe a black/opaque sheet, I am not sure. The background, then, is the black that normally appears between the bars.

[old_coot88]

I b'lieve Phil does have a 1077b Analyst.
It'd be cool if he could find a nice example of its stablemate B&K 415 Sweep Generator.
http://radiomuseum.org/r/dynascan_sw...rator_415.html
Since he's already got a nice scope, he'd be all set up to tackle some alignment.

[Phil Nelson]

Yes, I have a 1077B TV Analyst. I guess I need to use it more often. I hauled it out, and the contacts on the channel selector seem to be funky. After letting the Analyst warm up and spending a LONG time messing with a newish solid-state TV, I was able to find a signal with the Analyst channel selector set partway between channels 4 and 5. After exercising the channel selector some more, a signal eventually appeared at its channel 4 position. It was still barely usable with the modern TV, and trying out the signal with the CTC-4 just produced a mess. Besides cleaning the Analyst's channel selector, it might help to clean its control pots.

I also have a model 415 sweep/marker generator, which I picked up somewhere in my travels and never really used. I suspect it would also need some TLC before it's ready for prime time.

Phil Nelson

[DaveWM]

Phil, are you back to the orig coil?

so in the final analysis did it turn out to be a bad detector diode?

[Phil Nelson]

I haven't reinstalled the original L42 coil yet. I did reinstall the original T115 quadrature transformer.

It seems possible the detector diode was always bad, but when first looking at that output way back when, I only scoped the grid of the 1st video amp, which gives a skeletal luma-like signal, even with a new diode. I should also have looked at the cathode of the video amp, where color information (including burst signal) appears. Perhaps the signal coming out of the video detector network wasn't as bad as I first assumed.

Since that time, I have also replaced dozens of resistors in the chrominance circuits, the video board, and the video IF board. Those replacements corrected a number of wrong voltages. It's tempting to look for a single "silver bullet" solution, but I suspect that the TV started out with a bunch of problems (maybe including a funky diode), which I'm still in the process of correcting.

Phil Nelson