yes I see that as a plate bypass on the band pass amp, RCA schematic not sams.

I unsoldered C204 and tested it on my EICO 950B cap checker, and it is fine. I also disconnected the line that I was using to inject a video signal, in case that was introducing noise.

With the IF reconnected and a signal coming in through the antenna, I can see rolling color bars when using a color bar pattern. I was never able to see them before with the IF connected, only the static "colorless bars." The waveform at L42 is still clean and stable, so I conclude the oscillator is definitely running.

Tomorrow is another day . . . .

Phil Nelson

Maybe there's hope for the old girl yet.

http://antiqueradio.org/art/RCACTC-4FirstColorBars.jpg

(Ignore the dark horizontal band, which is a camera artifact.)

Today I went on a resistor hunt, checking every resistor in the color signal path, starting at the point where the video signal is picked off from the IF section and working through everything but the demodulators.

I replaced a number of resistors that were out of spec. Then I powered up the set mainly to confirm that I hadn't made things worse. I also wanted to scope the outputs of the quadrature transformer to see if anything like a signal could make it out that way. After several scope photos and a few adjustments, I was surprised to see real color bars for the first time on this TV.

Yes, I know the colors in that pattern are incorrect. There's obviously a lot more work to do, but it's nice to see a little improvement. I'll finish the resistor hunt tomorrow or Saturday.

Phil Nelson

Bravo!! That must be very satisfying. That set has proven to be a tough dog and I bet you're learning a lot from it. I'm sure it won't be long now before we see a gorgeous screen shot of Dorothy :thmbsp:

-Clark

Great work Phil! (Looking for a clapping-hand smiley):guitar: What does some reglar programming look like on it at this point?

I'd center the tint, and adjust one of the chroma transformers I forget the number (one with slugs on top and bottom). I had to adjust that on mine when the color came in, but it has been a few years.

What does some reglar programming look like on it at this point?Oh, regular programming looks ghastly, with wrong colors and rolling bars, not to mention lousy focus. Here is the witch reaching for Dorothy's ruby slippers:

http://antiqueradio.org/art/RCACTC-4RubySlippersRollingColors.gif

After I finish the resistor hunt, I will do some more voltage checks and look at the color AFC alignment procedure and other adjustments.

By the way, here are the outputs from T115, the quadrature transformer. First is terminal D, which goes to the G-Y demodulator grid. Nice and regular:

http://antiqueradio.org/art/RCACTC-4T115OutputTerminalD.jpg

Here's the output at terminal F, which goes to the 6AL5 phase detector (V120) and to the R-Y demodulator grid. Similar amplitude, but not the same pretty sine wave:

http://antiqueradio.org/art/RCACTC-4T115OutputTerminalF.jpg

Phil Nelson

I'm debating where to go next. I have replaced a number of marginal resistors, without any dramatic improvements. When injecting a video signal, I am able to get stable color bars (with wrong hues) by working through the entire color AFC alignment procedure on page 22 of the RCA service clinic manual.

However, in step 3, I am not able to find a voltage peak when turning the bottom adjuster of quadrature transformer T115. The voltage increases in one direction until the adjuster hits the wall. And I am still getting only zero volts on pin 6 of the chroma reference oscillator (V124A) rather than the desired -10 volts. Likewise with adjusting T112 (bottom) in step 6: I don't find a peak -- the voltage increases in one direction until the adjuster stops.

My sense is that the color AFC system just BARELY works at all, and only with a very strong color bar signal from a generator. When I switch to normal program content, things fall apart. The colors are all wrong and the image is poorly focused.

I have a replacement transformer, but it's not identical to the original. Instead of a 1000-pf cap in parallel with each secondary coil, it has a 1500-pf cap paralleled by a 680-ohm resistor. Maybe it's a plug-in-play replacement, maybe not. Its example circuit is taken from the RCA CTC-2B, which uses an amp between the oscillator and the transformer.

I'm debating two options:

1. Just stick in the new transformer and see what happens. (Pro: maybe it'll work. Con: the replacement is not easy, and if I wreck either the old or new transformer in the process, I'm worse off than before.)

2. Breadboard the CTC-4 oscillator circuit using the new transformer. (Pro: if it works in the CTC-4 circuit without modification, then we're home free. Con: if it doesn't work, then I'm torn between modifying the new transformer or trying to rewind the old one, and if neither of those options works, then I have a completely dead TV.)

Since I don't have standalone power supplies, perhaps I could power the breadboard circuit by tapping into the CTC-4's 6.3VAC filament circuit and +285VDC B+ circuit. It needs about -10VDC bias voltage; perhaps I could tap into the CTC-4's -20VDC supply and shunt some of that to ground with a pot.

Or, I could throw a tarp over it and work on something else, but it bugs me to leave the TV in this half-baked condition.

Edit: re breadboarding, I have an additional concern, which oldcoot_88 alluded to earlier, that it might seem to work on the workbench -- that is, the oscillate might oscillate and the transformer might pass a signal -- but the output might not be right in phasing & amplitude for the CTC-4.

Phil Nelson

Phil,
-
Regarding the -10v on the grid, that's derived by self-biasing of the tube itself when it's oscillating (same way the Hor osc. self-biases). It's not derived from a separate negative supply.
So if the tube is oscillating, the neg. grid voltage should be there.
If it's not, and there's still a strong signal going out thru T115 (indicating the osc is running), this is strange indeed.. unless maybe the osc quits when you put the meter probe on the grid. What kinda meter are you using?

Okay, I see your point about the self-biasing.

I'm using a newish Fluke digital DMM. The alignment instructions specifically state that you must use a VTVM to measure the -10V at pin 6 of the oscillator. It's certainly a sensitive spot. Later in the alignment instructions, they tell you to temporarily disable the oscillator by connecting a 1-foot piece of wire to pin 6. Connecting a short piece of wire does indeed disable the oscillator, so maybe it's quitting when I try to measure that voltage.

I posted the page with the alignment procedure here for reference:

http://antiqueradio.org/art/RCACTC-4ColorAFCAlignmentProcedure.jpg

At this point, I'm more concerned about output from the transformer than whether the oscillator is running.

Step 3(a) says that a peaking response to adjusting T115 shows that the oscillator is running AND T115 is passing the signal. I am not able to find a peak, so I gather that one of those conditions is not true (for instance, maybe the oscillator is running, and T115 is passing a sort-of signal, but the signal is weak or dirty).

Step 3(b) tells you to check for negative voltages at pins 2 & 7 of the R-Y/G-Y demodulators. I find about -20V at pin 2 and a smaller negative voltage (-10 to -15V) at pin 7.

In step 4, I am able to find a peak when adjusting L125, the hue coil.

In step 6, the adjuster for T112 runs out of room before I find a peak in the voltage.

So, running through the procedure is a mixed bag. Some adjustments work, others don't. The end result is a TV that can lock colors with difficulty on a very strong injected video pattern.

Phil Nelson

Okay, here's the breadboard circuit "before" photo:

http://antiqueradio.org/art/RCACTC-4T115Breadboarding.jpg

Here is the completed circuit lashed up to my CTC-4 chassis. Judging by the scope and my handy SW radio, the breadboard circuit does oscillate around the right frequency. The first photo shows the output at terminal F of the transformer:

http://antiqueradio.org/art/RCACTC-4T115BreadboardTerminalFOutput.jpg

This photo shows the output at terminal A. Note that the new transformer's labeling is slightly different (terminals A and D are reversed).

http://antiqueradio.org/art/RCACTC-4T115BreadboardTerminalAOutput.jpg

Just as with the original transformer, one output is a simple sine wave and the other is a little more complex.

Touching my DMM's probe to pin 6 of the oscillator tube in the breadboard circuit instantly kills the oscillation, so that little mystery is cleared up.

I have to run out for some errands. Later on, I'll try to measure the amplitude of the signal coming out of this transformer. I can also adjust the slugs to see how they affect the output, and try whatever else seems sensible.

Phil Nelson

Connect a 10 meg resistor to the tip of your meter probe and touch
that to the test point. You need to check the
impedance of your multimeter: mine are each 10 megs
so the resistor cuts the voltage in half.
The isolation needs to be at the probe tip.

Might want to slip the can back on T115 and be sure it's grounded, to 'liminate as many variables as possible.

Thanks, with a 10-meg resistor on the tip, I measure -9 volts at that pin. The manual for that meter must be around somewhere; I'll see if it says anything helpful.

The can is back on that transformer. It seems to work the same either way, but there's no reason to run it bare, given how twitchy the TV still is.

I guess I'll try installing the new transformer, but before doing that, maybe I'll disconnect the old circuit and run some leads out to connect the breadboard circuit just for kicks. Who knows if it'll work correctly with so much hanging out in the breeze, but it's only three wires.

Phil Nelson

Phil,
I'm looking at your pic of parts laid out over the schematic in post# 261.
The two resistors R248 and R249 are both 47K. But the new ones both appear to be coded 4.7K (yel-violet-red instead of yel-violet-orange).
I'm looking at it on two different computers, and they show the same color rendition.
-
Is there a chance R248 and R249 coulda got replaced with 4.7K resistors instead of 47K`s?
-
Or maybe orange bands are just showing as red.
Yet R250, 1K, appears coded correctly (brn-blk-red).

They are 47K resistors. I even checked them with an ohmmeter to make sure. The lighting in my workshop is a mix of daylight and different kinds of lights -- I'm never sure how photos will turn out.

Phil Nelson

Couple of questions... The quadrature transformer is designed to produce two outputs phase shifted by 90 degrees, but shouldn't both outputs be pretty pure sine waves at the correct frequency, rather than distorted, as shown on the scope?

Is that a Y-B 400 radio?

jr

Given the construction, I don't know why the outputs shouldn't produce the same waveform. I'm not a transformer designer, though.

Yes, that's a YB 400.

Phil Nelson

Could you post some more of the schematic directly downstream of T115? Just curious what it's loading into.

Here's the whole schematic in four parts:

http://antiqueradio.org/art/RCACTC-4SchematicPart1.jpg
http://antiqueradio.org/art/RCACTC-4SchematicPart2.jpg
http://antiqueradio.org/art/RCACTC-4SchematicPart3.jpg
http://antiqueradio.org/art/RCACTC-4SchematicPart4.jpg

The bit you're looking for is in Part 3.

In a nutshell, the outputs from T115 go to the grids of the R-Y and G-Y demodulators (V125). One of them also goes to the chroma phase detector (V120).

Phil Nelson

While messing around with the breadboard circuit, it occurred to me to use the scope to look at the phase relationship of the outputs from the quadrature transformer. I was able to adjust the transformer so that they were 90 degrees out of phase, but one output was still lumpy. Perhaps not surprising, considering that the breadboard circuit was crudely built, hanging off the chassis by various foot-long leads.

I disconnected the breadboard circuit, replaced a couple more resistors around the R-Y/G-Y demodulator, and reconnected everything with the old T115 transformer in place. I also dug up a second scope probe so that I could view both outputs at once.

Whaddya know, both outputs are nice sine waves! Using the top and bottom adjusters on T115, I got them into a 90-degree phase relationship:

http://antiqueradio.org/art/RCACTC-4T115OutputsPhased1.jpg

By switching the time base to channel B, I was able to get a purty-good 90-degree circle, as described in my scope manual:

http://antiqueradio.org/art/RCACTC-4T115OutputsPhased2.jpg

http://antiqueradio.org/art/RCACTC-4T115OutputsPhased4.jpg

This was all done without reference to other circuits or to what appeared on the screen. When I looked at the screen, I saw these color bands, which can be made to stand still:

http://antiqueradio.org/art/RCACTC-4T115OutputsPhased3.jpg

Not a correct picture, but my goal here was to confirm that:

-- The chroma oscillator works.
-- The quadrature transformer passes the 3.58-MHz signal.
-- The transformer adjusters affect the phase relationship of its outputs.

If these things are indeed working correctly, I'll cross 'em off the list and look elsewhere for solutions.

Phil Nelson

Looks like the 3.58 osc is quite a bit off freq. The reactance coil adj should be able to bring it back.

Hmm, adjusting the reactance coil doesn't really improve things. The speed of the rolling horizontal bars changes as I adjust either way, but it hits the end of its travel.

I can look at it some more tomorrow.

Phil Nelson

Is the readout on that radio accurate enuff to tell if the osc is running above, or below 3.58?
The value of that 270pf cap (C213) in the resonant tank could be altered a little bit up or down, depending on which way it needs to go. 10 pf might be a good starting figure.

No, the radio is not accurate enough to give more than a ballpark idea of frequency. Plus, when I had the breadboard circuit hanging off the chassis and two oscillators (or parts of them) running in near proximity, the radio was more confusing than useful.

Today I found a cheap frequency counter on craigslist, and it showed that you were right once again. The oscillator was running at the wrong frequency, somewhat high. This is as low as I could adjust the frequency using the reactance coil:

http://antiqueradio.org/art/RCACTC-4ChromaOscillatorHigh.jpg

The ideal frequency is 3.579545, not 3.579629. I tried clipping various caps in parallel with the 270-pf cap (C213). Adding a 100-pf cap worked the best:

http://antiqueradio.org/art/RCACTC-4ChromaOscillatorLower.jpg

The system locked on the lower frequency and I got color bars again:

http://antiqueradio.org/art/RCACTC-4MoreBadColorBars.jpg

I'm back where I was a while ago, in other words. But at least I got out of the rolling-bars rut! This evening I'll have time to tack the new cap in place and do some more checking.

Thanks,

Phil Nelson

Today I found a cheap frequency counter on craigslist, .......


Phil Nelson

Nice Fordham FM-8 Counter! - I had one for almost 25 years. Paid $150 back in '86....and still wish I had it!

Cheers,

This one was $20 from a pawn shop and it has already paid for itself in fun. I was curious to see what frequency I would find in my CTC-11, which has terrific color:

http://antiqueradio.org/art/RCACTC-11ChromaOscillatorFrequency.jpg

Let's see, what else can I run around and measure with my new toy?

Phil Nelson

Let's see, what else can I run around and measure with my new toy.

Great find! Have you measured the rest of your spare 3.58 MHz Xtals?

jr

Ramsey Electronics made several frequency counter kits in the late 1970s to early 1980s, and they also had a kit called a "frequency counter calibrator". It was a 3.58 MHz oscillator that you wired into your working color TV set, and it would phase-lock to the TV set's chroma oscillator. You then tuned the set to a network TV show (they were specific about that), and then adjusted the counter to display exactly 3.579545 MHz. It was interesting-all of the networks were within 1 Hz, and some of the local stations were off by several Hz or more.

Have you measured the rest of your spare 3.58 MHz Xtals?Yes, I had checked them all before, using only the scope and signal generator as dtvmcdonald suggested way back when. I checked them again today, using the frequency counter as well, and they all appear to resonate around the correct frequency (all slightly under the ideal, but that may just be an artifact of my test setup).

Phil Nelson

While I had the chassis on the bench, I decided to take another look at the breadboard circuit. It is being powered by the CTC-4 chassis but none of its signal leads are connected. Now I can see that it's oscillating lower than the ideal frequency.

http://antiqueradio.org/art/RCACTC-4BreadboardOscillator1.jpg

As before, one output of the quadrature transformer looks irregular and the other is a clean sine wave, when they are connected to a probe one at a time:

http://antiqueradio.org/art/RCACTC-4BreadboardOscillator2.jpg

It was interesting to connect the two scope channels to the two transformer outputs. When both outputs had some loading, the irregular waveform got more regular. With some adjusting of the bottom and top slugs, I was able to bring them 90 degrees out of phase, as I did before with the TV's native circuit:

http://antiqueradio.org/art/RCACTC-4BreadboardOscillator3.jpg

http://antiqueradio.org/art/RCACTC-4BreadboardOscillator4.jpg

This little exercise gave me a better grasp of how the transformer works. The bottom adjuster affects (mostly) the amplitude of one output, while the top adjuster affects (mostly) the phase of the second relative to the first. The closer they came to quadrature, the more the irregular wave seemed to smooth out. I was also able to observe an amplitude peak while adjusting the bottom slug, something that you're supposed to observe with a voltmeter in the service manual's alignment procedure.

Tacking a 100-pf cap in parallel with C213 gave me a TV that displays stable color bars (even though all of the colors are wrong), so I guess the oscillator is running in the right ballpark.

Phil Nelson

Phil,
Since you've already done the alignment procedure per the manual, presumably the tint ('hue') control shows some evidence of working when you turn it.
-
The display looks like possibly one of the two demods is really wonky or maybe not working at all.
Assuming the hue control is working, I would try this.. using a cap of .01 - .047 or thereabouts, short out each of the demods in turn, watching to see what the tint control does while one demod is shorted.
-
At the 12BH7 demod tube V125A, short the plate (pin 1) to ground*. This kills the R-Y signal. On a normally working set, the display will show some colors missing, but with the tint control still functional.
Now short the plate of the other (G-Y) channel (pin 6). You should get a different set of colors, and the tint control should still work.
-
If either channel shows little or no color, and/or the tint control has little or no effect, I would look further into that channel for the problem.
-
Of course if both channels pass the test, then the wonky-color fault lies somewhere else.
-
*Or to B+, doesn't matter since it's signal ground.

Thanks, that gives me some things to try. I haven't redone the alignment procedure since the latest replacements (which included a couple of resistors on the demodulators).

The tint (hue) control does have an effect. Colors shift around in the bars, but they are never correct.

Edit: I still haven't done the alignment, but here are quick photos showing the results of shorting the demodulators.

1. Demodulators normal (nothing shorted). Wrong colors, of course:

http://antiqueradio.org/art/RCACTC-4BarsDemodNormal.jpg

2. Demodulator pin 1 shorted:

http://antiqueradio.org/art/RCACTC-4BarsDemodPin1Shorted.jpg

3. Demodulator pin 6 shorted:

http://antiqueradio.org/art/RCACTC-4BarsDemodPin6Shorted.jpg

The tint control affects colors in both cases.

Gotta run some errands, but I can return to it this evening.

Phil Nelson

On second thought, before doing anything else, check for proper voltages on the 6AG7 demod driver tube V123. If good, check xfmr T114 for continuity on the secondary.
If good, scope the signal coming out of terminals A and D of T114. That's the 3.58 bandpass signal before it's mixed with the 3.58 osc. signal.

Now that the oscillator is running at the correct frequency, do what I did:

grab a pair of plastic diddle sticks, put one in each core of the quad x-former. Put a mirror in front of the set, so you can see the screen while you twist the diddle sticks. Tweak both cores with a color bar pattern on the screen till you see the correct bars, and make sure the tint control is centered when you do it otherwise you'll end up with not a lot of range when you're done. If you're able to get good bars I'd call it a day, if it looses lock during the procedure zero beat the oscillator again and repeat- remember that the phase detector and local oscillator are linked together, so a change in one affects the other.

If still wonky after adjusting T115 cores for best display, see what effect adjusting T114 core has.
Also check voltage on V125 plates (pins 1 and 6). Should be about 36V.
If not correct, check resistors
-
R256 (110K),
R258 (180K)
R262 (150K)
R283 (47K),
R282 (3.3K)
R286 (3.3K)


.

I spent some time adjusting T115 with two diddle sticks while watching the color bars, but didn't reach Nirvana. Lots of colors, but no clear path to the right ones. I think there is more going on here than mere misalignment. The lock doesn't seem very strong, either. The bars were prone to start pulsing, and tweaking the reactance coil did not always cure that in an obvious way.

I checked some voltages on V123, the 6AG7 demodulator driver:

pin 4: 0 (0 expected)
pin 5: 7.1 (6.2 expected)
pin 6: 175 (230 expected)
pin 8: 248 (280 expected)

The "expected" voltages are from the RCA schematic; the Sams manual gives lower voltages for pins 5 (5.5v), 6 (200v) and 8 (250v).

T114's secondary has continuity, measuring about 4 ohms from terminals A-B.

I checked the plate voltages of V125, the 12BH7 R-Y/G-Y demodulator:

pin 1: 33v
pin 6: 16v

I tried scoping the signal coming from T113, the bandpass transformer, and following it through the demodulator driver transformer (T114). Here is the waveform at the grid (pin 4) of the 6AG7 demodulator driver:

http://antiqueradio.org/art/temp/RCACTC-4DemodDriverGrid.jpg

Here's the waveform at the plate (pin 8) of the demod driver:

http://antiqueradio.org/art/temp/RCACTC-4DemodDriverPlate.jpg

Here's the waveform at terminal A of T114, the demodulator driver transformer:

http://antiqueradio.org/art/temp/RCACTC-4DemodDriverXfmrTermA.jpg

And here's the waveform at terminal D of T114:

http://antiqueradio.org/art/temp/RCACTC-4DemodDriverXfmrTermD.jpg

Out of curiosity, I put the frequency counter at these points. For what it's worth, it measured 3.80 MHz at the plate of the demod driver tube, 3.69 MHz at terminal A of T114, and 3.74 MHz at terminal D of T114.

Next stop: checking resistors.

Phil Nelson

check resistors
R256 (110K),
R258 (180K)
R262 (150K)
R283 (47K),
R282 (3.3K)
R286 (3.3K)I checked those (in place) without seeing anything alarming.

Maybe I will break down and install the replacement quadrature transformer. But first, I want to mess around with the breadboard circuit a little more, to see how close I can get to the ideal frequency. If there's any component tweaking to be done, I'd rather do it on the breadboard.

Phil Nelson

I checked those (in place) without seeing anything alarming.
Did you clip one end loose to check them? ('Specially the high value ones. The 3.3K ones not so much.)
I checked the plate voltages of V125, the 12BH7 R-Y/G-Y demodulator:

pin 1: 33v
pin 6: 16v

The 16V is 'waay low, less than half the expected. What about the grid voltages (pins 2 and 7)? Both expect the same, about -20V.

I lifted those resistors to re-check, and didn't find anything more than 10% off spec, but I replaced them anyway. Now I have replaced most of the resistors on the video board hanging off the side of the chassis.

None of the voltages on the 12BH7 (including plate voltages) are very close to spec. These circuits have a bunch of 2-watt resistors connected to the +380V line. I don't keep many 2-watt resistors on hand, so I'm off to the local store to restock.

Phil Nelson

Holy !@#$%^&. I didn't see those. Are those the three paralleled 82Ks feeding the pin 1 plate? Your voltage reading there was near spec.

And I didn't see R259 (120K) feeding pin 6 plate, which is 'waay low.

And don't forget the two paralleled 82Ks on pin 7 of V124B (B-Y amp). Kind of ugly construction in that neighborhood. Maybe they got a good deal on 82K/2W resistors.

Phil Nelson

Yeah, and considering that 2W carbons are know to char and drift downward, it'll be interesting to see what those actually read.

I continued the long resistor hunt, and did find some bad ones. This pair of 82K/2W resistors should measure 41K, not 122K:

http://antiqueradio.org/art/temp/RCACTC-4R295R296.jpg

Most of the other 2W resistors look like a different brand -- the usual dark colors -- and they looked pretty good, but many got replaced anyway.

After those replacements, the voltages on the 12BH7 demodulator looked closer to normal. The voltages on the grids reflect the amplitude of the signals coming from T115, the 3.58-MHz transformer, so when you adjust T115 in the course of alignment, they also change.

At that point, I had basically the same issue -- colors in the color bar pattern were incorrect, and no amount of adjustment made them right.

The replacement 3.58-HMz transformer had produced reasonable-looking waveforms in the breadboard circuit, so I decided to pull out the old transformer and try it.

Comparing the transformers side by side renewed my doubts. The new transformer has a lot more windings, closer together. Remember, it also has 680-ohm resistors in parallel with each capacitor, and the capacitors are 1500 pf, not 1200 pf as in the original.

http://antiqueradio.org/art/temp/RCACTC-4TwoQuadratureTransformers.jpg

The new transformer fits physically, but I had to reroute a couple of connections, since its output to the phase detector is terminal D rather than F, etc.

http://antiqueradio.org/art/temp/RCACTC-4NewQuadratureTransformerInstalled.jpg

(After installing it, I did reattach that hanging blue wire, seen in a previous photo running across the installation site.)

The bad news is that it doesn't work -- not even close. I ran through the color AFC alignment procedure and tried other cowboy adjustments while watching the screen, but one of the waveforms coming out of the transformer is very irregular and the two signals are badly mismatched in amplitude. Perhaps it is expecting an input signal level very different than what this TV provides. What I see on the screen is a very washed-out image of "colorless bars," with very faint indications of rolling bars (or whatever).

After double- and triple-checking everything on and around that transformer, I can't find anything wrong apart from this simply being an unsuitable part. I think I will pull it out and go back to the original. If the windings on this transformer looked more like the original ones, I might try removing the 680-ohm resistors and substituting 1200-pf caps for its 1500-pf ones. But that might just be time wasted.

Phil Nelson

Just a shot in the dark, but did you inspect lead dress carefully for any wires shorting? Looks like possibly a melt-thru of the insulating sleeve on that resistor going to lug #2 of the tube socket.

It's not beautiful, but I have checked it all carefully and there are no shorts or wiring errors.

Phil Nelson

I think your most telling post came around #134. In that post you put up
screen shots of the O'scope of the input of the video amplifier. This was also
smartly compared to a different set, a ctc 11, not that it matters.
What it does say, is that your scope, scope probe, are not distorting, or
un-necessarily loading the circuits.

The scope image also says to me that there is serious overshoots on vertical
rise items in the circuit. This can cause ringing, which could also be causing
harmonics, and other distortions around the burst signal as well.....
You should fix this first, I think it's giving you trouble down the line...

I think your problem is not in the color osc circuit, but before it..... You need
to figure out why your video detector circuit is allowing much higher
frequency response (is what I believe is going on) This could be an open
coil, or open cap in and around the Video IF and video detector area...

PS You already showed that your color osc works on yer breadboard
thing. Feed that osc a buncha noise and I bet it quits, or looks like
it quits. Your freq. counter acts like it does when noise is present
too, locking on different frequencies.....
It would be neat if'n you had a spectrum analyzer to see what's
getting to the vid. amp and ocs. grid.

just my 2 nuts worth....
(not those nuts)

.

Thanks, I am open to all suggestions at this point.

After I saw the signal coming out of the video IF section, I disconnected that output and injected a video signal from my pattern generator. I'm injecting it at the grid (pin 2-9) of the 1st video amplifier, V110. The idea is to get the chroma circuits working first, and worry about the video IF later.

I don't have a spectrum analyzer, unfortunately.

At this point I'm thinking you're correct, that the oscillator and color lock are working reasonably well. As I understand the lock system, the local oscillator puts out its 3.58-MHz signal and the phase detector compares that frequency to the incoming 3.58-MHz signal. If they are different, the reactance tube pulls the oscillator up or down to match the received frequency.

Here's a little test that I tried after getting my frequency counter. The counter showed different frequencies (at the oscillator) when I used different sources for the injected video input. I tried this with two pattern generators and a DVD player. I assumed this was because the 3.58-MHz component from each of those sources was a little different, and the CTC-4 was able to lock on those slightly different frequencies.

Then I connected those different sources to my CTC-11 and compared what I had measured at the CTC-4. For instance, here's my Leader pattern generator feeding a signal to the CTC-4, which appears to lock onto 3.579673:

http://antiqueradio.org/art/temp/RCACTC-4FrequencyFromLeader.jpg

And here's the same generator feeding a signal to the CTC-11, which locks onto 3.579679:

http://antiqueradio.org/art/temp/RCACTC-11FrequencyFromLeader.jpg

That is the same frequency out to five decimal places. The results were similar when I compared other sources with both TVs.

This suggests that my CTC-4's oscillator is running and that it can lock on the same incoming frequencies as my CTC-11 (which has great color).

Which leads me to think that its problems lie elsewhere -- maybe in the demodulator circuits downstream, or, as you say, somewhere upstream, where the incoming signal is getting attenuated or mutilated. The chroma circuits seem to work best with the strongest injected signal that I can provide. The schematic says that the CTC-4 expects +2.7V at the grid of the 1st video amp, and none of my equipment can provide such a strong video signal. When people want to inject video on a CT-100 or CTC-2B, it's necessary to provide a little video preamp.

Anyway, I'm open to fresh ideas. Yesterday I reinstalled the original quadrature transformer, which works just as it did before.

Regards,

Phil Nelson

Yah, from what I read, I don't think it was in that transformer.....
Bad looking signal at post 134 means the problem is not after it,
it's before it..... Trying to work on down stream stuff is really pointless
if the upstream circuits are feeding you trash....
That's just how I see it.......

.

Another shot in the dark, Phil. Have you tried subbing the vid detector diode itself?

I am injecting the video signal downstream of the video detector diode -- at the grid of the 1st video amplifier.

Phil Nelson

go back more.... detector and before.... Compare what you get with your reference ctc11....
You got a signal source of any kind you need - pick it out of the 11 and feed it to your 4.

That's a low pass network after the diode, look at that buncha coils, are they checking
reasonable ohms, etc..... Check all that stuff around that diode, back to the tube too....
Voltages in those stages ok....? Compare scope readings to the 11......
That big screen grid cap ok....?

.

I am injecting the video signal downstream of the video detector diode -- at the grid of the 1st video amplifier.
Understood. But the reason for asking was: since you're injecting at the grid, and the grid is DC-coupled back to the diode (i.e., no capacitive isolation), a shorted or partially shorted condition in the diode could be lugging down the genny's signal. Like I say, it's a shot in the dark. But easy to test by just lifting one end of the diode.

go back more.... detector and before.... Compare what you get with your reference ctc11....
You got a signal source of any kind you need - pick it out of the 11 and feed it to your 4.

Good point. Just be sure the two chasses are grounded securely together, and monitor the display on the good set at all times.

Have you swapped out the tubes for the video IF stages,
and the first video amp...? With known good ones....

Have you looked at the signal going into the first video amp
with the tube out...? At the grid....

I can't imagine enough room to set up 2 chassis and make
checks on them and go on for weeks at a time......
Divorce must be that thing over in the corner that
never seems to feel right..... Is it growing....
que - twilight zone music......

.

how about just driving with a solid color screen see if you can scope the grid and plates of the Red and Green demod tubes.

Will the genny he's using make solid-color blank displays?

test by just lifting one end of the diode.Well, that led me stumbling down a path that eventually led to a real color picture. After I lifted one end of the diode, I checked out the output network downstream of the video detector (which all looked OK). I experimented with injecting the video at a couple of spots other than the video amp grid, without getting anywhere. I was really tired of messing with injected video at that point, so I decided to replace the old diode and send a signal in through the antenna, just to see how bad things really were.

It didn't occur to me to test the diode until after I removed it. Maybe it has been bad all along, or maybe the heat of removal sent it south. It's definitely bad now, anyhow. I wonder if this 1N64 was a replacement, since the schematic calls for a 1N60:

http://antiqueradio.org/art/temp/RCACTC-4VideoDetectorDiode.jpg

I replaced it with a 1N34A, since that's what I happened to have on hand, and boom -- there was a color picture. Not a correct color picture, but better than I had seen before.

I noticed a persistent vertical jitter that hadn't been obvious when looking at color bars, and spent a long time getting rid of that. Replacing resistors on the 2nd sync amp as well as the vertical circuits finally took care of it.

Some colors are still wrong, and there's a lot more work to do, but at least you can recognize pictures of humans, with some colors in the right spots.

http://antiqueradio.org/art/temp/RCACTC-4DorothyFirst.jpg

Tomorrow I'll try the color AFC alignment once again, put the scope on some interesting places, and go from there. The resistor hunt is not over -- it has turned up a lot of bad resistors in this set, and I suspect there are more to find.

Thanks again for the advice & encouragement.

Phil Nelson

Huzzah!

Well that's an improvement.............
A nice rich hi contrast picture....
cool.......
Alright... Next tv......

.

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

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/RCACTC-4ColorDemodProcedure.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 (antiqueradio.org/art/RCA CTC-4 Television Service Manual.pdf), 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

Well if the diode was open, then removing the new one from the circuit
should produce the symptoms it had before...... There are other
components that most likely would have passed what you were seeing....
Try removing it, see what you get....? Whaddya got to loose....?
It must have worked a little.....
Scope it with some high frequency waves, see if it waffles at some point....

.

If t'were me (which it ain't:o), I would go ahead and get some good red purity on it, set the grey scale, and see what a regular vid with known color content actually looks like.
Then if need be, try some eyeball tweeking of a few of those adjustments (with the exception of the reactance adj, since color sync is already good.

Others' opinions may vary.

After more eyeballing and tweaking, here's some slightly improved color bars:

http://antiqueradio.org/art/temp/RCACTC-4BetterBars.jpg

As usual, my camera isn't very accurate. The bar that looks white is actually quite yellow, and the dark one between green and red is a dark magenta. I see now from the leftmost bar that the should-be white background was actually a little green. Yes, I should go back and do purity & gray scale, and then return to this level of tweaking.

Here's what a DVD scene looks like:

http://antiqueradio.org/art/temp/RCACTC-4DorothySecond.jpg

Again, the real screen is warmer and less washed out than the photo. You can see from the PAUSE letters that edge convergence needs attention, too.

The color sync is still not as robust as it should be. If you fine-tune a little bit off-signal, or even move the Hue control too far at the wrong time, faint rolling bars start to reappear. I'm tempted to reinstall the original L42 inductor on the oscillator. It looked scorched when I pulled it out, but it worked as well as the new ones in the breadboard circuit. That whole area needs to be neatened up, anyway.

The audio's not bad. Now that the rest is sort-of working, I can give that alignment a try. Still more stuff for the to-do list!

Phil Nelson

I'm going to make a stab at your problem based of my CT-100 experience:

Bad, very bad, IF alignment and AGC setting. The symptom is that black
magenta bar. Of course, a wrong type diode might do it.
Try viewing 50% or 25% saturated color bars, they won't show as much
problem with the magenta bar.

In any case, I would do really careful, really full IF and RF alignment next.

Yeah, get the proper diode in there first. There used to be some conjecture about the 1N34 at higher frequencies.

I ordered a pack of 1N60 diodes, and after ordering learned they will not arrive until June 11. I see that a local store has NTE110A diodes. I wonder if that is an appropriate sub?

http://www.nteinc.com/specs/100to199/pdf/nte110a.pdf

Phil Nelson

This listing indicates it's equivalent to the 1N60..
http://www.vetco.net/catalog/product_info.php?products_id=1729

Try viewing 50% or 25% saturated color bars, they won't show as much problem with the magenta bar. In any case, I would do really careful, really full IF and RF alignment next.Doing a full alignment ain't gonna happen soon, at least not by me, but you were right, simply turning down the chroma level on my pattern generator made the magenta more obvious in my crummy photos.

http://antiqueradio.org/art/RCACTC-4BetterMagenta.jpg

I plunked down 71 cents at the local store and installed a 1N60 diode in place of the 1N34A. Can't say that I noticed much difference. This scope trace from pin 2 of the 1st video amp looked identical before and after the diode transplant. (The top trace is the output from my video detector and the bottom trace shows the color bar pattern direct from the generator).

http://antiqueradio.org/art/RCACTC-4VidAmpPin21N60Diode.jpg

'That still looks more like a luma stairstep signal than a color signal," I thought, but as Username1 recently pointed out, there is more than one output path from the video detector (forehead slap). I put the scope on pin 4 (cathode) of the 1st video amp, and finally saw the kind of signal I had been expecting to see way back when:

http://antiqueradio.org/art/RCACTC-4VidAmpPin41N60Diode.jpg

That looks like a burst signal as well as chroma info. When you fine-tune the signal in and out on the tuner, the upper trace gets fatter & skinnier and the screen colors come and go.

I still haven't done the ground-up color setup procedures, but simply trying different sources and exercising the controls (including AGC) makes me think that I can eventually make this a watchable television.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Oh yeah. You've come a long way with this. It'll be great to see it properly calibrated.

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.

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_sweepmarker_generator_415.html
Since he's already got a nice scope, he'd be all set up to tackle some alignment.


:)

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

Phil, are you back to the orig coil?

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

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

If you have a Sencore VA62, it also puts out the color bar pattern.

Dave

Phil,
Have you checked/replaced any resistors in the IF strip yet? If not, I would start with the screen grid and cathode resistors of the three IF stages. And then check the screengrid & plate voltages of all three stages.
If all's good, then scope the screen grid of each stage. They should all be flatline. Any hash or signal of any kind on them would indicate a bypass cap problem.

That reminds me, I need to fix the color bars on my VA62. It makes beautiful B/W patterns, but the colors roll. Just call this the house of half-baked equipment :( .

In the IF section, I have replaced R115, R116, R118, R119, R120, R122, R123. That was in the context of working on AGC, and I see that it did not include the IF cathode resistors. It can't hurt to check out that section some more.

Phil Nelson

If, in checking voltages, you find any seriously out of whack on plate & screengrid of 1st and 2nd IFs, there's two honkin' 10K resistors in parallel (R320, R321). Since this pair is a B+ dropper, they could sure mess with those voltages.

That reminds me, I need to fix the color bars on my VA62. It makes beautiful B/W patterns, but the colors roll.
Phil Nelson

You might try turning the VA62 color bar frequency adjusting capacitor screw circled in this picture. I had to adjust it on mine to get the right colors in the bars. Remove the four case bottom screws, the four rear case screws and remove the bottom cover and case cover. Remove the two screws on the top of the control panel inside the case. The top will swing backward on a hinge like shown in the picture. Its worth a try...

Dave

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

any updates Phil, last time looked like you were getting close.

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

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. :D

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.

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.

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/VideoAdapterForRCACT-100Television.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

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.

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.

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.

I agree with this as a first place to check..... If you have a tuner subber use it,
or since you have the ctc 11 sitting there with nothing to do...... Take off the
rca plug from the tuner into the if strip, parallel it to feed both sets.... That way
if running 2 IF loads diminishes the signal you will see it on the 11 also at the
same time..... And it keeps the agc intact for the tuner.....

Double check the long feed line by running it to the 11 first to be sure it is not
getting any loss or noise etc.....

Remember to check grounds..... No spark shows.....

.

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/VideoAdapterForRCACT-100Television.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

see if you can locate the mixer plate adjustment. It may be a slug or a trimmer.The schematic shows trimmers and adjustable coils connected to pin3 of the 6X8 oscillator/mixer:

http://antiqueradio.org/art/temp/RCACTC-4Mixer-OscillatorSchematic.jpg

Some (maybe all) of those three trimmers may be accessible through holes in the tuner cage. I haven't found a diagram that shows all of that clearly. The Sams schematic does not show the adjustable coils (L42, L56)at all, and my photos of the tuner taken while cleaning don't show the internals clearly enough to guess whether they exist in my set.

Take off the rca plug from the tuner into the if strip, parallel it to feed both setsI was ready to run out and try this, when I remembered that the CTC-11 cable unplugs, but the CTC-4 cable is soldered on. I'm debating whether I want to unsolder it yet another time. It has already come off twice -- once for cleaning the tuner and again to lubricate the fine tuner when it got sticky.

Phil Nelson

Phil,
There is no mixer plate coil as there is in later tuners. Please scratch that whole idea. I should not have brought it up. Please don't mess with any of the trimmers or slugs, as they are strictly oscillator related.

While I have not worked on a ctc4, I have looked back at some of my books, and
they do mention color troubles with the bars not looking right on just some colors
like what you have..... So as mentioned before the tuner, and agc could be possible
sources of this trouble...... Subbing the tuner in some way quickly crosses that off,
checking on the agc specific to that set's setup procedure for agc may quickly
eliminate that.... These should definitely be looked at before any alignment is
attempted, because it would actually then end up being a de-alignment, and you
would be adding to your troubles...

Check non alignment trouble sources first....

.

When I look at you scope trace from the video amp cathode, it looks
awfully correct. RF-IF alignment may be causing problems at the color edges of
the bars, but across their center they should look quite good, both color and intensity.
Especially the magenta bar.

I would be scoping the video and color demodulator circuits very carefully.
Because the circuits are so very idiosyncratic, you many eventually need
to get scope traces from a similar generator on a working CTC-4. When my CT-100
had similar troubles, especially the magenta bar, scope traces in the color section
were clearly wrong. But ... the Ct-100 color section is a piece of caketo understand,
compared to yours.

In any event, scope everywhere after the video detector and post the traces,
so everybody can look at them and ponder them.

Another idea: Spend $800, buy the Hallicrafters CTC4 clone McVoy just listed on ebay, spend untold hours working on it, then compare waveforms!

Then you'd have a CTC-4 and a real rarity.

Another idea: Spend $800, buy the Hallicrafters CTC4 clone McVoy just listed on ebay, spend untold hours working on it, then compare waveforms!

Then you'd have a CTC-4 and a real rarity.

Yah.... You could do that....... Proly the best way to figure this one out....

.

I hate to cry Uncle, but I don't think I can take this set any farther without getting it aligned. I contacted the local collector club and asked if they know anyone in the Pacific Northwest who would try that if I delivered the chassis (or the entire TV), but got no response.

When I move the fine tuner, there is a very broad area in the middle of its travel where you bring in a fine B/W picture. You can only get color (true color, if it's a good day, or rolling bars, if it's not) at the opposing extremes of the tuner's range. This occurs on different channels, not only one.

On other sets, I have bypassed the tuner/IF and injected video and audio directly, but injection is not such a simple matter in a CTC-4, if I understood Pete's earlier remarks correctly. I guess I'll wheel this guy out of the shop and work on something else for a while.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Too bad Bob Andersen isn't closer. :D

http://videokarma.org/attachment.php?attachmentid=168681

On other sets, I have bypassed the tuner/IF and injected video and audio directly, but injection is not such a simple matter in a CTC-4, if I understood Pete's earlier remarks correctly.

Phil, the original 1956 RCA construction article for composite video inputs also included two circuits for the CTC4. The second might be the more useful as the other requires a 75-ohm composite signal. The second can be driven by typical consumer equipment composite video outputs, the yellow RCA connectors. The link (notice there are more inductors and caps in this-made-for-the-CTC4 design to develop the requisite curves):

http://antiqueradio.org/art/BROADCAST_NEWS_0656_4.jpg

Pete

Phil, the original 1956 RCA construction article for composite video inputs also included two circuits for the CTC4. The second might be the more useful as the other requires a 75-ohm composite signal. The second can be driven by typical consumer equipment composite video outputs, the yellow RCA connectors. The link (notice there are more inductors and caps in this-made-for-the-CTC4 design to develop the requisite curves):

http://antiqueradio.org/art/BROADCAST_NEWS_0656_4.jpg

Pete

Now this is a tempting solution to my alignment troubles (which are at least as bad as Phil's)! I just found what I think is the correct tuning knob, and cleaned my bench enough to fit it so this may be god telling me to get back to my CTC-4...

Well, that seems worth a try. At least it would be easier to evaluate the chroma circuits if I knew I was working with a clean input.

Thanks for the tip!

Phil Nelson

Phil, I think the solution is to learn to do alignments. Its not exactly easy,
but you can do it.

Just one hint: before you do anything else, get a respectable set of
calipers, good to a few thousandths of an inch. Measure and write down
the position of each adjustment. For thinngs that are inside, like hollow cores,
stick a plastic rod into the can and measure how far it sticks out.

That way you can always restore to where it is now.

Get a generator and scope and do it.

A CTC-4 is easy ... except traps, which are set and forget, it has only 5
adjustments for the IF, one per can.

In the second 21CT-660 preamp schematic from the Broadcast News article, no values are given for the coils L1, L2, L4 and their associated capacitors.

A note says:

L1C1, L2C2, L4C3 -- RCA stock no. 75251

Any guesses as to these values? I don't have references for those part numbers. Presumably each of these was a coil with a cap wired in parallel.

Regarding L3, the note identifies it as "North Hills coil no. 120 I set at 400 mu-h," so I suppose I can use an adjustable coil around that range.

Regards,

Phil Nelson

My Merit cross reference shows that the RCA 75251 can be replaced by a Merit TV-151, 4.5MHZ sound trap. It gives no specs though for the part. There are probably Thordarson and Miller replacements as well. Hope this helps.

...I think the solution is to learn to do alignments. A CTC-4 is easy.... (1) I can't disagree; it's an important skill. But I also think divide-and-conquer troubleshooting is an important tool. Eliminating the RF-IF and detector in this case would neutralize many possible failure points.

(2) In defense of the adapter, before building my first adapter for a CT-100, I reasoned that it was not distorting history because it was RCA who, at the infancy of color television, designed built, and used them.

(3) Phil, I do not have the skill set needed to mathematically determine the missing LC values from CTC4 alignment procedures and the circuit, but I'm hopeful it can be done.

(4) Thanks for the info Tim.

Pete

I found a source who has two of the three 4.5-MHz sound traps that I would need to build this preamp. In the schematic (below) they are L1C1, L2C2, L4C3.

I am wondering about substituting a modern component for coil L3, which is described as:

"North Hills coil no. 120 I set at 400 mu-h"

I see variable inductors at sources like mouser.com, but I'm not sure what (if anything) might work.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

http://antiqueradio.org/art/temp/RCACTC-4VideoPreampSchematic.jpg

The problem is that you don't know if modern or even NOS specified
replacenemt parts are in fact exactly equivalent. In my CTC-100 restoration
I found that for the notorious white coils they were not.
Do you have a scope and sweep generator? If so, you should
play with various parts to get a good result. This will result in you
learning how to sweep and play with parts in a non-stressful
situation.

You are unlikely to find an adjustable 400uH coil in a modern
catalog. Try Playthings of the Past and look in old Miller or Meissner
catalogs to see if they have what you need. A Miller 6195 will work.

Miller 6195 -- thanks for the tip. PTOP is the outfit that has 4.5-MHz traps in stock. They also have the Miller 6195 coil and they suggested another that might work, so I ordered them both.

Phil Nelson

Miller 6195 -- thanks for the tip. PTOP is the outfit that has 4.5-MHz traps in stock. They also have the Miller 6195 coil and they suggested another that might work, so I ordered them both.

Phil Nelson

Phil, could you please measure the L and C of the traps when you get them?

I have to comment that I can't think of a good reason to include 4.5 MHz traps in a video input circuit, unless the input was from a separate RF receiver. Why would anyone deliberately limit the bandwidth?

Edit: I can think of one reason - to create the same awful chroma delay and quadrature distortion that an IF amplifier produces, so that it matches the delay lines in the TV chassis.

I have to comment that I can't think of a good reason to include 4.5 MHz traps in a video input circuit, unless the input was from a separate RF receiver. Why would anyone deliberately limit the bandwidth?

Edit: I can think of one reason - to create the same awful chroma delay and quadrature distortion that an IF amplifier produces, so that it matches the delay lines in the TV chassis.

Wayne, I figured it was their cheap and dirty way of matching the 20-dB down slope of the CTC4 i.f. (I've never worked on a CTC4 and only am going by how I interpret what the 1956 article mentioned about the i.f. response.)

Pete

Phil, could you please measure the L and C of the traps when you get them?Sure I can measure. The parts are supposed to arrive in a few days.

Phil Nelson

Okay, here are the key video preamp parts. From left to right, we have three Miller TV-151 4.5-MHz sound traps, a Miller 6195, and a Miller 4566:

http://antiqueradio.org/art/RCACTC-4VideoPreampCoils.jpg

There are minor differences among the three TV-151s, but presumably they work the same. The last two are adjustable across the target value of 400 muh given for L3 in the schematic; I'm hoping that one or the other will work.

Tomorrow I'll gather the other components and start building.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Phil, you are a "trooper". I am impressed by the tenacity you put into your projects-and the results you get, which are well-deserved. Your CTC-4 is going to be a fine performer at some point.

Thanks for the encouragement. The jury's still out on this set, but at least I haven't run out of things to try.

Phil Nelson

Sure I can measure. The parts are supposed to arrive in a few days.Here's the data sheet for the 4.5-MHz trap:

http://antiqueradio.org/art/MeritTV-151DataSheet.jpg

The sheet identifies that as a 47-mmf cap. Two of my traps have a 47-mmf cap and the third has a 45-mmf cap. They all measure 2.2 ohms across the coil:

http://antiqueradio.org/art/RCACTC-4VideoPreamp4.5MHzTrap.jpg

I put one of them on a scope with a 4.5-MHz signal and was able to peak the amplitude by turning the adjuster. I don't know if that's where you want the adjustment to be in the preamp, but I was curious.

http://antiqueradio.org/art/RCACTC-4VideoPreamp4.5MHzTrapAdjusting.jpg

Haven't done much beyond that inspection, except to collect the needed parts. Perhaps I will create a new thread about building the preamp.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Back to the video preamp plan, with a question about the heater connection. I posted the original article at:

http://antiqueradio.org/art/RCA_Broadcast_News_June_1956_Video_Preamps.pdf

The plan I'd like to build is on the last page of the article, which says, "The heater connection is made to pin 4 of the adaptor socket."

The heater connection is to service the 12AT7 tube on the preamp. Here's the 12AT7 basing diagram, showing that its heater is center-tapped at pin 9.

http://antiqueradio.org/art/12AT7TubeBasingDiagram.jpg

The article tells you to connect the heater connection to pin 4 of the adaptor socket. That's clear, but I want to make sure I understand the way they drew the heater connections in the preamp schematic:

http://antiqueradio.org/art/RCACTC-4VideoPreampHeater.jpg

The drawing seems to show pin 4 of the adaptor connecting to pin 9 (the center tap) of the 12AT7. And pin 5 appears to be jumpered to pin 4.

Or am I reading the schematic incorrectly?

(The ground connection for the heater is made through pin 5 of the 6CL6 1st video amp tube, whose socket the adaptor plugs into.)

Phil Nelson

The article tells you to connect the heater connection to pin 4 of the adaptor socket. That's clear, but I want to make sure I understand the way they drew the heater connections in the preamp schematic:

http://antiqueradio.org/art/RCACTC-4VideoPreampHeater.jpg

The drawing seems to show pin 4 of the adaptor connecting to pin 9 (the center tap) of the 12AT7. And pin 5 appears to be jumpered to pin 4.

Or am I reading the schematic incorrectly?

(The ground connection for the heater is made through pin 5 of the 6CL6 1st video amp tube, whose socket the adaptor plugs into.

Looks ok to me... remember that the 12AT7 actually has two 6.3V heaters (one for each triode). They are wired in series between pins 4 and 5. If you used just those two pins, you would need to apply 12.6 V to light up the heaters. Pin 9 is the center tap between the two heaters... by connecting pins 4 and 5 together and applying voltage to that connection and pin 9, only 6.3 V is required, as the two heaters are then wired in parallel.
What could be missing on the schematic is the connection from ground of the TV to the ground of the added preamp... IMHO, the wire from pin 5 of the 6CL6 socket should also be connected to ground(s) of the preamp.

jr

Yup, the whole family of 12V twin triodes (12AT7, 'AU7 'AX7, 'AY7, 'AZ7 etc) have the centertap for 6V heater operation.

OK, after pushing my CTC-4 project aside for a long winter's nap, I hauled it back out.

Last year, I had decided to build a video preamp in hopes of injecting a usable video signal at the 1st video amp, so that I could bypass the set's RF/IF stages and debug my obvious color problem in a "clean" environment.

Long story short -- Yes, I did eventually build a video preamp; No, it didn't work as expected (owing to the inevitable leetle wiring mistake by me); and then I got disgusted and worked on other projects.

Recently, I got a helpful email from ChrisW6ATV, who asked whether I had ever tried injecting a video signal from my Sencore VA62A, using the Drive Output jacks at the lower right of the front panel. Chris mentioned that you can set the Drive Range level to 3VPP and basically accomplish what I wanted to build a video preamp for (that is, inject a somewhat stronger-than-standard video signal). He reported getting good color on his CTC-4 when injecting video in this manner.

Duh!

So, here we go. The VA62A makes good color bars, using the 10-bar pattern referenced in the RCA service manual ( http://antiqueradio.org/art/RCA CTC-4 Television Service Manual.pdf ):

http://antiqueradio.org/art/RCACTC-4SencoreVA62ColorBars.jpg

Here is that video signal scoped at the injection point (terminal J on the IF board; the junction of L105/R126/R324 in the RCA schematic):

http://antiqueradio.org/art/RCACTC-4SencoreVA62ColorBarsScoped.jpg

Here's the screen image when that signal is injected (after tweaking L127, reactance coil, to slow down the rolling enough for a photo). Yes, we have seen this barberpole effect many times before:

http://antiqueradio.org/art/RCACTC-4SencoreVA62ColorBarsDisplayed.jpg

Chris noted that you can pipe an external video source through the VA62 using its video jack in the rear. To try that out, I set a second generator to display NTSC color bars and sent that signal through the VA62:

http://antiqueradio.org/art/RCACTC-4SencoreVA62NTSCColorBarsDisplayed.jpg

Of course, you can also inject a DVD video signal through the VA62 in that manner, but I'll spare you any more ghastly-looking scenes from the Wizard of Oz. Trust me, they look awful.

Now that I can inject a known-good video signal at the right level, I'll back up and:

-- Run through color setup procedures as best I can
-- Check voltages throughout the color sections
-- Scope signals throughout the color sections

These things have been done (or sort-of done) before, but now that I can inject a nice strong 10-bar color signal, it should be possible to work through the demodulator phase adjustment procedure on page 49 of the RCA service manual. Other parts of the manual that reference that pattern may also make more sense.

As always, I'm open to other ideas. And thanks again to Chris for this bit of advice.

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

Phil,

Are you connecting the Sencore from terminal J to ground? The detector circuit output is across J and L. I looked at the CTC4 schematic and the connection of the generator from J to ground may not work because L is floating. Note that L is the sync separator feed. L also attaches to the cathode of the first video stage. V110 is a cathode follower.
The first cathode follower video stage does not invert the signal nor does it provide any voltage gain. It only provides an impedance match to the delay line.
So why not try and attach the generator from L to ground and see if that works? (You might want to DC isolate the connection to L with a 2uF 50v electrolytic with "+" to the cathode of v110).

Cheers,

Terry

So why not try and attach the generator from L to ground and see if that works?I tried connecting the generator from L to ground and the picture became completely unstable: horizontal wouldn't really lock, and I couldn't tell whether vertical locked or not. When I connect to J, the image is stable apart from rolling colors.

(You might want to DC isolate the connection to L with a 2uF 50v electrolytic with "+" to the cathode of v110).Not sure what this means. Can you explain what is connected to what?

Phil Nelson

And thanks again to Chris for this bit of advice.
Phil, after all of the great advice and tips I have learned from many of your restoration pages, I am glad to be able to offer even a little tidbit back in return.

Phil, Are you connecting the Sencore from terminal J to ground?
Terry, I also connected my VA62's video between point J on the RCA CTC-4 schematic and ground, and in my case, I got near-perfect video right from the start. The signal was, if I remember right, inverted video at about 3 volts peak-to-peak. Feeding it at point J seems to also put the needed signal to point L, through the nearby components. I also tried between point L and ground and did not get a usable signal; I did not try between points J and L (but did not need to, as noted above).

Okay Phil you've been invaluable to my own CTC-4 restoration SO let me tell you some things you likely already know I've learned in case they are of help.

It's important to remember the CTC-4 chassis is one of the only color sets ever made where the local oscillator *must* be running for correct black and white setup. RCA's Color TV Troubleshooting Guide (by John R Meagher, chief engineer of the RCA service division, 1964, RCA Publishing) specifically mentions unlike practically every other set ever made the cathode bias balance of the three guns will be upset unless they are fed a 3.579545 MHz signal even during black and white setup because of the high level demodulation almost never used elsewhere. And indeed in my set I saw this to be true. I was glad I had the book because no one else had this information and assured me what I was seeing was impossible.

John H.