My CTC-16XL Project.

[old_tv_nut]

It's the triode section of V24, labeled "Chroma Reference Oscillator Control." Sorry I did not use the schematic name. It's also called a reactance tube, because it amplifies the reactance of C111, 4 pf, by the gain of the tube, due to connecting C111 from plate to grid. Any stray capacitance between the grid and the plate is also amplified in a triode, and is referred to as Miller capacitance. Anyway, the purpose of this circuit is to vary the capacitance attached to the oscillator circuit according to the control voltage developed by the phase detector. It varies the capacitance when the control voltage from the phase detector varies the gain of the triode.

The phase-locked loop works this way: the phase detector develops a voltage depending on the relative phase (and frequency, within limits) of the oscillator and burst. If the oscillator is tending to be too high in frequency, the control voltage will increase the gain of the control tube, thus increasing the capacitance loading the crystal, and reducing its frequency, and vice versa. L31 is tuned to get as close to the right frequency as possible when the detector output is zero; then it can correct either way as things drift with temperature or other factors. That is why you short point J to ground when adjusting A16 for zero beat - you are making the control voltage zero.

[Yamamaya42]

Originally Posted by old_tv_nut

It's the triode section of V24, labeled "Chroma Reference Oscillator Control." Sorry I did not use the schematic name. It's also called a reactance tube, because it amplifies the reactance of C111, 4 pf, by the gain of the tube, due to connecting C111 from plate to grid. Any stray capacitance between the grid and the plate is also amplified in a triode, and is referred to as Miller capacitance. Anyway, the purpose of this circuit is to vary the capacitance attached to the oscillator circuit according to the control voltage developed by the phase detector. It varies the capacitance when the control voltage from the phase detector varies the gain of the triode.

The phase-locked loop works this way: the phase detector develops a voltage depending on the relative phase (and frequency, within limits) of the oscillator and burst. If the oscillator is tending to be too high in frequency, the control voltage will increase the gain of the control tube, thus increasing the capacitance loading the crystal, and reducing its frequency, and vice versa. L31 is tuned to get as close to the right frequency as possible when the detector output is zero; then it can correct either way as things drift with temperature or other factors. That is why you short point J to ground when adjusting A16 for zero beat - you are making the control voltage zero.

that it what I thought, all the related caps for it are on my hit list, if it said 10% i'm getting 5% if it said 5% i'm getting 2% and so on, and making sure if is says NPO that I'm putting new NPO ones in.
and the ones with no tol listed I'm putting in 10%
a bit of an overkill perhaps, but they are cheap.

Surgery went OK, took forever, spent more time laying around waiting than anything. got there at 9am, left at 3:30.
Now I wait till I can move w/o hitting the ceiling T_T

[Yamamaya42]

List of caps ordered (mouser)
75-564R60GAQ10 10pf NPO
594-D331G47U2JH63J5R 330PF
594-S221K25Y5PP63K5R 220pf
75-561R10TCCQ82 82pf NPO
75-562R10TSS10 .01uf
75-561R10TCCV39 3.9pf NPO
810-CC45SL3JD150JYNA 15pf
594-S104Z93Z5VL83L0R .1uf
598-CD15CD070DO3F 7pf

should have them next week, used slowest shipping, no use in spending $7 for ups ground, just to have them get here sat an sit and wait, Mouser is so close to me anyway, they will prob be here Monday via USPS basic ship anyway.

prob won't put them in and test till the weekend of the 8th.

[old_tv_nut]

suggestions:

Since the ceramic disc caps are unlikely to be failures, do some more troubleshooting before all the effort of replacing everything.

Make sure you cannot go through zero beat by adjusting A31/L16 with point J grounded. If you can get zero beat, there is no problem with the oscillator.

If you can get zero beat with J grounded, unground it and observe what happens. If the oscillator frequency changes only slightly, then look at point J with your scope and a voltmeter to see if the beat frequency appears coming out of the phase detector, with an average value about zero. If you now tune A31/L16, you should see the beat frequency change on the scope. If you get very close to zero beat, the DC voltmeter will start to follow the slow voltage swings of the detector output. If you see the detector output behaving normally , the phase detector is OK, otherwise it is suspect. So, if the detector output is OK and the oscillator is OK, and you still have no lock, that points to the control (reactance tube) circuit.

[Yamamaya42]

as mentioned before, i have no idea what was done to it before I got it, every resistor in the color pcb was replaced ( 2 with wrong values ) which now is correct, there is a VERY HIGH CHANCE that who ever did that also replaced caps, this is why I'm going over it like this.

[old_tv_nut]

You're right, I wasn't considering the perversity of the previous "fixer," but if you could identify the current cap values and try replacing only the obviously wrong ones first, you might save some effort.

[Yamamaya42]

Originally Posted by old_tv_nut

suggestions:

Since the ceramic disc caps are unlikely to be failures, do some more troubleshooting before all the effort of replacing everything.

Make sure you cannot go through zero beat by adjusting A31/L16 with point J grounded. If you can get zero beat, there is no problem with the oscillator.

If you can get zero beat with J grounded, unground it and observe what happens. If the oscillator frequency changes only slightly, then look at point J with your scope and a voltmeter to see if the beat frequency appears coming out of the phase detector, with an average value about zero. If you now tune A31/L16, you should see the beat frequency change on the scope. If you get very close to zero beat, the DC voltmeter will start to follow the slow voltage swings of the detector output. If you see the detector output behaving normally , the phase detector is OK, otherwise it is suspect. So, if the detector output is OK and the oscillator is OK, and you still have no lock, that points to the control (reactance tube) circuit.

I now have the caps I ordered, not had a chance to go over things to check / replace things yet, spent most last week resting, will try this weekend, I understand MOST of what you said to check, but the term “zero beat” is new to me, and I really am not sure how to apply it to the situation at hand. T_T
(looking it up did not help much )
PLZ forgive if it seems silly to ask, but, I grew up mostly in the digital/ solid state age, tubes were already way out when I was in the Electronics class in HS, “zero beat” may have been covered, perhaps, I may have just forgotten :/

[old_tv_nut]

The rainbow stripes are at a frequency that is the difference between the color burst frequency and the color oscillator in the TV. This difference is a "beat frequency." The term comes originally from sound and musical instrument tuning.

As you tune the color oscillator, you will see the beat frequency change, visible as the number of rainbow stripes. As you get closer to zero difference between the burst and the oscillator, the number of stripes will decrease and they will get broader. If you get close to zero beat (meaning the oscillator frequency is close to the burst frequency), the phase lock loop should pull in and stabilize the color, but if the detector or control circuit aren't operating, it will be unstable, and you will see the hue of the whole image change randomly. As you continue tuning the oscillator from too high to too low (or vice-versa), the number of stripes will increase again.

So, with point J grounded, there is no control voltage, and you should be able to tune the oscillator from too low to too high (or vice-versa) with zero beat somewhere in the middle.

With point J ungrounded, if the phase detector is working, a scope will show a sine wave at the beat frequency (of a couple hundred Hz or less) at point J. This corresponds to the rainbow stripes of color phase you see in the picture, because the oscillator frequency is off from the burst frequency by this much.

I looked for a good illustration on YouTube, but couldn't find one, so I hope my verbal description is understandable.

[old_coot88]

The term "beat frequency" has its origins in acoustics. When two sound waves are of different frequency, they 'beat' against each other, producing a third (or intermediate) note which is the difference between the two. But if the two are identical in frequency and phase, there is "zero beat".

The same thing occurs with RF signals. The beat is sometimes called a heterodyne, as in superheterodyne receivers; the incoming signal 'beats' with a local oscillator, producing the intermediate frequency (or IF) signal which is then amplified and demodulated.

[Yamamaya42]

Originally Posted by old_coot88

The term "beat frequency" has its origins in acoustics. When two sound waves are of different frequency, they 'beat' against each other, producing a third (or intermediate) note which is the difference between the two. But if the two are identical in frequency and phase, there is "zero beat".

The same thing occurs with RF signals. The beat is sometimes called a heterodyne, as in superheterodyne receivers; the incoming signal 'beats' with a local oscillator, producing the intermediate frequency (or IF) signal which is then amplified and demodulated.

Now that I sort of remember from classes, :p when we were thought AM/FM stuff, but that was sooo long ago I was a dumb teen then, a lot of that info on my master hard drive was flushed long ago to make room for other data! :p

[Yamamaya42]

this won't be until the very LAST stages, but this does look perfect for a replacement the focus rect, does it not? https://www.electronicsurplus.com/nt...nium-rectifier

or am I seeing it wrong.?

[Yamamaya42]

something is not right :/

c101,102,103,104
c98,99,100 now new.
still cant do color afc.

https://imgur.com/ZL7DFjY pin 3 of v23 (removed) phase det
https://imgur.com/IsFfC3h pin 1 of v23 (removed) phase det
https://imgur.com/rb81uJO pin 1 of v23 (tube in) phase det -did not bother with pic of pin 3 with tube in.

https://imgur.com/QuIObvz point J
not sure what i should be seeing there, but I don't think that looks right.

Edit,
What you said about " the number of stripes will decrease and they will get broader. If you get close to zero beat (meaning the oscillator frequency is close to the burst frequency),"
an am not seeing this, They roll up,/down but tuning the coil for the osc, never made them decrease/ get broader, they did stop moving at one point...

which makes me suspect it's off freq. ( a guess )
so when i saw this...
https://www.ebay.com/itm/RCA-CTC-15-...YAAOSwrHtcUie2
it was too good to pass up,
it may not fix anything... or, who knows...

[old_tv_nut]

Point J is the filtered phase detector output, so what you're looking for there is the few hundred Hz (at most) sine wave indicating how far off frequency you are. this is like a fairly low audio frequency tone, so, set the scope horizontal rate much slower, say 1 or 2 milliseconds per division.

[old_tv_nut]

pins 1 and 3 of the phase detector should have the gated burst, and that seems to be what you have, so it's OK. Pin 2 should have the continuous wave at 3.58 MHz from the oscillator. The schematic says 10 volts p-p - the copy is not clear if there is any pulse waveform or beat note waveform superimposed or just pure sine wave.

Yes, it is possible the crystal is bad, but I was under the impression that when the crystal dies the oscillator stops in these sets, and you definitely have the oscillator running

[old_tv_nut]

Rolling up or down shows that you are changing the frequency, but if you can't get it to slow down to zero beat with point J grounded, there is something wrong in the oscillator (or maybe control) circuit. The caps you replaced would not be critical unless completely dead or very wrong values. Look for trouble with the cap values in the oscillator and control circuits.

C112, C118, C114, C111, C108. Due to C108, there should be no 3.58 MHz energy whatsoever at point J, only the low frequency beat note.