RCA 21" triage CTC20 CTC16x CTC12

[DavGoodlin]

Originally Posted by Yamamaya42

I had to replace the 40uf cap (C1c) in mine, before AGC/SYNC would behave.

I will try replacing that one on the AGC control, then check ESR on all the 'lytics in-circuit to be thorough.

I understand that multi-section lytics can present odd issues such as high impedance in the common ground, resulting in coupling of ripple between stages. My CTC11 worked OK on its originals until 2008, when it got the bends and I shut it down.
My other CTC11 had all its caps replaced, seldom used but works reliably.

To bridge all the 'lytics while observing pic, I used a 40mf @ 450v but that did not accomplish much. The alternative is to replace all 3 cans' caps*, which needs done regardless.

*I was thinking I could skip that step to see what set gets the "full monty" first!

[nasadowsk]

Originally Posted by DavGoodlin

16x update: Germanium diodes video and sound detectors tested good, that is 0.3volts fwd drop and no reverse current.

The HV is behaving, providing proper focus yet strangely low cathode current of 128 ma.(normal is 195 per std info). H-drive amplitude possibly affected by sync issue?

The CTC16x locks in very barely horizontally, using crosshatch or color bars it appears the first video is weak going to the 6KA8 so the scope is needed. You just know its not a tube, either

Once had a blanker issue on a CTC-25 clone cause low h output current....

[DavGoodlin]

Originally Posted by nasadowsk

Once had a blanker issue on a CTC-25 clone cause low h output current....

It was a temporary issue, 6LQ6 cathode current is now dipped at 175 ma., which is still low but after an hour of narrowing the noise inverter issue, the fly is fairly warm which is concerning as always. RCA figured to "box-in" the fly to keep dust and airborne slime off the HV rect, etc. The downside is well known and Im considering a reversible modification to address that.

The Tuner-IF-1st/2nd video are cleared of causing the current issue of poor V-H lock, video appears to change every 16 seconds and I verified it by scoping the input to the 6KA8 AGC/Noise inverter. A series of sync pulses float slowly to the left on the scope, with a bunch of odd spikes appearing for an interval. All tubes have been swapped, with no changes noted.

The only clue left is 28 volts of DC voltage on the plate (keyed AGC side) of the 6KA8 where schemo shows 0.2v of DC, which gets the keyer pulse from the fly, the other voltages are within 10% of RCA schematic I'm using. I went after a few electrolytics including the 2mf on 6LF8 screen bypass and 40 mf on the AGC control, still no improvement.

I highly suspect the 560K and 22M resistors around the 6KA8 and in particular, ANY capacitor coupling on the flyback pulse leg.

[old_tv_nut]

"Video appears to change every 16 seconds"

This is an indication of AC line rate hum somewhere. 16 seconds is the beat period between AC power and NTSC 59.94 Hz field rate. It will show up with a full-standard NTSC source, but not with old test gear that locked the vertical to the AC line.

Bad power supply filtering? Maybe a heater-cathode short?

[DavGoodlin]

Originally Posted by old_tv_nut

"Video appears to change every 16 seconds"

This is an indication of AC line rate hum somewhere. 16 seconds is the beat period between AC power and NTSC 59.94 Hz field rate. It will show up with a full-standard NTSC source, but not with old test gear that locked the vertical to the AC line.

Bad power supply filtering? Maybe a heater-cathode short?

I am in process of changing the electrolytics a few at a time, though none are heating up, they may be past reforming again. Im using the VA-48 for video, possibly explaining that periodic differentiation-style spikes going to the 6KA8 grid. It cannot even show a good air signal via the ch 6 broadcast MOD I'm using.

The H-K short must be somewhere possibly back-feeding video, from output 12BY7? I also need to throw my iso-trans/non-brightener on the CRT, it (had) tested good except with a H-K short in blue gun Gray scale is easy to set, but with an H-K short (considering B+ is on heater winding) THAT would present an obvious clue!

[DavGoodlin]

After replacing a few more electrolytic caps and the AFC diodes, no real improvement. I figure it's (the bad part) not affecting the vertical like the horiz anyway.
1966-RCA-CTC16XL a.jpg
Then I decided to scope H-osc sync coming from the horizontal AFC diodes, it looks like the trouble may be what is coming out of the AFC network into pin 2 of 6CG7 H-osc, also where a series RC network 12k resistor and .15mf goes to ground.

I assume this network is meant to "filter" the wave shown below.
1966-RCA-CTC16XL b.jpg

[DavGoodlin]

Some more caps replaced and still the same issue. The 47pf feeding AFC diodes from 6KA8 plate was a suspect along with seven others identified in the "RCA color tv service manual" to which I am referring any schematic notes to. I have an 820pf and 27pf yet to try.

Vertical lock is OK but horizontal is affected by the phantom stuff riding on what should be a cleaned up, semi-sine shaped horz sync pulse. It is no wonder the video pulls and loses lock-in, sometime locking with a blanking bar on the left side.

[Yamamaya42]

Originally Posted by DavGoodlin

After replacing a few more electrolytic caps and the AFC diodes, no real improvement. I figure it's (the bad part) not affecting the vertical like the horiz anyway.
Attachment 201953
Then I decided to scope H-osc sync coming from the horizontal AFC diodes, it looks like the trouble may be what is coming out of the AFC network into pin 2 of 6CG7 H-osc, also where a series RC network 12k resistor and .15mf goes to ground.

I assume this network is meant to "filter" the wave shown below.
Attachment 201954

W/O knowing what v scale you were set to, no way to know what P-P voltage that is, :O
The Sams says 30v on the other side of R111, so without knowing what the scope was set to, I'm assuming it's abnormally high, and the clipping at the top does no look so healthy as well.
Never having had such problems on mine, I never had to poke around there, so I;m not sure what it SHOULD look like, but I'm guessing the flat tops is abnormal.
I guess I have been lucky with my 16xl and it has behaved in that aspect, or is it that you are unlucky?

[JohnCT]

Originally Posted by DavGoodlin

Vertical lock is OK but horizontal is affected by the phantom stuff riding on what should be a cleaned up, semi-sine shaped horz sync pulse. .

Maybe I missed it, but have you tried another source besides the VA48? I have two and both had issues with failing bypass electros in the power supply and on the generator boards.

John

[DavGoodlin]

Originally Posted by JohnCT

Maybe I missed it, but have you tried another source besides the VA48? I have two and both had issues with failing bypass electros in the power supply and on the generator boards.

John

My scope at input (6KA8 pin 6) shows a very noisy video with AC riding it. I assume the noise inverter is meant to clip video, leaving clean sync pulses.

EDIT: Relatively good 6 V p-p video going into 6LF8 pin 2 and then odd stuff at the plate pin 9 at much higher amplitude. The screen grid bypass cap was replaced early on, as it causes these issues in older chassis using a 6AW8.

I will put the VA 48 on a '70 Zenith 16" color and see what its does. It was fine when I was working on a Zenith last month. I have three more orange and maroon drops to try then I give up for now.

EDIT : The Zenith shows a normal picture with fill range of output, and the RCA 16x looks equally un-synced with an air signal .

This is going nowhere, so I change chassis and go back 3 years to a simpler RCA design with a few less "features" in its circuits.

[old_tv_nut]

[QUOTE=DavGoodlin;3229725 ...I assume the noise inverter is meant to clip video, leaving clean sync pulses...[/QUOTE]

The noise inverter is expected to clip noise pulses that go in the direction of sync but are momentarily bigger than sync, like impulse noise from motors. It does not clip video, which is opposite polarity of noise pulses.

[JohnCT]

Originally Posted by DavGoodlin

This is going nowhere, so I change chassis and go back 3 years to a simpler RCA design with a few less "features" in its circuits.

Whatever the problem is, it isn't the design. I worked on many many hundreds of 16s when they were still in daily use (although getting older), and they were always rock solid on any quality of signal.

John

[DavGoodlin]

Originally Posted by JohnCT

Whatever the problem is, it isn't the design. I worked on many many hundreds of 16s when they were still in daily use (although getting older), and they were always rock solid on any quality of signal.

John

Agreed, these look super when something wacky isn't going on. I just need a break but I appreciate the help everyone.

[Yamamaya42]

“Relatively good 6 V p-p video going into 6LF8 pin 2 and then odd stuff at the plate pin 9 at much higher amplitude.”

This does makes sense, as Sams shows the input to the 2nd video amp ( pin 2 v4 ) to be 5v ptp, and shows no wave for output of the 1st, pin 9, as that point still has the bias on there, I'd expect it to be garbage if you hook up there, but the fact that pin 2 is clean, means that it must be OK by default, one could also hook to the far side of color link cap (c22) and see what you have there.

But by default, if as you say the signal at pin 2 v4 is nominal, then 1st video should be fine, and path 21 is screwy if the signal is crap down there or something

[DavGoodlin]

The video circuits, if the waveforms are simpatico, can be used to compare what the 16x looked like. I replaced 14 caps and 4 resistors with nothing worse but no better.

I hope anyone trying to get one of these sets restored learns something from the trial and error. Im using the RCA factory schematic, which shows a waveform going into the 6LF8 pin 2.


I can pull out Sams 818 to double check my voltages on the 16X but at this point, we need to go back in time.

The CTC12B set I have was built in the 34th week of 1962. Edit: Model 213G216MU "Lynnhaven" in walnut.