RCA CTC-16XH Smearing Issue

[Jon1967us]

Currently trying to iron out a smearing issue that appears on high contrast areas, especially text. I believe this in not an uncommon problem, but I've tediously made several attempts to trace the cause with no success so far. I understand this is typically due to problems with open coils or leaky caps in the IF and Video stages...

Example 1:

Example 2:



Some test info and findings:

• CRT tests good on emissions, about 1/3 into the good scale on a B&K 465. I don't recall what the cutoff was, but the picture is bright and clear
  
• All tubes in IF section test good and have no shorts and aren't gassy
  
• Performed step one in RCA service guide for peaking (max/min) IF stages with non sweeping frequency and this did NOT affect or remedy the smearing. I assume this stage of alignment is to get your stages "in the ballpark" before further refinement in the sweep gen alignment procedure?
  
• Injected composite video into circuit, just before the 1st Video Amp, and observed smearing, leading me to believe the defective component should be after that stage.
  
• Injected composite video into circuit, just before the 2nd Video Amp, and observed a reversed fainter image, but didn't notice smearing - not that there couldn't have been any, I just didn't see it, leading me to believe offending component to be before this stage.
  
• All peaking coils' in IF and Video tested ok in terms of resistance, cold solder joints. Adjoining resistors were also tested - a couple were out of tolerance and replaced, but did not affect the smearing.
  
• Replaced the video detector diode - no effect.
  
• Replaced coupling caps in these sections - no effect.
  
• Switched the converter box - no effect.
  
• Tested continuity and resistance of Delay Line and noted about 130Ω. Even shorted the delay line to see color shift but no change to the smear.
  
• Saw no cold solder joints on the underside of the IF or Chroma boards.

IF section Sams schematic:
Pt 1

Pt 2

Thoughts/questions

• One thing that I observed was that when I jumped a couple of the peaking coils there was no effect to the picture. Could this mean that there might be shorted turns in one or more of the coils? I don't have an LCR meter, but I do have a Sencore YF-33 Ringer Yoke FLyback Tester. Not sure yet how to use that to test peaking coils.
  
• Adjusting coils, traps or transformers in the IF/Video stages doesn't affect the smearing
  
• Could it be poor cutoff on the CRT? The CRT is bright and clear though.

[zeno]

Wow, wish everyone worked a problem like this ! Well done.
If you have an anylist break the chain at the video amp & inject
video. This will cut the possible stages in half. Its why I scream
" learn the block diagram" for newbies.
I would not suspect the CRT. Those little square peaking coils
did open but I never saw one short. I am sure you covered it
but be sure all the ground lugs for the PCB's are good & any shielding
like the IF is also.
On the Sencore most of there gear came with 2 manuals. One was
a basic manual & the other was a large book that showed every
creative thing you can do with it. Look into that.

73 Zeno
LFOD !

[old_tv_nut]

Peaking coils will only affect very short transients, near edges. Not the case here.

That streaking is a very long time constant, lasting for most of a scan line.

The important clues are:

• Injected composite video into circuit, just before the 1st Video Amp, and observed smearing, leading me to believe the defective component should be after that stage.
• Injected composite video into circuit, just before the 2nd Video Amp, and observed a reversed fainter image, but didn't notice smearing - not that there couldn't have been any, I just didn't see it, leading me to believe offending component to be before this stage.

Scratching my head a bit to identify a component with such a long time constant.

Wish they listed the inductance value of L10, but assuming it's OK, how about R57 220 ohm and C21, .001uf (screen bypass) or C24 and R61 (connect to the sync separator).

[old_tv_nut]

Also check C1, 40 uf, in the AGC/sync separator circuit.

[Jon1967us]

Originally Posted by zeno

Wow, wish everyone worked a problem like this ! Well done.
If you have an anylist break the chain at the video amp & inject
video. This will cut the possible stages in half. Its why I scream
" learn the block diagram" for newbies.
I would not suspect the CRT. Those little square peaking coils
did open but I never saw one short. I am sure you covered it
but be sure all the ground lugs for the PCB's are good & any shielding
like the IF is also.
On the Sencore most of there gear came with 2 manuals. One was
a basic manual & the other was a large book that showed every
creative thing you can do with it. Look into that.

73 Zeno
LFOD !

Can you recommend a particular Analyst?

[Jon1967us]

Originally Posted by old_tv_nut

Peaking coils will only affect very short transients, near edges. Not the case here.

That streaking is a very long time constant, lasting for most of a scan line.

The important clues are:

• Injected composite video into circuit, just before the 1st Video Amp, and observed smearing, leading me to believe the defective component should be after that stage.
• Injected composite video into circuit, just before the 2nd Video Amp, and observed a reversed fainter image, but didn't notice smearing - not that there couldn't have been any, I just didn't see it, leading me to believe offending component to be before this stage.

Scratching my head a bit to identify a component with such a long time constant.

Wish they listed the inductance value of L10, but assuming it's OK, how about R57 220 ohm and C21, .001uf (screen bypass) or C24 and R61 (connect to the sync separator).

All these caps have been replaced. OOT Resistors too. I may have to check C22, the 7pF. Don't recall if I replaced that one.

[Jon1967us]

Next plan is to jump into more extensive voltage testing as well as video insertion. Maybe see if I can trace down to a bad tube pin joint, supply voltage or something overloading a section. Open to other ideas.

[old_tv_nut]

Originally Posted by Jon1967us

All these caps have been replaced. OOT Resistors too. I may have to check C22, the 7pF. Don't recall if I replaced that one.

Such a small cap cannot produce such a long smear. C7 couples only high video frequencies to the chroma circuits.

I would nose around the AGC / sync separator / noise inverter, since feedback circuits can do weird things and make it difficult to figure out where in the loop the problem is. Check voltges in that area, and substitute V6 if you have a spare. The fact that the smear disappears when you couple to the 2nd video is a strong indication that the problem is in first video or the above stuff that is fed from the first video.

[old_tv_nut]

Since the smear starts on black lettering, it's possible the AGC circuit is conducting during trace when it should be gated off. It would be good to know that the 450 volt keying pulse on pin 9 of V6 is there, as well as whether V6 is good.

Also, in case the smear really is still there when you inject into the second video, check any caps in the following video circuits that are .01 uf or larger.

[old_tv_nut]

You didn't say you have a scope - this is a case where looking for the right waveforms could be much faster to isolate the problem.

[Yamamaya42]

My CTC-16XL does the same thing, but to a MUCH lesser degree, it's hardly noticeable on mine, but it is there, it seems that it's common, but not sure what the cause is.

[Jon1967us]

Voltage tests on V4 indicate wrong voltage on pins 7, 8 and 9.

pin7: -5 vdc / .5 vac / Should be 0 vdc and 3.5 vac

pin 8: 177v / Should be 95v

pin9: 184v / Should be 130v

Resistances are wrong on pin 7 to B+ output according to the Sams.
Pin 7: 2.1kΩ and 3.4kΩ (Sams states R depends on polarity of probes)
R should be 130Ω

So, something's overloading the grids and plate on the Vid1 section and the tube can's cut off correctly. I have some ideas suspect dropping resistors and DC blocking caps before the Vid 1 stage, maybe even going back to the AGC...

Will also bust out my scope to verify the waveform on the Vid input. Maybe try different video injection points. I don't have an analyzer, so I have to do it by composite from the set top box.

[old_tv_nut]

For pin 8 to be 177, the 100 v source that feeds it must be way too high. Check all the power supply voltages (but especially the 100 v source and R215, 47k 1W) and check pin 8 again to make sure you read it right.

Pin 7 should be 0vdc as you state. However, the 3.5 V on the schematic is not an ac meter reading but a scope peak-to-peak reading of the video waveform. AC meter reading will vary drastically depending on video content. The 30~ notation means the scope sweep rate is set to 30 Hz, so you see the two video fields as illustrated.

[old_tv_nut]

Where do you see that the resistance reading for pin 7 should be to B+? I think it should be 130 ohms to ground, and the note on polarity variation is because of the video detector diode.

[Yamamaya42]

Originally Posted by old_tv_nut

Where do you see that the resistance reading for pin 7 should be to B+? I think it should be 130 ohms to ground, and the note on polarity variation is because of the video detector diode.

correct, it is to ground,

as stated on page 8 of the SAMs

it does not say anyplace that its to be from b+