CTC-16 Green Picture

[Jon1967us]

CTC 16 Green Screen



Symptom:

Picture only displays in green.

Sams:

https://earlytelevision.org/pdf/rca_...ams-0736-4.pdf

Injecting NTSC bars:

Grayscale: https://ibb.co/QMRLcYz
Color:https://ibb.co/BcWKhyn

One sees indication that chroma waveform is present in the second picture. Color waveform osc. seems to be working, crystal checks (was substituted with no effect) ok as well, yet no color.

When set is just warming up there will be blue lines in picture that will briefly appear. Occasionally, I will see a full 3 color barber pole pattern, but only momentarily. I used to be able to tap on the color sync/color killer detector and color would return. It no longer does. I reflowed all ground lugs on the Chroma board. I reflowed tube socket connections for V17 and V19. Visual inspection having been performed several times and no cold solder joints seen. All resistors on Chroma board are within tolerance. Have checked quite a few coupling caps and they always appear to be within spec. No adjustable coils on board have opens. Pretty much every component I’ve removed from the board as a possible suspect has tested fine and leakages on caps were fine. Sprayed all pots with contact cleaner thoroughly. All tubes between IF and Chroma test fine.

Screen voltages at CRT:

Green a little high (measured 192 - should be 180)
Red low (measured 108 - should be 170)
Blue low (measured 100 - should be 175)

Waveforms and voltages:

Burst Amp V18 Waveform pin 1 ok.

Color Killer V17 waveform pin 1 appear ok.

Chroma waveform osc. V20 appears to be in working order, and voltages appear normal.

Chroma Sync Phase Det V19 waveforms at pin 1, 3 appear ok, but voltages at pins 1, 3, Color Killer Det 7, 9 are too low.

Chroma bandpass amp, V17 pin 2 - appears the sync pulses aren’t getting removed and chroma signal is weak ~2-3v p-p.
https://ibb.co/2k5wsvP

Haven’t checked waveforms downstream yet, at the demods or color amps. Seems like the problem is centered around the Chroma bandpass and Killer Detector.

Injected composite video at video amp and symptom doesn't change.

Was wondering if this symptom/cause is familiar to anyone.

[old_coot88]

Look for an intermittent in the area of the 3.58 reference oscillator (V20). If the osc. is not running, the chroma signal is displayed only as green. Since you occasionally get a 3 color barberpole, the osc. is starting intermittently and out of sync.

Check especially around the reactance coil (L29). Since this coil sticks up pretty high it tends to get bumped a lot and any open condition there will kill the oscillator. (This coil is the color sync adjustment also.)

[Jon1967us]

Originally Posted by old_coot88

Look for an intermittent in the area of the 3.58 reference oscillator (V20). If the osc. is not running, the chroma signal is displayed only as green. Since you occasionally get a 3 color barberpole, the osc. is starting intermittently and out of sync.

Check especially around the reactance coil (L29). Since this coil sticks up pretty high it tends to get bumped a lot and any open condition there will kill the oscillator. (This coil is the color sync adjustment also.)

Thanks for your reply and seems plausible, but isn't this pic an indication the osc. running, or no?

Color Bar Input at antenna terminals:https://ibb.co/BcWKhyn

[Electronic M]

I had this issue with my CTC-16 Stockholm. Same exact symptoms too...In my case it turned out to be the well known jumper wire on top of the board rotting open. It looked fine but voltage and continuity checks led me to it.

One of the jumpers basically passes B+ to the chroma video outputs and when it rots Green ends up dominant.

[old_coot88]

OK entirely different symptom from dead oscillator. Apologies for bad info.

[Jon1967us]

Originally Posted by old_coot88

OK entirely different symptom from dead oscillator. Apologies for bad info.

Happy to have you chime in. Right after your suggestion I gave a look in that area and happened to be lucky enough to find what Electronic M was about to suggest before he suggested it!

Sure enough, found that there was indeed a jumper with a corroded end. I excised that end, extracted the detached lead and soldered a new lead to that one. This was the jumper going from R199 to R184 with 405v on it leading to the G-Y and R-Y amps.

Once that was done I returned the chassis to the cab and proceeded to coax out a barber pole, straightened that out, and tweaked on the chroma section with composite injected into the Vid amp, pin 7. Having dialed that into a decent ballpark, I then injected into the antenna and dialed in the IF (yeah I know, I was monkeying before with the dang cans, trying to jump start the chroma back up after failing, and threw it all kinds out of whack)

The color is back and the picture would be nicer if only my Focus wasn't on the fritz. The focus problem recently cropped up. I heard some snapping and arcing a couple days ago. I suspect a shorted turn on the coil or a bad resistor. Haven't really dived into that yet.

For now we have color and a decent picture. Tomorrow? We know how that goes.

[zeno]

While you are in there check around the 6GU7's.
You probably found the right jumper but be sure. IIRC it was
in the filament. Was common in the olden days.

73 Zeno
LFOD !

[old_coot88]

Zeno's right. Those 2 ground stakes on the far right end of the chroma board need to be re-flowed. They're Schroedinger's stakes and exist in one of two states: either cracked or gonna crack. When that happens the 6GU7 heaters go out, initiating a cascade. Plate voltage rises, and since the plates are DC-coupled to the CRT grids, CRT beam current rises, dragging down the HV till raster blooms and fades out, stressing the bejeebers out of the 3A3 and flyback.

Re-flowing these should be done as a matter of course on every CTC-15, '16, '17 and '25.

[dieseljeep]

Originally Posted by zeno

While you are in there check around the 6GU7's.
You probably found the right jumper but be sure. IIRC it was
in the filament. Was common in the olden days.

73 Zeno
LFOD !

AFAIR, there was two wires on top of the board, one carried B+. When I got a chassis in for repair, even for a flyback replacement, I wound resolder the ground stakes and replace the two wires on top of the board and reinstall them on the bottom.
Also, the 620uh peaking coils would get damaged by arcing from those wires.

[Jon1967us]

Originally Posted by old_coot88

Zeno's right. Those 2 ground stakes on the far right end of the chroma board need to be re-flowed. They're Schroedinger's stakes and exist in one of two states: either cracked or gonna crack. When that happens the 6GU7 heaters go out, initiating a cascade. Plate voltage rises, and since the plates are DC-coupled to the CRT grids, CRT beam current rises, dragging down the HV till raster blooms and fades out, stressing the bejeebers out of the 3A3 and flyback.

Re-flowing these should be done as a matter of course on every CTC-15, '16, '17 and '25.

I did read in one of my books something about "a set shouldn't leave the shop without reflowing the solder at the ground joints on the chroma and IF boards"
I did reflow the joints and check them but only on the chroma board, and AFTER my focus went out. Having done that, the focus is still poor and the focus control has only a subtle effect on the picture. I'm not sure if what you described above is what happened or if my coil is bad or the 66M bleeder or something in that circuit has gone bad. 6GU7 filaments are present, currently.

I've read that focus voltage @ the CRT needs to be about 20% of CRT HV, so ~5KV. Need to test that.

[Electronic M]

Sometimes the focus rect tube will test good but not work well. I tend to parallel a modern 5KV diode across the focus rect tube as an early troubleshooting check/long term fix.

A few things to watch out for in focus circuits. Corrosion inside the CRT socket and or bad contact between socket and CRT, the 66M resistor, the 6KV ceramic cap on the focus coil can have issues...The collector that reported it had an tuning eye type cap checker and said it didn't fully open the eye on the capacitance test.

[Jon1967us]

Originally Posted by Electronic M

Sometimes the focus rect tube will test good but not work well. I tend to parallel a modern 5KV diode across the focus rect tube as an early troubleshooting check/long term fix.

A few things to watch out for in focus circuits. Corrosion inside the CRT socket and or bad contact between socket and CRT, the 66M resistor, the 6KV ceramic cap on the focus coil can have issues...The collector that reported it had an tuning eye type cap checker and said it didn't fully open the eye on the capacitance test.

When I heard the quick arcing/snapping/zapping sound, when the focus went out a couple days ago, it seemed to be coming from the chassis or HV area, not the CRT socket, but I'm not 100% sure. Still need to check voltage at CRT and definitely that cap and the bleeder.

[zeno]

Old trick for arcing was turn off the AC & open the shop doors on a
humid day. Sometimes just blowing on it will trigger it.
Trouble is everything in the focus is known to go bad.
Arcing was usually the socket or filament winding. Keeping one of the
stick rectifiers around is handy for test or eliminating the tube & socket.

73 Zeno
LFOD !

[DavGoodlin]

Make sure the HV is adjusted so that exactly 1 volt drops across the 1K resistor in 6BK4's grid circuit when pix is dark. As brightness increases, the voltage should decrease to about 0.5-0.7 volts. Tab books' RCA color TV service manual volume 1 by Carl Babcoke explains this operation thoroughly.

HV value alone is not a good way to adjust shunt regulators, as it ignores current through the 6BK4 which is a better indicator of correct range of regulation as brightness changes. Visual check by dropping brightness to black screen while watching the cathode glow brighter in the 6BK4/6EL4 tube. That may indicate excess current.

I have found on my CTC20 Rupert that .01 mf (1 kV?) disc cap with the spark gap was leaky and allowed HV to vary so much that focus would not track with brightness. All it takes is one good and random HV snap then that cap is toast, even with a spark gap. Good luck

[Jon1967us]

Originally Posted by zeno

Old trick for arcing was turn off the AC & open the shop doors on a
humid day. Sometimes just blowing on it will trigger it.
Trouble is everything in the focus is known to go bad.
Arcing was usually the socket or filament winding. Keeping one of the
stick rectifiers around is handy for test or eliminating the tube & socket.

73 Zeno
LFOD !

It's humid here by the coast. Could've been some condensation, or maybe an increased load due to having the supply voltage to the color amps out of circuit that could've compromised the focus components - hard to say. Checked (dialed down) the screen drives against service mode, but that didn't affect focus.

I did measure 4KV at the focus CRT pin - so that may be a little low...

https://ibb.co/jy0BFp8

Not psyched to pull the chassis for the 168th time, but I need to check the windings and other stuff for drift and leakage

Pic
https://ibb.co/0qBwfJr