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Old 10-24-2018, 04:21 PM
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1960s Sweet 16 RCA television help

Hi all,

I'm looking for some guidance on a 1960s RCA Sweet 16 sportabout I'm in the process of restoring. It was working fine until just a few months ago, when it developed some disturbances, almost like an AC buzz but visual, in the picture. It would react to tapping on the cabinet, sometimes getting better or worse. Then, I noticed that adjusting the Horizontal Hold pot actually caused the picture to disappear entirely, but it would always come back when I moved it back.

So, with my AC theory in mind, I went ahead and replaced the electrolytic can and the glorified paper safety cap across the AC line. Those replacements appear to be fine, no obvious trouble. It's turning on faster than before, and I'm getting perfect sound.

However, my picture is gone.

I noticed that the HV rectifier tube in the cage was still cold after running it, and there was no high voltage whine, so I replaced that tube. Now, the whine is back, it gets louder when I adjust the horizontal hold, but the picture is still absent. Placing a screwdriver against the HV rectifier cap in operation is giving me a healthy blue spark, so I know my flyback is fine, and I've also verified that the resistor across the HV rectifier tube socket is up to speck, and that the lead to the picture tube appears to be fine, though my continuity there did seem to cut in and out at times, so perhaps that warrants another look. The tube is definitely still under vacuum.

I'm thinking my issue either has something to do with the Horizontal Hold pot, the picture tube HV lead, or how I'm installing that cable. The amount of clip that actually goes inside the tube is much smaller than I expected, but I also can't see any way for it to go further. Regardless, it's not falling out or sparking. Considering where I started, does anyone have any suggestions for where I should look next? I don't have a HV test probe, though I'm willing to get one if it's necessary.
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Old 10-24-2018, 06:10 PM
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Check for arcing at the CRT...Slip a screwdriver under the cup and see if it sparks decently.

Might be good to compare the horiz osc waveform width to sync pulse width on a scope to confirm it is on freq...If not severe reduction in HV can result.

Your pre-recap symptoms depending on exactly how they manifested could be attributed to caps or bad grounding of circuit boards, etc. Another possibility is bad solder joints on the osc coil.
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Old 10-24-2018, 07:00 PM
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Post the chassis number for us 1st KCS###
Healthy spark at plate cap leaves
1) bad tube
2) open filament winding or resistor. Check resistor with tube OUT.
3) open anode lead
4) bad CRT

Is this one of those early solid state sets ?? If so IIRC the anode lead has
resistors built into it, special OEM part.....

73 Zeno
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Old 10-24-2018, 07:18 PM
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Chassis number is KCS152C. I'm including the SAMS schematic for reference.

I did a little more poking around, ran the same test with the anode lead disconnected from the CRT and received no spark from the end of the lead when I touched a screwdriver to it. Considering I already noticed that the continuity check on my multimeter seemed to go in and out on that wire, I'm leaning towards replacing it as my next step. I'll also go ahead and ohm out the resistor with the HV rectifier tube out to verify that it's good, but from what I saw in the wiring I believe my reading should be accurate.

This is not a solid state set, so fingers crossed that the lead is just a wire. No evidence on the schematic from what I see.

I would be surprised if the CRT was bad. It was perfectly bright just a few months ago, so to go from normal to no picture that quickly without something catastrophic happening would be bizarre. I'm no expert, though.
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Old 10-24-2018, 07:19 PM
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Is the schematic showing up? I'm not seeing it. I have to run but I'll post it later tonight if not.
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Old 10-24-2018, 09:03 PM
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Okay, here's a copy of the schematic.

Also, this slipped my mind, but I did notice one of the leads to the electrolytic can was broken off when I was removing it. I've fixed it, and didn't consider it worth noting since I was planning to move it to a terminal strip anyway, but if memory serves it was the positive lead to the triangle symbol. Either that or the semicircle. Either way, Unless the symptoms persist once I get a picture, I'm tempted to chalk at least part of the "percussive maintenance" response to that lead being broken or almost broken.

Does anyone have any recommendations for what kind of wire I should get to replace the anode? Are CRT anode leads still sold, and will a standard piece fit most CRT sockets?

Also, I was a little confused on some of the information regarding my 1G3GT HV rectifier. If I'm understanding it correctly, that tube is not part of the series filament string, and is powered directly by the flyback transformer. I verified that my old 1G3GT was bad by testing for continuity between pins 2 and 7, which I believe is where the filament is. Those two points tested no continuity, and further testing on a tube tester also showed no life.

Also, since I'm on the subject, and I apologize for asking these novice questions, what's the standard on numbering tube pins? If I've read correctly, I find the key on the center post, or gap on a miniature type tube, and count clockwise from there. Am I doing that right?

If everything I'm doing so far checks out, My next step is going to be to test that resistor and, assuming that checks good, verify that there is an intermittent break in the anode lead.
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  #7  
Old 10-24-2018, 10:06 PM
dieseljeep dieseljeep is offline
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Quote:
Originally Posted by Dubis7 View Post
Okay, here's a copy of the schematic.

Also, this slipped my mind, but I did notice one of the leads to the electrolytic can was broken off when I was removing it. I've fixed it, and didn't consider it worth noting since I was planning to move it to a terminal strip anyway, but if memory serves it was the positive lead to the triangle symbol. Either that or the semicircle. Either way, Unless the symptoms persist once I get a picture, I'm tempted to chalk at least part of the "percussive maintenance" response to that lead being broken or almost broken.

Does anyone have any recommendations for what kind of wire I should get to replace the anode? Are CRT anode leads still sold, and will a standard piece fit most CRT sockets?

Also, I was a little confused on some of the information regarding my 1G3GT HV rectifier. If I'm understanding it correctly, that tube is not part of the series filament string, and is powered directly by the flyback transformer. I verified that my old 1G3GT was bad by testing for continuity between pins 2 and 7, which I believe is where the filament is. Those two points tested no continuity, and further testing on a tube tester also showed no life.

Also, since I'm on the subject, and I apologize for asking these novice questions, what's the standard on numbering tube pins? If I've read correctly, I find the key on the center post, or gap on a miniature type tube, and count clockwise from there. Am I doing that right?

If everything I'm doing so far checks out, My next step is going to be to test that resistor and, assuming that checks good, verify that there is an intermittent break in the anode lead.
There's probably nothing wrong with the anode lead. The schematic shows a 1000 ohm resister in the HV rectifier socket, check that.
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Old 10-24-2018, 11:15 PM
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They used the same anode connector for most CRTs made throughout the years. If the lead somehow is bad one from a 1980's-2000's junk SS CRT set can be subbed. If your game to swap the connector to a new lead then non-resistive spark plug wire from an auto parts store could also work. Any wire with insulation rated 30-50KV will work in most B&W sets. The average CRT has 1mA max anode current so even thin conductor wire can handle that.
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Last edited by Electronic M; 10-24-2018 at 11:19 PM.
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Old 10-25-2018, 08:22 AM
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Pin numbers are read from the bottom clockwise. # 1 will
be the first one AFTER the gap or key.

The filament runs off the flyback not the regular string.
All of the HV stuff is running at 16 KV above ground so you need
20 KV or more wire. After that flexability helps. The anode lead
can be stripped if almost any junker. The 1 K resistor should be
carbon also. If you restring the filament try to keep it the
way the old one is.

73 Zeno
LFOD !
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Old 10-25-2018, 10:13 AM
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Quote:
Originally Posted by Dubis7 View Post
Chassis number is KCS152C. I'm including the SAMS schematic for reference.

I did a little more poking around, ran the same test with the anode lead disconnected from the CRT and received no spark from the end of the lead when I touched a screwdriver to it.
Something you need to know here is that the CRT coatings form a capacitor that is charged by the short pulses from the HV rectifier. So with the lead disconnected from the CRT the HV will not develop, no spark.
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Old 10-25-2018, 10:30 AM
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Quote:
Originally Posted by Dubis7 View Post

Also, I was a little confused on some of the information regarding my 1G3GT HV rectifier. If I'm understanding it correctly, that tube is not part of the series filament string, and is powered directly by the flyback transformer. I verified that my old 1G3GT was bad by testing for continuity between pins 2 and 7, which I believe is where the filament is. Those two points tested no continuity, and further testing on a tube tester also showed no life.
Very few tube testers check HV rectifiers well. They don't apply high enough voltage and the tubes can't pass very much current even when good. The filaments are higher resistance than normal because they run on low voltage and draw little current.
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Old 10-25-2018, 10:34 AM
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You should check the voltages on the CRT socket, they have to be correct also in order to get light on the screen.
You might have got on the wrong track looking at the HV because some of your tests weren't conducted right.
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Old 10-25-2018, 10:55 AM
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Thanks for all the advice. I went back and did another filament continuity test of the original tube, and it looks like my original tube wasn't faulty.

I also went ahead and restested that 1K resistor under the rectifier socket. The results were mixed. An ohms test across the resistor with the tube removed is reading just about 1K, within tolerance, but a continuity test across the same resistor is showing no continuity. At the same time, doing another reading of the anode wire is showing stable continuity when I flex the cable, so I'm inclined to believe that's actually fine, but I'm also getting no continuity across that cable when I measure from the opposite side of the resistor. Is that normal, or is the resistor suspect?

If my high voltage turns out to be fine, will it be possible to be seeing the same results if I'm not getting a signal to the tube? I'll have to go back and check with the anode lead plugged into the CRT, though I'm not sure of a safe way to pull back the rubber cup and look for the spark. Gloves perhaps? If anyone has a method for that, I'd love to hear it.
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Old 10-25-2018, 11:15 AM
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Quote:
Originally Posted by Dubis7 View Post
Thanks for all the advice. I went back and did another filament continuity test of the original tube, and it looks like my original tube wasn't faulty.

I also went ahead and restested that 1K resistor under the rectifier socket. The results were mixed. An ohms test across the resistor with the tube removed is reading just about 1K, within tolerance, but a continuity test across the same resistor is showing no continuity. At the same time, doing another reading of the anode wire is showing stable continuity when I flex the cable, so I'm inclined to believe that's actually fine, but I'm also getting no continuity across that cable when I measure from the opposite side of the resistor. Is that normal, or is the resistor suspect?

If my high voltage turns out to be fine, will it be possible to be seeing the same results if I'm not getting a signal to the tube? I'll have to go back and check with the anode lead plugged into the CRT, though I'm not sure of a safe way to pull back the rubber cup and look for the spark. Gloves perhaps? If anyone has a method for that, I'd love to hear it.
Take a small thin bladed screwdriver, use a clip lead clipped to the blade and to the chassis and work the blade under the rubber cup. You should hear a arcing sound even before the blade is fully inserted, if HV is present.
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Old 10-25-2018, 12:09 PM
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Okay, I just ran that test, and I did see a blue spark at the anode cup, so it's passing high voltage. I didn't hear anything, but I saw it, so I presume that's enough of an indicator?

Anyway, I suppose my next step is going to be to pick up one of those tube extenders so I can check the voltage going into the base of the CRT. I'll go ahead and re-verify that I didn't screw up any wiring or values when I replaced the capacitors. I've checked it multiple times, but since it was working before I got in there and isn't now, it's worth taking another look at what I did.

Does anyone have any suggestions on where to check next, in case I missed something?
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