1960s Sweet 16 RCA television help

[Dubis7]

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.

[Electronic M]

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.

[zeno]

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
LFOD !

[Dubis7]

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.

[Dubis7]

Is the schematic showing up? I'm not seeing it. I have to run but I'll post it later tonight if not.

[Dubis7]

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.

[Notimetolooz]

Originally Posted by Dubis7

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.

[Electronic M]

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.

[zeno]

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 !

[Dubis7]

Alright, I just checked the pins on the base of the CRT while it was turned on and everything was plugged in. I removed the connector enough to wiggle a multimeter probe in there while maintaining electrical connection.

Most of the pins seem to check normal, but pin 3, which is listed as 260V boost, is showing closer to 90V. Could low boost voltage be causing my problem? I'm not actually sure what it does, but it doesn't seem like there should be that much variance. It looks like that's coming off of a lug on my flyback, and going through resistor R79. I suppose my next step is to verify that R79 hasn't drifted above tolerance.

I remember hearing that, when testing a resistor, the correct procedure is to unsolder one of the leads and isolate it from the circuit. Would that be correct in this case?

[zeno]

Time to back up a little...........
Remove the plate cap from the 1G3 & let it hang away from metal.
Take a well insulated screw driver & get it near the cap ( wire end).
You should get about a 1/2 inch arc, NOT just a spark if you touch
it or get very close.

Boost comes from the flyback. Its stacked on top of regular B+ voltage
( think of 2 batteries in series ) & filtered by C50 & C51. R79 must also
be good.
On your set if pin 3 of CRT goes to low it will kill the raster BUT not effect the HV.

Next is to get good HV ( that nice arc ) you need the following stages running.
1) horz osc
2) horz output
3) damper
Any defect will lower & or kill the HV & boost.

Bottom line it nice big arc = good till that point
Small arc = problem is before HV rect.

73 Zeno
LFOD !

[Dubis7]

Gotcha. That's the first test I ran, so I can say with certainty that it was a very small spark. The screwdriver didn't have to quite touch the metal inside the cap, but it had to come very close to get a spark. I'd estimate a maybe 1/16" spark? Maybe less. So it sounds like my HV is low.

I keep thinking back to that Horizontal Hold control. It may be a red herring, but I was noticing some significant power supply defects when I adjusted that control before I recapped in. Specifically, a DTV converter box on the same outlet would actually reset itself as if the power were interrupted if I turned that control all the way up. Obviously I didn't repeat the test after I figured out what was happening, but I hope I didn't damage something doing it. A short somewhere, maybe? I checked the control, and it seemed to test fine, though I need to go back and make sure I'm testing the three leads correctly. I used to know how to do that, but it's been so long I've forgotten the specifics. I did notice that, at the extreme end, it drop from kohms down to ohms, which seemed extreme. I wonder if I've got a bad control, though that wouldn't explain the low voltage elsewhere.

I think I'll still give R79 a looksee. Worst case, I've practiced testing a resistor, and I might find a problem.

[Electronic M]

If the yoke is open or shorted it will greatly reduce the HV and throw off everything else in the horizontal section...Ever pull the H yoke leads on a CTC-15 and power it up?..I have.
Check your yoke like tolooz sayz. That magnet wire is coated in lacquer or some such translucent insulator and you'll need to scrape, melt, or dissolve it off before you can solder it...A short jumper if a section burned off will be fine...Just don't make it longer than it needs to be and don't place it where it could arc to other things.

It could be that pin 9 was being used as a terminal/tie point for other things in the horizontal osc or that it shorted to another pin...Sometimes if the osc is tuned close to the point where it screams adding a small bit of load to the circuit from a meter can push it over the edge in to the scream zone.

[ronl]

Looks like some carbon was created when the arc occured.make sure you scrape and or sand that black stuff off.its conductive.RonL

[zeno]

You have -20 volts on the G-1 of the output so the osc
is running. Testing around the osc will often throw it off
freq so that may be the scream you got.
If yoke wire broken you may be able to patch it if you can dig
out both ends. After that you are pretty much don to a bad
flyback, yoke or damper tube.

HINT ! When yokes get a shorted winding its usually on the
inside windings. Pull the yoke back off the bell. That will give
it air to burn better & you will see it often. Saves time !
If its not burning you may see on the CRT where it was burning.

73 Zeno
LFOD !