I've run it for a couple of hours now and nothing is overheating, so I'm comfortable with the modification.

I've run it for a few hours now with nothing overheating, so I'm convinced the modification is OK.

Excellent and a very neat early color TV to own.

I may have solved the mystery. Today I turned on the set and forgot about it. When I returned 3 hours later the focus had deteriorated. I let it cool down, turned it back on, and the focus was fine, though the control was full up.

When I first started restoring the set, I noticed that it had a solid state rectifier in place of the 3A3 focus rectifier. Because I wanted the set to be as original as possible, I took it out and installed a 3A3. The high voltage died. I assumed I had a bad 3A3, replaced the solid state rectifier, and planned to get back to that issue later.

There are 4 parts in the focus circuit; the rectifier, a 1500 pf doorknob on the cathode of the rectifier, the focus pot, and a 10000 pf doorknob on the wiper of the pot. First I replaced the 10000 pf doorknob. Still had poor focus after a couple of hours. Then I put a 3A3 in place of the solid state rectifier. High voltage died, plus the 3A3 got so hot that the glass cracked.

I had to quit for the day. I think that the 1500 pf doorknob is leaky, more so when it gets hot. The excess current drain caused by the leaky cap destroyed the 3A3, and, when the solid state rectifier is in place, it loads the flyback enough to reduce the width.

Tomorrow I'll test the theory by replacing the doorknob. Stay tuned.

Sounds likely, my CT-100 had a leaky cap in the focus circuit that was dragging down the focus voltage after about 10 minutes. Cap replaced, now everything's fine.

I'm going to stay focused.

That fixed it. Plenty of width without the capacitor across the damper.

Good show. So maybe the lesson is that it's not always a bad idea to leave a vintage set on unattended :D

Excellent diagnosis and troubleshooting.

I remember the old adage comparing TV repair with a medical doctor's job:

With the medical doctor, if the patient is dead, there is nothing he/she can do. With a TV, the deader the set is, the easier it is to fix.

I suppose the Sparton wasn't dead enough to make the fix easy.

Well, I still have a problem. The focus still goes out after a couple of hours, with the focus voltage declining from 2.5kv to about 1 kv. I'll experiment tomorrow. I put back in the original 10000 pf doorknob, and it may be bad too. Also, there is a possibility that there is a problem in the dynamic convergence transformer, through which the focus voltage goes.

Check the focus pot too, I don't know how the schematic in this set is but on the CT-100 when the focus cap on the wiper of that pot goes bad is smokes the pot.

Final report: the problem with the loss of focus turned out to be a 6BG6 with corroded pins. For no particular reason the filament stopped working, so only one 6BG6 was working, resulting in only about 12 kv high voltage and 1.5 kv focus voltage. After changing the tube, the set works well and is stable.

Excellent news, I look forward to viewing this set at the next ETF convention.