We had an old tiny 1972 B+W 200mm/8" Magnavox TV in our kitchen for years. When it finally died a year ago, I replaced it with a small color set, but always wanted to get the set going again. The set could also be 12V battery operated.
Started out tonight checking the 12V power supply by substituting a
14V bench supply. Same symptoms, very blurry picture. The output of the regulator was right at 12.2 volts, and the B+ Boost was at 305 volts, almost right on the 311V shown on the Photofact 1295. Dragged out my High Voltage probe, it seemed to be only 4.5 KV on the CRT, at any brightness setting. The other CRT voltages were right on.I figured the HV transformer had died, but decided to substitute the solid-state HV rectifier with an old solid-state unit from a 25 kV rectifier from a color set. I figured even with the higher forward drop of the substitute diode since there were many more individual diodes in the 25 KV rectifier, I might get lucky since the current drain would be much smaller. The diode was about 4 inches long, 3/4 inch diameter, with bare ends like the caps on an old horizontal output tube (for those of you old enough to remember what they looked like).
After cutting out the old diode, and putting the new diode in the circuit with clip leads and some judicious use of plastic spacers, I turned the set on. 9.2 KV high voltage right on the spot with the Photofact 9.2 - 9.7 KV. Now I had to figure out how to mount the new diode permanently, since it was twice the length of the old diode and would not fit in the original plastic clips.
I finally connected the new diode to the old leads with alligator clip lead insulators covering the ends of the diode. Additional electrical tape provided further insulation, and I used still more tape to put the diode in the middle of an open space away from anything. Done- so I thought ......... wrong !!!!.
Now the sound was garbled, and it had been fine earlier. I noticed that whenever I put a grounded screwdriver between the diode and the circuit board, the sound improved a little. I tried a small piece of grounded aluminum foil between the diode and the board and the sound was perfect. Apparently there was sufficient radiation from the plastic body of the diode in its substitute location to interfere with the sound circuits which were the closest on the circuit board to where I mounted the diode. A grounded piece of foil covered with electrical tape to keep it from shorting to anything proved a complete cure, and it ended up in a position that should not impair air circulation.
Now my wife can have her favorite set back if she wants. I'll bet though that she will prefer the newer color set.
H. R.(Bob) Hofmann