High Voltage Trouble Shooting

[Don Lindsly]

You state damper voltages all measure OK.

There is not much between the damper cathode and horizontal output plate, just the linearity coil and flyback. If nothing is running red and there's no smoke, then look for an open flyback, open lin coil or bad connection.

Don

[vts1134]

Originally Posted by Don Lindsly

You state damper voltages all measure OK.

Er um, I took another extended reading today and found that voltage on two pins were jumping around on my meter. I don't have the worlds best meter, and it's digital so it can some times be hard to read. I found that the horizontal on the yoke, which is supposed to read 17Ohms according to the schematic, is reading close to 90Ohms. After removing the crt and peeking at the yoke it looks a bit melty.

Could that cause the voltage problem with the horizontal output plate voltage?

[Don Lindsly]

Some digital meters will be deceptive on ohms because the high inductance fools the meter. Use a Simpson, Triplett or Analog VTVM for DC measurements on high inductance components. The results will be more believable.

Try the same thing for DC on old TV boost and B+ circuits.

Don

[vts1134]

Assuming the reading on the yoke is correct with my DVM, could the much higher impedance cause the damper tube to waver on it's voltage and in turn cause the flyback voltage problem? If it's not even possible for the yoke to cause that problem then I'll begin look elsewhere.

[Don Lindsly]

I have found the best procedure for troubleshooting high voltage is:

1. Visual inspection, burned stuff, broken wires, unlit tubes, bad fuse, damper or HOT running rep plates, etc.
2. Make sure all tubes are OK
3. Check for RF on HOT plate cap using an insulated screwdriver and draw a purple arc.
4. Check for grid drive and screen voltage. If no RF, check for HOT plate voltage.
5. If there is RF, remove the 1B3 cap and look for a much bigger RF arc at the transformer cap. If no arc, replace cap. If significant arc, look for trouble under the 1B3 like shorted doorknob cap, filament winding or CRT air.
6. Check damper plate and cathode. Note damper cathode (boost) voltage
7.Disconnect yoke horizontal windings. Recheck boost voltage. If it increases much, the yoke is likely bad. If it stays the same or decreases, rule out the yoke for now.
8. If there is no smoke or overheating, let the TV run for a half hour. Turn it off and feel the flyback coil close to the core. If it is hot, the flyback is likely arcing internally. If not:
9. Recheck HO Tube control grid drive for proper level and frequency.

These steps should get you close. If not, it needs a couple of quick scope checks like frequency and ringing. It requires only a mediocre scope. HPs and Teks are overkill.

I prefer a VOM or VTVM for troubleshooting old tube stuff. They are less sensitive to low level RF riding on the B+ like boost and provide accurate DC resistance readings for inductors.

Don

[vts1134]

Thanks so much for the well written step by step, it's very helpful.
It looks like it is the yoke. Disconnecting the horizontal windings made the boost voltage increase sharply. Looks like I need a DY-1.

[Don Lindsly]

You can "tack" any yoke in to see if it fixes the problem. Even a mismatch will restore the high voltage and picture if everything else is working. Just find one that fits around the CRT neck.

Otherwise, carefully remove the CRT socket and tack in a test yoke using clip leads and let it lie on the bench. If the old yoke is bad, there will be lots more high voltage and boost with the test yoke. Then do the mechanical work to get installed.

Don