High Voltage Trouble Shooting
 

Can any one give me a couple of pointers in troubleshooting an early B&W set with no high voltage at the crt anode? I've searched around the site but the only thing I could find is to remove the horizontal output tube from the set and meter the top lead. Can any one suggest any other general steps to take when testing a flyback, high voltage rectifier, and other components in the high voltage section. Having an emphasis on safety while working in that area of the set being the first priority of course.

The high voltage is generated by a flyback transformer in the plate circuit of the horizontal output tube. Check the tube, make sure it has proper voltages, and a goodly amount of drive.

When the tube cuts off with a negative grid pulse, the plate current stops but the inductance of the flyback transformer primary causes a violent polarity reversal. This creates a large voltage in the secondary, as well as a bit of juice to light the high voltage rectifier (maybe a 1B3GT?). Put the oscilloscope on the grid and see what you have. Refer to the waveforms in the manual.

you should see B+ at the cap lead with it disconnected, dont try to read it with it attached. as the other poster said check for a drive pulse with a scope a the grid of the HOT or, if you lack a scope, check for a neg voltage, should be around -35v. check the screen voltage. replace the HOT, Damper, HV rectifier with new tubes.

this is where a B&K analyst comes in handy, besides use as a video signal, it has its own built in plate drive (hook up to the plate cap lead), grid drive,(hook up to the grid of the HOT) for trouble shooting. Do you need it, no, but if you have one it comes in handy.

check for an open resistor in the filament lead of the HV rectifier (if it uses one).

You can also use a neon bulb held near the plate lead (the B&K has one mounted on a plastic stick, you dont want to get your hands near the plate cap).

lastly try unhooking the CRT anode, just to be sure is not loading down the HV.

oh and saftey, be aware the the HV rectifier can bite as well, the bottom of it is connected directly to the CRT anode so care when probing down around the base of it (yea I got bit once by that). Also the CRT anode can "recover" even after being discharged so be aware of that. A HV prob is a must IMHO, always assume the anode is charged.

A make and model/chassis number would help to provide specific answers.

The make and model that I'm working on currently is an RCA 730TV1. One other question I had on the subject is measuring the flyback itself for continuity and resistance, sort of a quick spot check to make sure it looks ok with the power off. Any suggestions for the "I always take a reading of this to make sure it's not (toast/open/etc) first"? By the way I guess the first thing would be visual inspection to see if it's torched. I've seen some pictures here of flybacks that look like they went through the toaster oven at a local sub shop, wax melted off, charred to a crisp. I've not come across a fly personally in that condition. The one in this set is near to immaculate ascetically speaking.

In my instance at hand I've got no voltage at the plate cap lead on the HOT. The voltages every where else on the HOT read normal, and the voltages on the damper do as well. I'm not sure if I have another damper tube on hand, but all of the tubes in the set are checked and are either new, or tested very good at my local antique electronic shop.

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

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.
http://farm7.static.flickr.com/6056/...9c00ef8bb8.jpg
Could that cause the voltage problem with the horizontal output plate voltage?

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

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.

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

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.

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

Unfortunately I do not have a spare yoke so I'll have to wait for an exact replacement to test with.