By Frank McCoy
This case happened almost 40 years ago when tube televisions were the only kind. However, the solution is still apt in some cases today.
I was a TV technician in a mom and pop TV repair shop; and I might say proudly, one of the best. Only once did I ever get a problem that stumped me completely.
A small Admiral TV came in with a vertical frequency problem. The usual solution was to replace the vertical oscillator, the feedback capacitor, and the low-frequency filter component for separating out vertical sync. I did so, and the TV worked just fine ….
… for about two hours. Then the vertical went wild!
So … I put it back on the bench … The set worked just FINE. I left it running on the bench; and about three hours later … away it went!
I touched a probe to the vertical circuit … and the problem went away for twenty minutes!
It turned out that touching almost any part of the circuit with a voltage or scope probe would trip the thing back into running; and then it would work again. I checked the parts I replaced; and all were well within spec. The filter was new, the tube was new, the feedback capacitor was rated at 2000 volts (spec. called for 1000) and NPO, so it shouldn’t be changing value with heat.
After a while I got angry and replaced EVERY component in the vertical circuit except the tube socket and vertical transformer; and I knew it was neither of those. Same problem.
We finally called the customer, explained what was going on, and they told us to scrap the set.
It went under the bench; and as far as I know is still there.
About six years later, in ANOTHER shop, I had a similar problem. What the? I never would have figured it out, except that I by accident touched the vertical feedback capacitor with my finger while the set was turned off … and burned it!
The factory original capacitor was a paper type, about 1/2″ around and two
inches long to handle the high voltage. The replacement was a tiny DISC ceramic capacitor about 1/4″ circular and 1/8″ thick. Though the replacement was twice the voltage rating and NPO (or supposedly non-temperature-sensitive) it just wasn’t built physically to handle that much current! It got HOT! Way beyond specification. I replaced it with a modern film capacitor and had no more problems.
The moral is: Capacitors have CURRENT ratings as well as voltage and capacitance. This is especially so with modern high-current power-supplies. I’ve seen some blow up like small firecrackers when the AC current through them was too high. Remember: In HOT locations, you might need a capacitor rated for two or three times the actual current. Also, try and measure the RMS AC current through the filter to be sure the ratings aren’t exceeded. Most especially do so when designing. You might need a much bigger capacitor than you thought would be necessary just to keep the AC ripple low and down to specification.
In this case it was SPIKE current through a relatively tiny capacitor. Who would have guessed that a .002 uf capacitor rated at 2000 volts in a 60hz circuit could possibly have enough current through it to melt the wax and burn fingers?