By Paul Baio
When dealing with electronics and in the TV repair business, experience is golden - there's no substitute. Here are some tips that my colleagues and I have learned over the years, on using a good DMM like a Fluke 85 to repair older television sets.
1. After repairing an older, quality television like a Sony, Mitsubishi, Toshiba, or anything that cost a lot of money, you end up with a lesser quality picture due to the age of the CRT and now having lower emissions.
Measure the filament voltage at the CRT neck (you need a meter that can measure True RMS AC voltage in the 15 kHz range), and you should see 6.3 V ac. If the voltage is lower -- and remember, tenths of a volt matter here -- then reduce the filament resistor by halving it, then halving it again, or jumping it out altogether if you have to in order to get the filament voltage up. You can go as high as 6.5 V ac if you have to, in order to heat the cathode up enough to boost the CRT emission and increase contrast on the picture tube. This will yield a noticeable increase in the quality of the picture, getting a few more years out of the aging set.
Use your meter and make sure not to exceed 6.5 V ac, or you risk opening the filament in the CRT, rendering it useless and dark forever. If the filament power supply is no longer functional, you can also do a quick boost by taking a two to three foot length of quality wire and wrapping it a single turn around the body of the flyback transformer. Then, connect the two ends to a 4.7 ohm, five watt cement resistor in series with the filament and follow the above procedure to halve the filament series resister. My friend and mentor, Dean, gets credit for this very valuable tip.
2. Another great tip comes from my experienced friend Richard. He could fix just about any television set with a schematic or without one (usually, we didn't have schematic diagrams or parts lists on older sets). Often a tech would come across an open resistor while repairing a TV set, maybe in the power supply to the vertical output circuit. You replace the vertical output IC to find no V cc present on the IC. You trace it back to a shorted diode and an open resistor fed from the horizontal output transformer. The diode we can replace but what value was that resistor we just took out?
Metal film is the most common type of resistor found in home electronic, so if you take a very fine, small metal file, and rub it across the body of the open resistor, you will expose the diagonal stripes of the metal film resistance metal. Put your ohmmeter meter probe on one lead of the resistor and put the other probe on the now exposed body of the resistor. Move it along from the lead at the meter to the other end of the resistor body until the continuity is broken.
At that point, write down the resistance you found up to that point. Now, put your ohmmeter probe on the other resistor lead and do the same thing, moving it along the exposed body of the metal film resistor until you lose continuity. Write down that value and then add them up.
Remember that the test leads will add resistance to your reading. Here is a tip from the Fluke 80 series manual:
Now compare your total to a resistor preferred-value chart and come up with a standard value to compare your total against. For example, a total of 23.7 ohms is close to a preferred value of 22 ohms, which is a standard resistor value. Insert a 22 ohm resistor into the circuit of the correct power range, and the circuit will work.
This is no way a guarantee that the value will be absolutely correct, but electronics is rarely an exact science and getting very close is usually good enough. This method works great when you just don't have the schematic and need to get the job done.