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Early Electronic Television

Advice for Restoration of Old TV Sets

These tips are for those who are restoring postwar tube-type TV sets. I don't recommend that you tackle TV restoration unless you have prior experience with radio or television restoration or repair, and at least basic test equipment. It is almost impossible to restore an old set without a schematic diagram, so the first step should be to acquire one. Sams Photofacts are the best, since they contain voltage listings for all of the tubes, and other important information.

Television restoration is much like radio restoration, only with many more circuits, some of which are inter-related. For that reason, I follow the same sequence each time I restore a set.

Cleaning the set. Before doing any electrical repair, remove the chassis from the cabinet. If the speaker or picture tube are attached to the chassis, remove them. Take out all the tubes (valves), and remove the cover to the high voltage (EHT) cage. Then, put a liberal quantity of dishwashing detergent all over the chassis, and spray the chassis with water. Using brushes of various sizes, remove as much of the grease, grime and dirt as possible. Then, rinse the soap off. Water won't hurt the components,  but be careful not to soak paper labels, which might come off. Then, let the chassis dry for several hours. Don't apply AC (mains) power to a chassis until it is thoroughly dry.

Replacing Paper Capacitors. Many of the paper capacitors in your set will be bad, and many more will go bad after power is applied. So, all of them should be replaced before you do anything else. Though this is time consuming, you will actually save time in the long run, since debugging will be much simpler. My experience is that about 70 percent of the sets I restore come to life without further work after replacing the capacitors. Be sure to use capacitors with the same or higher voltage ratings.

If your set is rare, or if you want to retain its original look, you can install new capacitors inside the old shell. Here are instructions. Otherwise, simply cut out the old one and solder in a new replacement.

Don't replace any of the mica capacitors, since they are rarely bad. They are usually in tuned circuits, and you may have to re-align the set.

Applying Power. Put the rectifier tube (valve) in (if the set has one. Also, if your set has series filaments, you will need to install all the tubes (valves) at this point), and apply about 1/2 the normal AC (mains) voltage to the set. If you don't have a variac, you can rig a lamp socket in series with one leg of the AC (mains) input and install a 100 watt bulb in the socket. This will drop the voltage to the set. By changing the wattage of the bulb you can vary the voltage.

Put a voltmeter on the B+ line at the rectifier. You should get about 1/3 to 1/2 of the normal voltage. Leave the set on for a while, then feel the electrolytic capacitors to see if they are warm. If everything is fine after an hour or so, raise the AC voltage, and again check for warm electrolytics.

Horizontal (line) Sweep. The next step is to get the horizontal (line) oscillator to work. Install that tube (valve). If you have an oscilloscope, connect it to the control grid of the horizontal (line) output tube (valve). You should have a waveform of 20-40 volts at a frequency around 15 kHz (10 kHz in 405 line sets). If you don't have a scope, use your DC voltmeter to look for a significant negative voltage on the control grid. If you don't have horizontal sweep, you need to find out why.

First, swap the oscillator tube (valve) with another of the same type. Then, measure all the resistors in the circuit. Replace them if they are more than 20% off. If that doesn't work, you will have to work your way through the circuit using the Sams voltage measurements until you find the problem. The same process will be used to debug each section of the set.

Now, install the horizontal (line) output, damper,  and HV (EHT) rectifier tubes (valves). Hold a screwdriver near the cap of the HV (EHT) rectifier. If you have a healthy arc, then the sweep is probably working. If you have a HV probe, measure the DC HV (EHT), or, if not, hold a screwdriver from the chassis to near the HV (EHT) connector to see if you have at least a 1/4 inch arc.

Looking for a Raster. At this point you can install the picture tube. Measure the cathode and grid voltage on the CRT. By turning the brightness (and sometimes the contrast) control, you should be able to get the grid voltage close to, but below, the cathode voltage. If the tube has an ion trap, rotate it and move it in and out along the neck until you have an image.

Vertical (frame) Sweep. Next, install the vertical (frame) oscillator and output tubes (valves). Debug these circuits to get a raster with good linearity.

Tuner, Video IF, and Video Amplifier. Now, install all the rest of the tubes (valves). Connect a VCR to the antenna input, and set it so it can be used to tune off-the-air or cable channels. Put the TV channel selector on channel 3 or 4, depending on which channel your VCR puts out.

In most cases, you will have some sort of picture. If not, you will have to go through and find where the problem is.

In my experience, IF alignment is rarely required. Don't attempt to do an alignment unless you have a sweep generator and the alignment instructions. You will  only make the picture worse if you randomly adjust the coils.

Sync. Check to make sure the horizontal (line) and vertical  (frame) lock. If not, debug the sync separator.

Audio. Connect the speaker and listen for sound. You may have to adjust the fine tuning to bring in the sound, especially on sets without intercarrier IF.