Old TV Components
The components used in old TV's are essentially very similar to the components used in modern TV's; that is, they perform the same function as their modern counterparts. Where they differ greatly however, is in their size. Because of the high power requirements of valves, the components employed had to be able to cope with the large currents and higher voltages to be found in these old sets. These "High power" components can be more difficult to find than the more common "Low power" components that are found in most modern equipment. Luckily, there are still places where these components can be found. There are dealers about who specialise in genuine old components, and these can be beneficial if a "Totally original" restoration is being carried out. However, if it isn't too important for the restoration to be totally original right down to component level, then more reliable, modern components can be used. These can be obtained from most Electronic Components companies such as (In the UK) RS Components, Farnell or CPC.
Wax Capacitors
High voltage (Up to 400VDC) low-medium value (0.01uF - 2.2uF) capacitors to replace the horrible leaky wax variety can be quite difficult to find, but I can recommend the MKT1813 series made by Vishay Roederstein. These can be found on the RS Website by using their search facility and entering "MKT1813" (Without the quotes.) If you cannot find them with this, use the menus to select: Electronics - Capacitors - Polyester Capacitors - Polyester Capacitors, Axial Leads.
Higher voltage (Up to 1500VDC) low-medium value (0.001uF to 0.4uF) capacitors can also be found at RS in the form of the Arcotronics KP1.72 series. These capacitors are very useful for replacing the chunky high voltage capacitors often found in the line output stage of old TV's. These can be found by searching for "KP1.72" (Without the quotes) or using the menus to select: Electronics - Capacitors - Polypropylene - Polypropylene Axial Capacitors.
Electrolytic Capacitors
General purpose and smoothing / resevoir electrolytic capacitors can also be found on the RS Website by using the menus to select: Electronics - Capacitors - Electrolytic. This type of component can vary greatly, so you will have to search for the particular item that you are after in RS's menu system.
Other Low Value Capacitors
Low and very low value (Up to .001uF) capacitors (Often used in I.F. / R.F. stages) can be found quite easily from most component suppliers, but again, these can be found at the RS Website by selecting: Electronics - Capacitors and then whichever type of capacitor that you need. (Ceramic, Polystyrene, Polycarbonate etc.)
Resistors
The resistors used in most old tellies can be replaced with standard modern components without any problem. Mostly they are 1 or 2 watt and are of standard values. In very early televisions, manufacturers sometimes used completely non standard values, but they are generally of low tolerance so the next nearest modern value will usually be fine. Big dropper resistors can be a bit of a problem to replace. In years gone by, certain component manufacturers made 5, 10 and 20 watt resistors specifically designed to replace faulty dropper sections, but now these are no longer manufactured and mostly these will have to be sourced from an old-component dealer or replaced by second hand parts from another TV.
Transformers, Coils and Chokes
Faulty transformers are one of the worst components to source and can often be the difference between keeping an old set going or having to write it off (at least until the right transformer comes along.) Generally speaking, transformers, particularly line output or EHT transformers, have a tough life in a television set, and so it's not too surprising that they should be one of the most unreliable components. There are companies about that will attempt to re-wind old transformers so long as they (or you) have all the data about the windings and tappings. Amateur radio rallies and junk sales can occasionally turn up spare transformers, but probably the best source for a rare transformer is of course, a scrap telly of the same type as the original.
Small coils, such as are found in the I.F. and R.F. stages of a set, can often be repaired or rewound. Very often if the coil has a shorted turn that you can see, the two wires can be prised apart and then treated with varnish or some other insulator. If the coil has gone open circuit, it can often be the connection between the ends of the wires and the pins that connect the coil to the rest of the circuit. In this case, a touch with a soldering iron can fix the coil without the need even for it to be taken out of circuit.
Valves
Despite what people think, most valves, even those found in the earliest televisions can be quite easily sourced from specialist valve dealers, scrap radio or television sets, at amateur radio rallies or by searching for the valve type on the Internet. Most old TV restorers keep a stock of valves in their collection for such occasions.
Cathode Ray Tubes
A faulty Cathode Ray Tube is another component that can cause a restorer to have to write a set off, at least until a replacement turns up. If the tube has low emission (Not enough electrons are being released from the cathode,) a temporary cure to brighten up the picture is to increase the heater voltage slightly. This has the effect of releasing more electrons from the good parts on the surface of the cathode. Unfortunately this method doesn't last very long and the voltage will keep needing turning up until the heater blows and then the tube will have to be replaced. Another method for rejuvenating a low emission tube is to strip the top layer from the cathode. This can be done using a tube booster designed for the job or by making a simple circuit up for the particular kind of tube that you are using. Again, this method may not be terribly satisfactory. If you strip too much off the cathode by running the booster for too long, then the tube will be U/S and will have to be replaced. Even if you manage to get a bit more emission from the tube, the surface of the cathode may have been stripped to a very thin layer and it may fail again after a short time.
There are companies around, who may (depending on the type,) "Re-Gun" your tube. This method can be highly effective and is the next best thing to a brand new tube. The down side is that it can be very expensive and the tube will have to be packaged and sent away with the utmost care if accidents are to be avoided.
New tubes do very occasionally turn up, and you are a lucky man (or woman) indeed if you can find a brand new tube of exactly the type that you are looking for.