Here we examine the various components employed in valve radios and likely faults to be encountered with them.
CAPACITORS



Here we see the common WAX COATED CAPACITOR,many manufacturers made these including,Plessey,T.C.C and Hunts. The big problem with these are they tend to leak. The way to test these is to clip the earthy end of the capacitor switch on the radio and check using a digital multimeter on the clipped lead. It should be zero volts. If it is not short the clipped lead to chassis and lift off and try again,if the voltage is not zero(assuming there is high voltage on the other end of course) the capacitor is clearly leaking.

The silver component above the wax capacitors is an ELECTROLYTIC CAPACITOR. You will usually encounter these in all sorts of shapes and sizes,from large upright silver cans to small red and yellow ones in the more modern sets. Electrolytics tend to dry up when subjected to heat,and as a result go low capacity. You can test these using a capacitor tester or an avo 8 meter. NOTE DISCHARGE THE CAPACITOR FIRST! You can compare the avo meter reading on a known good capacitor of the same value. The ohms range you use will depend on the amount of capacity you are checking i.e something like 0.0001 uf will need the use of the "high ohms range",something like 10uf will be ok using the "low ohms range".

I have included the TRANSFORMER on the left of the picture to jog your memory,for if you replace the capacitor across the primary winding.USE ONE RATED AT 1000VDC OR HIGHER AS THIS COMPONENT HAS TO COPE WITH HIGH PEAKS OF AC VOLTAGE.IT WILL BREAK DOWN IF TOO LOW VOLTAGE ON IS PUT IN.

Capacitors can also go short circuit and open circuit.

I have known the connection tag rivets become high resistance and result in arcing. For my more experienced readers you will probably remember the Philips G11 colour television and the problem this could have with the line output transistor...those were the days!!



VARIABLE CAPACITORS such as the main tuning capacitor can suffer from the two sets of vanes shorting out intermittently when turning it. You can carefully bend these back into alignment but it takes a steady hand and quite a bit of patience. It pays to fully open the capacitor,use a soft paint brush and using a vacuum cleaner suck all the dust out from in between the vanes. This dust can become conductive with little pieces of metal from the vanes themselves. If this does not cure the problem try lightly oiling the ball race and earthing strips on the shaft. Still no go? Looks like bent vanes,turn radio on tune into a station if you can,and keep turning until you hear a crackle through the speaker.pull the mains plug out.now check with your analogue meter while turning the capacitor(i know you need three hands) and see if the needle goes over,if it does you definately have shorting vanes. By visually looking at them you should be able to see which one(s) is the likely culprit and CAREFULLY bend it straight. This is difficult but it can be done.



Mica trimmer capacitors or PADDERS as they sometimes are called,can sometimes go short circuit because the mica insulation cracks or rips. This can cause a silenced set. The picture above shows six of them on the bottom of the chassis.

RESISTORS

These can take the form of a small carbon type to a large wirewound dropper resistor.



First of all we will deal with the CARBON type. These usually have bands painted on them or in the case of older resistors a spot system. Both types of resistors can tend to go high in value or open circuit. In the case of the older resistors the legs tended to be wrapped round the body,these can become loose and become detached from the body causing an open circuit.



WIREWOUND resistors are used when you need to dissipate a lot of power. They are available from about 3 watts to over 100 watts. The bigger they are the hotter they get something to consider when putting them in a small radio.the faults you encounter with these are usually a break in the wire causing an open circuit.



VARIABLE resistors such as volume controls and tone controls usually have three connections,two for the track and a centre one for the wiper. Problems here are usually an open circuit track or a noisy volume control where dirt contaminates the track and stops the wiper making an efficient connection. A good dose of switch cleaner usually works here.



THERMISTORS are resistors that change in value when a voltage is applied. They can be low resistance when cold and high when hot or vice versa. These are usually used as surge limiters. Faults with these are usually things like high resistance and they can tend to crack in half,not unlike the three legged deguass thermistors found in colour tvs that so often fail.

TRANSFORMERS(I.F.,AUDIO OUTPUT,MAINS)

I.F. TRANSFORMERS can be quite difficult to repair if they become faulty. Their worst enemy is "green spot" where corrosion eats through the copper wire and causing it to become open circuit. It can also cause windings to short out too. This is usually caused by storing the radio in damp conditions like a shed,garage or damp cellar. Another problem can be caused by the cores breaking within the core housing,this usually causes the if's not to peak when trying to align them. Also the small mica capacitors within the metal can often "drift capacity" or go completely open circuit. This also will prevent the if's from peaking.



AUDIO OUTPUT TRANSFORMERS like the ones above mainly suffer from open circuit primary windings,this can be caused by"green spot" or the output valve drawing too much current. Another point worth mentioning is that if you operate a valve radio without the speaker connected this has been known to break down the output transformer or cause shorted windings.



MAINS TRANSFORMERS are quite reliable i have found. But they dont like damp conditions. This can cause shorted windings causing the transformer to overheat and burn out. Another problem is a burnt out transformer caused by the VOLTAGE TAPPING PLUG being on the wrong voltage. Always check this.

LOUDSPEAKERS

There are different types of speakers that were used in radio,HORN SPEAKERS,MOVING IRON,INDUCTOR DYNAMIC AND MOVING COIL. The first three are more or less obsolete but may still be encountered in older sets from the 20's and 30's. They tended to have a very high impedance of around 2000 ohms so they could be driven direct from the valve without the need for a transformer. The flat metal plates that sit on top of the coils has to be totally flat or there will be little or no volume produced. Also because the wire in the coils is very thin,it does tend to suffer from "green spot" and open circuit coils.




MOVING COIL SPEAKERS like the one above,suffer physically more than electrically in my experience,they sometimes have OPEN CIRCUIT SPEECH COILS,but this is not common unless the speaker has been grossly overrun.

They are more likely to suffer from RIPPED CONES due to careless handling or foreign objects being pushed through the speaker grill/cloth.

Another problem that can happen is usually associated with damp conditions,where you get bits of rust falling down the gap between the magnet and the speech coil causing A SCRATCHING COIL.

Another very common problem that i always seem to get is an OUT OF CENTRE CONE,where the coil is not dead centre within the coil gap in the magnet,this causes a sticking cone or the coil rubs the magnet as its going out. The effects of this may only be realised at low volume levels though but are quite unmistakeable.

VALVES


Here we have a selection of valves with the general age of use going from left to right.

Valves suffer from a variety of problems a major one being CRACKED GLASS,valves are quite robust electrically but not physically. The B9A,B8A AND B7G valves tend to suffer from cracked glass near the pins where they come through the glass. LOCTAL VALVES LIKE THE EF50 suffered too.

Another one is where AIR HAS GOT IN causing a valve with a white deposit on top,this causes the valve either to not work or spark internally. Valves are VACUUM devices and any air that gets in renders them useless.

Valves can become LOW EMISSION caused by the cathode being stripped of its emissive material because of over biasing. This is usually caused by positive volts on the control grid(g1) with respect to cathode,this causes the valve to be overrun.

On the older valves the BAKELITE VALVE BASES tended to become DETACHED from the glass and could snap the wires FROM THE BASE TO THE VALVE. Use Araldite epoxy resin here.

The TOP CAP on the older valves sometimes becomes detached from the glass,sometimes snapping off altogether leaving a broken wire at glass level,this is a bit risky to solve,but i have had success this way,gently use a needle file edgewise in an upwards direction and try to file a little bit of the glass away to reveal the wire. I have done it and it works but not always. But its worth a try.

The HEATERS inside the valve can sometimes go open circuit but with older valves and the E SERIES valves such as EBC33,EL84,ECC85 etc,this was uncommon. It tended to happen more with the U SERIES valves such as UL84,UF89,UY50 because the heaters were used in series from the mains supply via a dropper resistor,and the resistance of these was lower when they were cold and the sudden surge of mains tended to make them glow very brightly for a split second then go dimmer. I suppose if this keeps happening over a long period of time i would weaken them eventually. By far the most problematical valves with heater problems were the B7G battery valves such as DF91,DAF91,DL96 etc these valves used heater voltages of just 1.4v and were super critical,if this voltage was deviated from the heaters used to go....puff...and that was it. Something to bear in mind if you are brave enough to repair these sets.

INTER ELECTRODE SHORTS were fairly common with the ECL86,ECL82 type valves,they did not seem very tolerant of being overrun and tended to develop these problems for some reason. If anybody knows please email me using the contact page,i would be very interested to know why.

The bigger older valves such as the TDD4,FC4A,ACTP seemed to suffer from INTERMITTENT ELECTRODES WHEN TAPPED. The only cure here is to replace the valve.