Vacuum Tube (Valve) Radio and Audio Repair Guide: A logical approach to fault finding

It cannot be stressed too highly that only a systematic approach to fault finding can guarantee a speedy and efficient repair. Haphazard testing is a waste of time, for there is a certain order in which checks must be made, which is from the power supply stage back to the aerial socket. The following sections of this guide dealing with the various sections of receivers have been arranged in the same order, to facilitate servicing. If you study the chart ( FIG. 1) you will find that it commences with ‘set dead’, and recommends actions to be taken, starting with the power supply. This, then, is the subject of the first detailed section, followed by AF amplifiers and output stages, detector! AVC/AF amplifiers, IF amplifiers, frequency changers, and so on. The approach, in all cases, is first .to deal with the basic, conventional circuitry found in the majority of sets, followed by a discussion of ‘unusual features’, found in perhaps only certain makes, or at fairly well-defined eras in the development of receivers.

Obviously, it is possible to join the chart at a lower point, e.g. ‘hiss but no stations’ but after remedial action has been taken it is still sound policy to run through the check list again from the top. A radio which has not been used for some time, due to the fault given as an example, may well develop others after being put back into service. Something as simple as replacing a leaky capacitor in the output stage could forestall an HT overload leading to an expensive or impossible mains transformer replacement job.

Initial tests

When you are faced with a radio set about which nothing is known, it is advisable to carry out some simple checks before plugging it into the mains. The presence of a mains transformer or dropper will indicate whether the set is for AC only or AC! DC operation, unless it is so old as to be one of the comparatively rare DC-only models. If there are any suspicions be guided by the tube (valve) numbers and/or the absence of a rectifier. Assuming, how ever, that the set is for AC or AC/DC operation, test across the mains lead with the ohmmeter, with the radio switched ‘on’. AC-only sets should show a reading of 30 ohm to 70 ohm approximately, whilst AC/DC sets will be much higher, usually from 500 fl upwards, depending on the current rating of the tube (valve) heaters. Should a dead short be registered, check for perished insulation on the lead, particularly where it enters the set.

Many sets had a capacitor wired from live input to chassis, or in AC/DC sets, rectifier anode to earth. A value of 0.01 uF to 0.1 uF is common, its purpose being to reduce ‘modulation hum’ — a 50 Hz buzz derived from the mains and apparently tuned in with a strong station — and it is not uncommon for them to go dead short. To replace, use one rated at a minimum of 1000 V DC or 300V AC.

In AC-only sets the presence of a large value capacitor from mains to chassis can render the latter slightly live even though the component is in first class condition. A shock from this source can be unpleasant, because although not dangerous in itself, it can frequently lead to a hand being grazed as it is snatched hurriedly past a sharp object!

FIG. 1 Fault-finding chart

If all seems to be in order on the mains input, turn to the HT side, and check the resistance of the main smoothing capacitor to chassis, using an ohms range of about 100kg maximum. The needle should flick right over for a second or two, then drop back fairly rapidly to above 10 k-ohm at minimum, indicating that the capacitor is charging from the meter battery, and thus reason ably efficient.

The standing resistance to chassis may be due to a slight short somewhere in the set, or to the presence of ‘bleeder’ resistors used to drop the voltage for certain tubes (valves). If this test is satisfactory, plug the set into the mains, bearing in mind that with AC/DC sets the neutral side must be connected to chassis for safe working. Switch the test meter to the 500 V DC range and connect to the previous HT point and chassis. Within about three minutes maximum, readings should be obtained, and the set should produce some kind of noise.

You are now in a position to employ the fault finding chart and to refer to the appropriate section of this guide. Obviously one cannot claim to be 100% comprehensive, but the faults and cures listed are those which have been experienced most often by the writer in 50 years of practical experience in the radio trade.

Please note the reference on the chart to ‘check loudspeaker’. Many sets were fitted with switches, or plug-and-socket connections to enable the internal speaker to be silenced while an extension unit was in use. Always give these devices a close look, and if necessary connect a test speaker to make sure that complete silence is not due to a simple fault in this part of the set. In fact, always look for the simple things before commencing high powered fault finding. Fuses can be open circuit even if they appear to be in perfect condition, on! off switches do stick in the off position, and ‘dry’ soldered joints can mysteriously make themselves a nuisance after 50 years of working well enough. You could say that Murphy’s Law — if anything can go wrong it will go wrong — and its many sub-clauses might have been propounded with vintage radio in mind but this is not really fair, because so many tubes (valves) receivers have lasted for what would have seemed totally incredible lengths of time to their makers. The last tubes (valves) receivers were made in the early 1960s, which is long enough ago in all conscience, but what worker, assembling a set in 1929, could possibly have imagined it still being in working order 70 years later? In fact, what other domestic appliance even approaches the radio set in large-scale longevity? On this optimistic note let us commence practical servicing work.

Look for previous work!

Experience — bitter experience — has taught the writer always, on principle, to mistrust any previous repair work done on a set. All too often an enthusiastic but careless repairer will have changed components but put in replacements either of incorrect value or of a completely unsuitable type. This may sound both pessimistic and cynical but over the years the writer has encountered the problem so many times as to make it almost an occupational hazard. Always look out for suspiciously new-looking components and check them out before doing anything else to a set.