Transistor Radio Faults

There are three main types of problem that can stop a transistor radio working. In order of likeliness they are:

Other components can and do fail occasionally, but these will generally become apparent as fault-finding progresses.

Electromechanical faults

Transistor radios are designed to be lightweight and cheap. Because of this the construction will not be as substantial as typical valve sets. Indeed the construction of many sets is fairly flimsy, which probably explains why many of the sets that have survived are in fairly poor condition.

One of the worst problems to deal with is the result of leaking batteries. Sometimes the damage this causes is so bad that the set is only useful as a source of spare parts. If the damage is limited to the battery compartment it can probably be cleaned up and repaired. However if the PCB and other components are affected you should consider whether the set is really worth repairing.

The brown residue can be cleaned away with form cleaner and an old toothbrush. If the battery connectors are discoloured and not rusty, household lime-scale remover may restore them. Replacement battery leads with PP9 and PP3 style connectors are available, although they are a different style to the originals. Springs for battery holders can be obtained by dismantling modern battery holders, and appropriate fixed contacts for the other end of the batteries can generally be fabricated.

If the set is totally dead (no sound, crackles or pops), check the on/off switch. A variety of different types may be encountered, many of which are not particularly robust. Cleaning and retensioning the contacts usually solves the problem.

Another cause of dead sets is the earphone socket. Many British sets used fully enclosed types with a metal bezel and plastic insert, of similar diameter to car aerial input sockets. The switching contact, designed to disconnect the speaker when the plug is inserted, often fails or becomes intermittent. These sockets cannot be dismantled easily, so the simplest solution is to link out the relevant contacts so that the speaker is in the circuit whether or not an earphone is plugged in.

In many sets the PCB is only held with one or two screws or clips. These do not provide sufficient support if the set is dropped, so cracked or broken PCBs are fairly common. The damage is often close to the fixings or near heavier components such as transformers. Broken tracks can be repaired by soldering lengths of tinned-copper wire across the broken section, preferably from one component connection pad to another. Inspect tracks in the area of the damage carefully, with a magnifying glass. You will sometimes find that the crack has extended further than you thought. If necessary the board can be strengthened with epoxy resin, but this can make subsequent repairs more difficult.

Many sets from the Far East use small edge-operated volume controls with an integral on/off switch. Although the controls are reasonably reliable, the mounting method relies on solder and maybe a couple of screws or rivets to the PCB, so damage in this area is not uncommon. The same applies to any PCB mounted control if the board is not adequately mounted or the set is not handled with due care.

Broken ferrite rod aerials can be repaired with superglue if the break is clean and somewhere near the middle. There will be a slight reduction in performance with this sort of repair, but not enough to worry about. If the rod is broken into several pieces it should be replaced. The fine wires from the coils on the ferrite rod aerial can become broken, and the break is not always visible. If in doubt, check with a meter.

The IF transformers are nicely visible in the back of the set, so are an open invitation for someone to fiddle with them when the set stops working. Invariably the wrong tool is used and too much force is applied, resulting in broken cores. For larger transformers in British sets, the repair techniques discussed for valve sets can be used. For the smaller transformers in Far Eastern sets the only option is replacement, however these usually seem to survive being fiddled with.

Some Roberts sets are wired up using thin insulated single-core wire, and the standard of soldering on these connections is often fairly poor. It is not uncommon to find one or two wires that have fallen off and a number of other joints that have only remained attached due to a miracle!

There is any number of other possible electro-mechanical faults, but hopefully this section has given you a general idea of the types of things to look out for.

Electrolytic capacitors

The electrolytic capacitors used in early transistor radios are now well past their intended lifespan, so it is not a surprise to find that some of them will be causing problems. There are a number of different types and makes, and opinion as to which are the worst depends on who you ask. It would be best to view any electrolytic capacitor in an early transistor radio with suspicion.

There are come visual clues. If the rubber piece at the positive end is squeezing out of the can, you know the capacitor is no good. If there are signs of leaking, or the can is corroded, again the capacitor needs to be replaced.

There is a range of red and black plastic capacitors that have a yellow self-adhesive label around them showing the value etc. The label will nearly always be falling off, but this does not indicate that the capacitor is faulty (it just indicates that the adhesive on the labels wasn't much good!). This type of capacitor is, in my experience, no more or less reliable than any others.

Repairers of hybrid televisions are highly suspicious of the moulded black plastic capacitors used in a number of chassis (such as the Thorn 1500). They don't appear very often in transistor radios, but it would probably be advisable to replace any that are found. In televisions they can go off like little bombs, but there isn't enough power in a PP9 battery to do anything so dramatic in a radio.

An electrolytic capacitor can fail in two ways. It can become electrically leaky; this gets progressively worse until it is virtually short-circuit. Or it can dry out, with the effect that the capacitance decreases and the impedance increase to the point where it is doing nothing useful.

The solution, of course, is to replace it with a modern equivalent having similar capacitance and similar or higher voltage rating. Modern components are somewhat smaller but the leads are long enough to fit. A modern component will be a visible repair, but that does not concern most transistor radio repairers.

There is no need to replace all the capacitors in a set, although some dealers who are selling restored sets with warrantees may do so for reliability. Generally it is only necessary to replace any capacitors that are identified as faulty during the faultfinding process. If, during this process, most of the capacitors in a set are replaced, you might as well do the rest on the basis that they are probably not much better.

Short-circuit capacitors can be identified easily with a multimeter on a resistance range while they are still in the circuit. Leaky capacitors will probably show up during voltage checks, since the voltage across them will often be lower than indicated on the service data. To prove the point with these, disconnect one end and solder a replacement temporarily into the circuit. Dried-up capacitors can by proved by simply connecting a replacement across them.

Other components

Most of the other components in early transistor radios tend to be reliable. Even Hunts capacitors, which cause so many problems in valve sets, normally behave themselves in transistor sets. Resistors do not tend to change in value, and transformer windings usually remain intact.

Transistor sets work at much lower voltages, currents and temperatures than valve sets, so the components are under considerably less stress.


Transistor faults, testing and replacement is detailed on the next page.

This website, including all text and images not otherwise credited, is copyright © 1997 - 2006 Paul Stenning.
No part of this website may be reproduced in any form without prior written permission from Paul Stenning.
All details are believed to be accurate, but no liability can be accepted for any errors.
The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
Appropriate precautions must always be taken to minimise the risk of accidents.

Last updated 14th April 2006.