Portable radios

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This page covers the practical aspects of diagnosing and fixing portable radios.

Summary

Many fixable problems with portable radios are common to many types of electronic gadgets and only require a rudimentary understanding of electrical theory. A slightly deeper understanding may allow you to localise some of the less obvious problems. This is covered in the companion page How radios work, which also explains any abbreviations or technical terms you may not be familiar with in this page.

Safety

Warning03.png
Mains powered radios (except those powered by a separate "wall cube" type adapter) are likely to contain dangerous voltages when plugged in. Don't plug them in while the case is open unless you (and any bystanders) understand how to manage the risks.
Virtually all vintage valve radios operate at high voltages. A large smoothing capacitor may retain a dangerous voltage after the radio is switched off.

Types of radio

The oldest radios you might come across use valves. Individual components are wired together rather than being mounted on a printed circuit board. They are often AM only (MW and LW, possibly SW) but post war ones may also receive FM.

Vintage transistor radios date mainly from the 1950's or 60's and use germanium transistors, which are less robust than more modern silicon types. Later radios use silicon transistors, and increasingly through the 1980's and 90's used integrated circuits for some or all of the internal functions.

All the above types of radio almost invariably follow the superhet design.

Digital (DAB) radios are the latest type, introduced in the late 1990's. Some also incorporate FM.

Common problems

Telescopic aerials on FM and DAB radios are relatively easily bent or broken. Close inspection will show whether there is any chance of a repair. As a temporary measure, replace it with a length of wire (say, 50cm, or similar in length to the original aerial). You should be able to secure it at one end under a screw securing the base of the aerial.

Look out for other purely mechanical problems such as the linkages, pulleys and strings which operate the tuning dial on an older radio. Corroded contacts in the battery compartment is common in most types of battery operated equipment, resulting from dead and leaking batteries being left in the device.

Opening a radio is not always straightforward but is nearly always possible once you find the right screws. Sometimes you may come across a combined radio/cassette/CD player which won't come apart easily. You may have to search for hidden screws underneath a label, rubber foot or speaker grill, and the order of disassembly (and reassembly!) may be critical.

Once inside, look for common problems such as broken or shorting wires, signs of burnt or overheating components and bulging electrolytic capacitors.

If the radio shows no signs of life, check for a power supply problem.

If you can identify the audio amplifier as a separate module and can identify the audio input to it, touching this with a finger should produce a hum from the loudspeaker, or touching it with a metal screwdriver while you hold the metal shaft should produce a crackle. This indicates that the audio amplifier is working and the fault is probably in the tuner. (Never try this on a valve or other mains powered radio.

Push button switches can become unreliable with use. Beneath the external button there is often a standard tactile switch soldered to the circuit board. Sometimes a simple solution is to insert some packing, such as a folded piece of paper, between the external button and the tactile switch, allowing a little more pressure to be applied. Replacements are available very cheaply on eBay in a range of sizes. As a short term measure you may be able to swap an unreliable switch with another on the same circuit board operating a function that is rarely if ever needed.

AM/FM radios

Vintage valve radios are likely to contain a thick layer of dust over the internal components which may impede cooling. Clean it out with a vacuum cleaner.

A valve radio which has seen much use may benefit from new valves. These may be available on eBay, particularly the miniature all-glass types generally used in post war radios.

AM only and AM/FM radios contain a number of tuned circuits which are adjusted by the manufacturer to the same frequency or frequencies by means of trimmer capacitors having an adjustable screw head and coils with a ferrite core that can be screwed in or out. DO NOT adjust these - you need specialist equipment and the service handbook for the particular radio in order to do so, without which you are very likely to make the radio much worse rather than any better. As a last resort, having understood how a superhet works, adjust one at a time, noting the exact position of the screw head before the adjustment and returning it to the same position if there is no significant improvement. If you succeed in making an improvement, make sure the improvement is maintained across the tuning dial.

A crackly volume control is a common problem in older radios. This will be a potentiometer and may well be a standard widely available part, relatively easily fitted. As a short term measure you may be able to effect an improvement by squirting switch cleaner into it through a gap in the case. Operating the control repeatedly from one limit to the other should then help to shift any dirt on the carbon track.

In the case of a vintage transistor radio, check the type codes on the transistors and look them up on the Internet. If they are germanium rather than silicon types, these are much less robust and it is possible that one has failed. There's a chance this might still leaving the radio working, but only just.

If you can identify which lead of a transistor is which, try testing the voltage between the base and the emitter, which should be around 0.3V for germanium types and 0.7V for silicon. There should be a greater voltage between the emitter and the collector, probably at least 1V and more likely several volts. If there is no voltage that is a strong indication that the transistor has failed.

If you remove transistors for testing (you can use the transistor test function on many cheap multimeters) you should unsolder them using the minimum of heat in the case of germanium types as too much heat can damage them. It's a good idea to grip each lead with pliers between the transistor body and the circuit board while soldering or unsoldering. This will act as a "heat shunt" to conduct away much of the heat.

Given a circuit diagram or sufficient experience to identify the various sections, you may be able to narrow down a fault with a signal injector and/or an oscilloscope, but if you know how to use them and to read a circuit diagram you probably won't be reading this page!

DAB radios

DAB radios ought to be very reliable but experience shows this not always to be the case. The reasons are poorly understood in the Restart community, as faults in the digital circuits often cannot be pinned down accurately. Suggested causes are:

  • Brittle solder joints or solder whiskers (lead-free solder used in modern equipment is more brittle than older lead/tin solder and prone to the growth if tin "whiskers" which can cause short circuits).
  • Manufacturing defects in the integrated circuits.
  • Inaccurate or unstable power supply voltage (most digital circuits require an accurately controlled power supply of 3.3 or 5V with a margin of only +/-5%).

However, you should at least be able to narrow the fault down somewhat. Here are a few simple tips:

  • Check the output voltage(s) of the power supply.
  • Check the ribbon cables joining the various modules are properly seated, and try reseating them.
  • For a DAB radio also having an FM waveband, check whether it works on FM. If so, the fault is in the DAB tuner. If not it's in the audio section, which is common to both wavebands.
  • If there is no sound from the speaker, check whether the headphone socket works. If so, the fault is in the headphone socket itself, the loudspeaker, or the wiring between and around them both.