Portable radios

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

Summary

If there's no life at all in a portable radio, it's probably a fault in the power supply section, possibly a swollen electrolytic capacitor. Any slight hum or hiss from the speaker will tend to indicate that the audio amplifier is working. On a digital radio, if the display is blank or if it can't tune any stations, that indicates a fault in the tuner section.

Many fixable problems with portable radios are common to many types of electronic gadgets, such as loose or broken connections or signs of overheating. These 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

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. If it needs re-threading around the pulleys you will need to examine it carefully to work out how it's meant to go. 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.

If there is no sound a very simple test is to try with headphones (assuming there is a headphone socket). If it works with headphones it could be a problem with the speaker or the headphone socket. (The headphone socket incorporates a switch which mutes the speaker when you plug headphones in. A fault with this could be the problem.)

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 or card, between the external button and the tactile switch, allowing a little more pressure to be applied. Replacements are available very cheaply on eBay. Amongst several standard sizes 6x6mm is common, available in a range of heights. 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

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, or by coils (often in an aluminium can) 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.

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 those and to read a circuit diagram you probably won't be reading this page!

Vintage valve radios

A vintage valve radio that hasn't been opened in many years may contain a thick layer of dust over the internal components. This may impede cooling. Clean it out with a vacuum cleaner.

Pre-war radios and ones from the early '50s tend to contain large valves with bakelite bases and very often an International Octal base with 8 pins and a central keyed spigot. Miniature all glass valves very largely displaced these in the post-war period.

Cheaper radios of the "AC/DC" type didn't have a mains transformer but instead rectified and smoothed the raw mains to provide the high voltage (HT) for the valves. In these, all the valve filaments were wired in series and also fed direct from the raw mains, through a large dropper resistor. Valves intended for this mode of operation frequently have type numbers beginning with "P" or "U". The direct connection to the mains means that the chassis may be live, and great care must be taken to avoid a potentially lethal electric shock if plugged in with the cover off.

Other radios had a large mains tranformer with two or more secondary windings, one for the HT and one or more LT windings for the valve filaments (typically 6.3V). The presence of high voltages (300V or more) means that care must be taken equally in working on these. In both types of radio a valve is used to rectify the AC drerived from the mains.

The simplest diagnostic procedure is to check whether the glow of the filaments can be seen in all the valves when the radio is switched on. However, if in an AC/DC radio a glow can be seen in any of the valves, they must all be glowing even if the glow is obscured by a screen or by the silvering in the top of the valve.

For other radios, you can test the filament with a multimeter on a resistance range. Look up the valve type number on the Internet (or search for the Mullard Data Book for many post-war valve types). This will enable you to determine which two pins are the filament. You should get a reading between five and a few hundred ohms (higher values for valves in AC/DC radios).

The silvering in the top of a valve is the "getter" - a reactive metal deposited on the inside of the glass during manufacture to absob any residual gas. Should this have turned white it means the glass is cracked, making the valve useless.

The filament of a valve heats the cathode, which is coated in a special material to facilitate the emission of electrons. This gradually looses efficacy. A valve radio which has seen much use may benefit from new valves, particularly the audio output valve driving the loudpeaker if the output volume is low, or the rectifier valve in the case of poor sensitivity. These are the two most likely valves to suffer from low emission.

Spare valves should be available on eBay, particularly the miniature all-glass types generally used in post war radios.

A noticeable hum from the loudspeaker probably indicates that the smoothing capacitor or capacitors need replacing. These will generally be electrolytic types. Replacements must have at least the same voltage rating and the same or possibly slightly higher capacitance. As with all electrolytic capacitors, it's essetial to connect them the right way round otherwise they will quickly fail, possibly explosively.

A crackly volume control potentimeter is another common problem. Sometimes it can be improved simply by repeatedly winding it each way. If that doesn't work you could try squirting a little switch cleaning fluid through any openings in the case and repeating. In fact potentiometers have come in standard sizes for decades and you may well be able to get a standard replacement. Apart from the physical size and type and length of spindle, a replacement must have the same resistace value and the same "Log" or "Lin" marking.

Vintage transistor radios

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.

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 of 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. For a radio also having FM (and possibly other wavebands), check whether these work. If not, the fault is probably in the audio amplifier or power supply rather than the DAB tuner.

Here are a few simple things to try before going deeper:

• Try fresh batteries and check that the battery contacts aren't corroded. DAB radios normally tell you when the battery is low and work fine until then, but this is an easy thing to try.
• Try a hard reset. You will probably find a reset button behind a pinhole in an inconspicuous place, such as under the battery compartment cover. Insert a bent paper clip in order to actuate the reset button. If you can't find it, take out the batteries and leave them out for a minimum of several minutes.
• Check the output voltage(s) of the power supply. Check the polarity and voltage needed, typically anything from 5V to 12v. Most wall cube power supplies have the positive as the middle pin, though some are negative on the inner pin.
• Check the ribbon cables joining the various modules are properly seated, and try reseating them.
• If the display seems to indicate that the radio is working but 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, otherwise it will be in the audio amplifier.

In the case of a failed display, in principle this should be replaceable. OLED displays are used on some radios, generally blue and self-luminous against a dark background. Early ones had a limited lifetime. A diligent search, particularly on Far Eastern sites such as Aliexpress and Bangood may lead you to a replacement. Replacements may also be found for LCDs (in appearance, more like most cheap calculator displays). Sometimes just the backlight fails, but you may well have to replace the entire display.

This blog post should give you some useful leads in sourcing a replacement display.

Sometimes a DAB radio will display "No Ensemble". If you are sure you have a good signal and the aerial is in good order, try a hard reset. This can be effected as described above. If you have successfuly reset it, the radio will rescan on subsequently switching it on. Select and play your favourite station and check that the fault doesn't reappear after powering the radio off and on again. In that case, a flash memory chip holding the results of the scan or those particular memory cells in the chip may have failed. This is often an 8 pin surface mount chip on the tuner module, and if you can identify it you may be able to obtain a replacement. However, the same chip may hold firmware which you woud have to copy across. Some DAB radios might use a button cell backup battery instead in order to retain the scan results. If you can see one, check its voltage. It will probably be a lithium type which will need to be replaced if it reads less than 3V.

Many DAB radios use a DAB tuner module made by Frontier Silicon, used across different brands of radio. Searching on the model number of this module may be successful if the model number of the radio itself draws a blank for your problem.