Transistor Radio Fault Finding

Fault finding procedure

The defective stage in a transistor radio can usually be tracked down by working systematically and logically. The following procedure and checks should help you get started.

Connect the battery (make sure it's a good one) and switch the set on. If you get a pop or crackle you know that the power is getting into the set, the output stage is doing something (but not necessarily working) and the speaker and earphone socket is OK.

If the set is silent, the first check is whether power is reaching the set. You could trace your way through the set wiring with a multimeter on a voltage range, but it's much easier to connect the meter on the 200mA DC current range in series with the battery.

If the current consumption is between about 8mA and 20mA, power is reaching the set and it is probably drawing around the right amount of current.

If the current is a lot higher, power is reaching the set but something is wrong. Quickly feel around the output stage to see if anything is getting warm, and then disconnect the battery otherwise it will run flat fairly quickly. If anything is warm, that's the area where the problem lies.

If the current is zero, power is not reaching the set. Check the on/off switch, battery leads, and any switching between internal and external power supplies that may be included.

If we have power reaching the set but it is still completely silent, check the speaker with the meter on the lowest ohms range. The resistance should be similar to the impedance marked on it. You may hear a pop as you connect and disconnect the meter - this is more likely with an analogue meter. To further check the speaker you can disconnect it from the set and momentarily connect a 1.5V battery to it. You should get a good healthy pop as the battery is connected and disconnected. Or you could connect the speaker to a known working radio.

If the speaker is OK, check the earphone socket (if fitted). It is quite common for the contacts that open to mute the speaker when the earphone plug is inserted to remain open when the plug is removed. The open-type sockets used on Far-Eastern models can usually be fixed by retensioning the contacts, whereas the enclosed type used on British sets cannot normally be repaired. If this is the problem, bypass it for now so that the speaker is connected directly to the output stage.

If the set is consuming about the right amount of current and the speaker and its connections are OK, we would really expect to get some sort of pop or click from the speaker when the set is switched on and off. Still silent? A fault in the output stage is the most likely. The output coupling capacitor (single transformer or transformerless output stages) could be dried up. Measure the voltage drop across the output transistor emitter resistors (if fitted). It'll be fairly low - tens of millivolts probably - if all is well. Check the biasing of the output transistors (voltages are normally on the service data). The driver or output transformer could be faulty, but this is unlikely unless they are physically damaged. Resistance checks will prove the point.

If the set is consuming the right current and makes a pop or click when switched on or off, try operating the other controls. Does it crackle when the waveband switches or volume control are operated. If so, the audio stages are probably OK. To check further, set the volume control to maximum, hold a screwdriver with your finger against the blade and touch the tip onto the centre connection (wiper) of the volume control. If the audio stages are OK you should get a buzzing noise. If not, voltage checks around the audio stages may help to identify the fault. Dried up capacitors between the volume control and the AF stage or between the AF stage and the driver stage are a possibility too. Remember to tap the transistors with the handle of a small screwdriver to see if they are intermittent.

Once we've got the audio stages working, we're well on the way. Does the set use AF117-type transistors? If so, we already know the likely problem. Tap each one with the handle of a small screwdriver and see if you get any crackles or bursts of life. If it does, that's the problem. If not then it may well still be a faulty transistor, but the quick check didn't work for us this time.

Voltage checks should help narrow down the problem stage - again the expected voltage should be on the service data. If not, compare the voltages between the two IF stages - the circuits are similar so the voltages should be too. Generally the emitter will be a little above 0V, the base will be about 0.2V higher than the emitter and the collector will be a little lower than the supply voltage. For the mixer-oscillator the base voltage may be the same as the emitter or possibly a bit lower, if it is oscillating. The main clue here is that the collector voltage is a bit lower than the supply voltage, indicating that the transistor is passing some current, so a bit of voltage is being dropped across the IF transformer and oscillator coil in the collector circuit.

By now you should be getting fairly close to having a working set, or at least to knowing which section is causing the problem. When working on the defective section, remember to keep your eyes open for broken connections, cracks in the board and other electromechanical faults. If the audio and IF are OK, and the AF117-type transistors are behaving, it is most likely that you have a physical fault in the waveband switching, tuning capacitor or ferrite rod aerial coils.

Don't be put off if the IF amplifier and/or mixer-oscillator are contained in metal cans and are just shown as modules on the circuit diagram. Once you've established that the fault is in the module, you'll need to remove it from the set (often not easy), dismantle it and try to work out what's wrong. The circuits of some of these modules are available, generally without component values (see my Valve Data CD-ROM). The circuit is nothing special though. They normally contain AF117-type transistors, so we know what the problem is already! Remove each transistor, test it, zap it with a charged capacitor if necessary to blow the short, and then replace it. Check the diodes and replace them if they are unwell. Reassemble the module and refit it, then the set should work fine. There's nothing else in the module to go wrong.

If an AM/FM set works OK on AM but not on FM, the problem is almost certainly in the FM front-end. Because of the high frequencies and very short connections in there, you are unlikely to be able to make any sort of meaningful voltage measurements. The only check you can really make is that power is reaching the front-end when the set is switched to FM. Most sets in the era of interest will use AF114 and AF115 transistors in the FM front-end, and the most likely problem is the usual one with this range of devices. Physical faults are unlikely since the whole thing is protected inside a metal can.

If an AM/FM set is distorted on FM but OK on AM, check the FM detector circuit. Either the electrolytic capacitor has dried out, or one of the diodes is getting tired. If there's a small variable resistor for balancing the circuit, make sure this is OK too. If replacing the capacitor doesn't work, try replacing both the diodes.

If, after following the above suggestions, you are still getting nowhere, don't despair. Leave it for a few days, then come back to it when you are fresh. You could ask in the Discussion Forums on my website, giving as much detail as possible about the checks you have done, and see if anyone there has any suggestions (normally someone will be happy to help you).

Don't dismiss the radio as being unrepairable too quickly. At least not until you know what's wrong with it. There isn't that many parts in there, and with patience and practice it should be possible to work out which one is causing the problem.

There is always the possibility that someone has been at the set before you. Look for signs of previous repairs or bodges, and check these carefully. Transistors removed for testing and refitted incorrectly? Components replaced with incorrect values? Wires disconnected and reconnected in the wrong place? Alignment fiddled with?

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The types of equipment discussed on this website may contain high voltages and/or operate at high temperatures.
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Last updated 14th April 2006.