Typical audio stage design and common faults.



Most audio output valves in use are of the PENTODE type,that is a valve with five electrodes.earlier valves were TRIODES such as the celebrated PX4 and PX25 valves. These were quite robust and tended not to suffer from inter electrode shorts and cathode stripping like EL84,UL84 etc. It is important that the first grid (G1) IS NEGATIVE WITH RESPECT TO CATHODE IF IT IS NOT EXCESS CURRENT WILL FLOW AND EVENTUALLY STRIP THE CATHODE AND CAUSE LOW EMISSION. One of the first checks i do on any radio is this. Pay particular attention to C3 which in an older set will usually be of the waxy type and almost certainly leaking causing positive volts on G1 and could have already damaged the valve causing LOW VOLUME and DISTORTION. Another cause of LOW VOLUME could be C2 usually an electroytic capacitor of about 25uf or so going open circuit or low capacity.


First thing to check here is the HT on the ANODE of the output valve. If there are no volts here but there are volts on the smoothing capacitor,then you may have an OPEN CIRCUIT PRIMARY WINDING ON THE OUTPUT TRANSFORMER. This robs the output valve of its ht supply. Another thing to watch out for is the INTERNAL LOUDSPEAKER SWITCH. If the set has one. These can become dirty/corroded and can deaden the sound by becoming open circuit. The LOUDSPEAKER itself could have an open circuit speech coil of course. If you have access to a SIGNAL INJECTOR feed a tone via a capacitor of say 0.01uf into the G1 (control grid) of the output valve,if you hear a clear note,the fault must be further back in the stage possibly the af amplifier. You could also check R3 as this would kill the af signal going into the output valve. Now feed an af signal into G1 of the af amplifier(the valve preceding the output valve) and see if you hear a tone now. If you dont then the af valve may have no ht,check R1,R5. The VOLUME CONTROL is under suspicion too. If you still have no sound change both valves(if applicable).check and clean the VALVE HOLDERS too. Of course if C1 happened to go short circuit it will kill the sound too.


THESE ARE USUALLY CAUSED BY WAXY CAPACITORS GOING LEAKY. Pay particular attention to C1 going open circuit. NOTE if you have to replace this capacitor use one rated at 1000vdc minimum as very high ac voltages are built up across the output transformer and could break the capacitor down.

WIRING also can play a part if its not routed properly.

KEEP CAPACITOR LEGS AS NEAR TO THE VALVES AS YOU CAN as this also can cause instability,but this usually applies around the frequency changer valve.

LOW HT can also cause instability and whistles,check the ht on the output valve and af amplifier valve,R1 or R5 may have gone high in value.

another cause of instability can be the METALLISED COATING on the outside of valves particularly the rf amplifier and frequency changer valves flaking off. You can place tin foil around the valve and hold in place with fine fuse wire then connect the wire to the appropriate pin on the valve.

VALVES themselves can be also a source of instability,you can check this sometimes by simply tapping the valves one by one. If a valve is found to be faulty, replacement is the only cure.


The WAVECHANGE SWITCH can be a common cause of crackles. It can be caused by HT TRACKING when you get arcing between contacts on the wafer. Of course the WAVECHANGE SWITCH could simply be DIRTY and need a clean. When cleaning these switches try to not soak the switch with switch cleaner as this could give rise to ht tracking.

VALVE BASES are commonly the culprits too. Clean the valve legs and bases with a good quality switch cleaner.

DRY JOINTS can cause crackling too,its best to tap the underneath of the chassis with a small screwdriver or probe and see if this causes any crackling noises.

Check the VOLUME CONTROL AND TONE CONTROL as these tend to become crackly when turned,a quick squirt of switch cleaner here usually suffices.

The cause could always be EXTERNAL too,MAN MADE INTERFERENCE as it were can be caused by unsupressed motors notably model trains,vacuum cleaners,power tools etc,also unsupressed spark plugs in motor cars. Lightning is also a well known cause of interference on an A.M. radio.

VALVES sometimes cause crackling too when tapped,this can be caused by loose electrodes the only cure being replacement.


This is almost always due to POSITIVE VOLTAGES ON THE G1 ELECTRODE,this causes the valve to overheat due to excess current flowing and may even cause the valve to glow red hot or blue. In this condition the valve life is rapidly shortened as the emissive material around the cathode becomes stripped due to being overrun.