Fault finding and likely faults in the power supply section


The above power supply is a simple one. Being a mains transformer set the valve heaters are in parallel and should have 6.3v ac across the heaters. If the voltage here is low it can damage the valves i am told.


In cases where there is NO VOLTAGE output at the positive connection of C2 first thing to do is check the mains plug for a loose wire or a loose/missing/blown mains fuse.

Check the MAINS SWITCH for open circuits. If everything is ok here,next check if there are any fuses going to the transformer that may be open.

Also check the MAINS TAPPING PLUG because this sometimes has a fuse within it.

Next comes the MAINS TRANSFORMER itself. As stated before if you check across the primary winding(where the mains goes in),the resistance reading should be between about 20 to 50 ohms depending on mains voltage. If there is no reading here you have my deepest sympathy as it looks like you have an open circuit primary winding,remember though do your resistance check at the transformer pin connections NOT the mains lead. On the positive side in my experience mains transformers rarely fail.

Its worth mentioning here as well that older radios used a MAINS ENERGIZED LOUDSPEAKER which also acted as a SMOOTHING CHOKE with good results. The act of switching on the radio and allowing the ht to build up from the rectifier and through the field coil produced an electromagnet,and this was enough to drive the moving coil speaker. If this goes open circuit it robs the radio of its ht supply and worse deadens the sound,i believe you can get the coils rewound but this usually is not an option. You might be able to find a good second hand one with a similar coil but usually the easiest answer is to use a modern speaker preferably 4ohm impedance and using an aluminium clad resistor in place of the open circuit field coil,a resistor usually around 2000 ohm should do the job nicely.


If you have a raw HUM coming from your loudspeaker its usually because C1 is open circuit,if C2 goes open circuit the hum should not be quite as bad but still there.another clue to C1 going open circuit is you will get LOW HT on the output.

Another cause of hum of course can be the VALVE RECTIFIER possibly with a heater/cathode short.only cure here is substitution.