CHASSIS TROUBLESHOOTING

When working on old valve radios, get into the habit of using a 240V isolation transformer at all times. These sets were all manufactured before OH&S became fashionable, and the exposed 240v AC wiring can be deadly. While an isolation transformer won't protect you under all circumstances, it will protect you from the majority of situations - ie where you are likely to come between the active terminal and an earthed object.

I also put a new three-wire power cord on all sets I restore, and make sure that the earth is connected to the chassis of the set. (This is not possible with some AC/DC sets where one side of the power input goes directly to chassis).

When removing the top cap connections to the older valves, don't just grab and pull up as you'll often end up with the top cap of the valve as well. Carefully prise open the clip slightly before pulling off and you won't have to worry about trying to resolder and re-glue the top cap back to the valve - assuming that there is a long enough piece of wire sticking out of the valve on which to solder.

Oscillator issues

There are a number of ways to check if the oscillator section of the mixer valve is actually oscillating. Three of these are:

Bias

Always measure the cathode and grid DC volts on the output valve to ensure that it is properly biased. The valve data books will give the correct bias volts. The grid must be negative with respect to the cathode, and the two most common measures to obtain this bias voltage are:

Other

When removing the top cap connections to the older valves, don't just grab and pull up as you'll often end up with the top cap of the valve as well. Carefully prise open the clip slightly before pulling it off and you won't have to worry about trying to resolder and re-glue the top cap back to the valve assuming that there is a long enough piece of wire sticking out of the valve on which to solder.

Don't just plug in and turn on those untested indirectly heated rectifiers such as a 6X5GT otherwise you are likely to smell a frying back bias resistor and power transformer. Always first test to ensure that there is no cathode to heater short. As one side of the heater circuit is usually earthed, a cathode to heater short will put a direct short circuit on the rectified HT output.

If you are having problems with crackles and pops and/or varying volume, suspect the two mica capacitors, usually approximately 100pf, around the detector valve. One is usually used to feed the IF signal from the anode of the final IF stage to the AGC detector diode, while the other is used as an IF filter at the base of the secondary of the final IF coil.

On the more modern sets fitted with AGC circuitry, always measure the AGC line to ensure that the AGC circuit is working correctly. The AGC line should be approximately zero volts or slightly negative with no signal, and rise to between neg 5 and neg 10 volts with a strong signal.

To repair small tears and holes in speaker cones, buy some craft glue and black sewing interface material from a craft shop. The material is very cheap and reasonably close to the texture and grey colour of most speaker cones.

If you have determined that a winding on an aerial oscillator or IF coil is open circuit, carefully remove the coil from the set after first drawing a small diagram showing how it is connected. Then remove the coil from the can, and using a powerful magnifying class, inspect the connections from the coil to the terminal posts and also the outer layer of the coil. I have found some where the coil wire was poorly soldered to the post and had come adrift, and a few others of the older, unwaxed type, where corrosion, often visible as a small blue/green spot, has eaten through the wire.

If you don't bulk replace all capacitors but rather just test them for leakage and replace as necessary, you should also check the actual capacity of the capacitors with a capacitance meter. I have seen quite a few that measure, as good when tested for leakage, but in fact have been open circuit and have no actual capacity.

When there is a need to align the IF stages of older superhet sets, don't assume that the IF frequency is 455Khz. Always check the circuit as the actual IF frequency used by the many manufacturers varied greatly, both higher and lower than 455Khz, before it was standardised at 455Khz in the late 1940s.