Faults found around the frequency changer stage and rf amplifier stages of a valve radio.
CRACKLING RECEPTION

As mentioned before,the wavechange switch is prone to crackles and can be caused by a condition known as HT TRACKING. This occurs because the ht current finds an easy path to earth or somewhere via another contact on the wafer switch. This is because the dirt and grease on the paxolin wafer becomes conductive and the ht makes a "track" or groove in the paxolin wafer where it arcs away and causes crackling/rustling noises from the loudspeaker. Difficult to cure. For some reason this seems to happen more with the "gram" contacts than anywhere else. If this is the case you could consider disconnecting these. It could just be that the wavechange switch has DIRTY CONTACTS in which case clean with switch cleaner,only a small amount though.

The FREQUENCY CHANGER VALVE seems prone to failure too usually going off when tapped. Replacement is the only solution here.

I have also known the RF TRIMMER CAPACITORS to intermittently short out,but this is rare.
More commonly the main TUNING CAPACITOR can have SHORTING VANES causing complete stoppage of reception on parts of the band. Its worth getting a soft paint brush and vacuum cleaner and sucking all the dust out which can be conductive with tiny bits of metal in it from the tuning capacitor itself. If this does not work then you could consider trying to bend them back,but this is a risky business...you could cause more damage than before. On the positive side though i have done it with considerable success. The trick is to UNPLUG THE RADIO,then using a multimeter connect it across the capacitor and when the vanes touch together the meter needle should go right over,this should tell you whereabouts roughly this is happening,then disconnect your meter,switch on and probe the capacitor vanes one by one to see which one is the likely culprit and VERY CAREFULLY slowly bend it very very very slightly. You are quite safe doing this,there is no dangerous voltage on this capacitor.

Daft as it may sound check your AERIAL/CONNECTION as this could cause crackles too.

SOME BANDS NOT WORKING

In my experience this is mostly caused by a faulty FREQUENCY CHANGER VALVE.check by substitution first.

LOW HT on the valve can also cause it,check the anode feed resistors for being high resistance.

Once again WAXY CAPACITORS are a likely cause,change all these as a matter of course.

POOR ALIGNMENT can cause weak reception on some bands..align the I.F. stages first then the R.F. stage using a signal generator. If you are unsure of the I.F. frequency CONTACT ME USING THE CONTACTS PAGE STATING MAKE OF RADIO AND MODEL NUMBER and i will do my best to help you.

NO STATIONS

This can be caused by an OPEN CIRCUIT RESISTER feeding the anode of the frequency changer/rf amplifier valve. Check the ht volts here.

If you are really unlucky and i hope you are not,the first if transformer winding could be open circuit possibly caused by "GREEN SPOT" a type of copper corrosion that is caused by damp conditions that eats away at the lacquer insulation then the copper winding eventually breaking the wire and causing an open circuit. This will effectively silence the radio producing no stations.

The RF AMPLIFIER VALVE or the FREQUENCY CHANGER VALVE can cause this too. Substitute these.

Sometimes but rarely the AERIAL TUNING COIL can go open circuit causing no signals or low level signals.

If the I.F TRANSFORMERS ARE BADLY MISALIGNED this could easily cause the problem too. Realign these using a signal generator and circuit diagram using the correct I.F. frequency. IF YOU ARE UNSURE OF THE CORRECT IF FREQUENCY PLEASE USE THE CONTACTS PAGE STATING THE MAKE OF RADIO AND MODEL AND I WILL DO MY LEVEL BEST TO HELP YOU.