CompuServe Messages

Yes – depending on the local oscillator configuration, you may be experiencing several things. warmup, of course, does not count; tubes change physical size during that time, and all bets are off. If your oscillator is unbuffered, then the coupling capacitor to the mixer stage(s) could be varying in value, such that the load on the oscillator is "pulling" it. capacitors at ANY place in the oscillator itself are suspect; screen decoupling, emittor bypass, anything in a tank circuit (like those usesd to "pad" the variable capacitor)… check any coils (for that band) for "baked" (and therefor flaking) insulation – a turn cutting in and out can 'pop' you up and down a small amount as it varies the inductance of the coil. Clean ALL the dust out of it – dust can make a reciever do ANYTHING. You should do that before you even TRY do diagnose it. Likewise lubricate the capacitor bearings and use contack cleaner on all the switches that change the bands. Same basic kind of reasons – dirt gets in there, you end up with a series resistance (that varies with the square of murphy's law) that upsets the Q and operating frequency of the oscillator. New tubes may be called for, and if so, either the oscillator, the mixer, or the last (only?) front end stage may need it, or any combination. If there are covers on the tubes (little metal shields) make sure that you use the tuner cleaner on the base area where they attach to the chassis; they are supposed to provide a shield for the tube, but if the connection to the chassis is bad, they can provide a varying capacitance to the tube elements, which is a bad deal. I could go on, but these are the things that I do when I rejuve an old radio – and I (so far, crossing fingers) always get then stable and happy. Just did one that was given to me in Montana… thing works great now.