Drift happens as radios heat. Usually after temperature stabilizes drift stops. It can be caused by the oscillator tube or just heating of coils & caps in a tuned circuit.
Do you know if your radio had drift before being recapped?
Drift is not the same as fade. If you are having fade measure voltages. Grid #1 of the audio output tube may be losing negative bias? This in turn could also cause drift due to higher current drain & voltage going down.
Norm
:Hello, I have a Admiral 10A1 chassis radio/phono. It has been recaped and all tubes tested or replaced. All the stations come in loud and clear, but after a few minutes they will either drift or fade away. This has tuning lugs instead of a tuning capacitor. It even has the original loop antenna and I can pick up far away stations. It also has shortwave and I can pick up lots of ststions. What could be causing this problem? Could it be a faulty tube? You can view the schematic here online under Admiral 10A1.
:Thanks
:David
Nothing wrong with new 6F6 metal tubes except you can't see inside and they get very hot. It tubes draw too much current you can see red elements in glass tubes.
Measure pin #5 from pin #8. Pin #5 must be 20 volts or so negative. If it's not your 6F6's will draw too much current. This will cause distortion and reduction of sound.
Good 6V6's would be better than 6F6's. They are later design and take less bias than 6F6's.
Norm
:Hello, the AM was not working at all before recapping. Yes, I think it is fading more than drifting. When I first got the rado it had 6V6 tubes in it instead of the 6F6 tubes that it calls for. These are the audio output tubes that you are refering to? I found a nice matched pair of 6F6 metal tubes that I put in, I wonder if I should try the 6V6 tubes or 6F6G glass tubes?
:Thanks
:David
May not be too much voltage but rather current? If grid #1 isn't negative enough tubes will draw too much current. This can be caused be leaky coupling cap, grid resistor high in value or leakage in output tubes. You will know if this is a problem when volage is read between grid & cathode.
Norm
:I put in the 6V6 and they were glass tubes and got very hot. I pulled them and put in the 6F6G ST larger size glass tubes for better cooling. I did not check the voltage yet, but I am guessing I am getting to much? What would cause the jump in voltage to these tubws?
:Thanks
:David
Your fading problem is most likely in the RF stage. When the radio fades out, touch the grid of the 1st audio tube and see if the amplifier hums....or connect an audio source to the volume control. To connect an audio source to the volume control, use something like a portable CD player. The CD player's headphone wire will have center leads and an outer shield (usually). Connect the shield to the chassis side of the volume control. Connect the center wires to the other end terminal (not the center terminal). When both center wires are connected together (left and right channel), harmonic distortion can occur due to the phasing of each channel not being that of the other. Keep this in mind if you hear distortion in the music you are listening to. If you hear distortion, try using only one channel (disconnect one of the center wires). Also make sure that your CD player's volume isn't up too high...otherwise it'll distort even if you have your radio's volume control low. If the amplifier hums when you touch the grid, or you are able to put audio through, then it is fine. If you re-capped the amplifier, and the resistors are in good order, and none of the wiring has been changed, it is probably working well.
Fading in the RF section could be caused by a leaky condenser (possibly mica if you replaced all others, though mica condensers rarely fail....sometimes mica condensers aren't really mica condensers, so test to be sure). Fading could also be due to a faulty oscillator tube. It could even be due to a drifted or unstable resistor, or a combination of a faulty resistor and a weak oscillator tube. Another problem could be a lack of shielding on the RF tubes...allows random oscillation. Be sure, too, that none of the RF tubes has an intermittant heater...one that turns on and off due to a break in the filament and thermal expansion...moves broken wire.
After thoroughly checking all of the above, be absolutely sure that the AVC condenser has NO leakage, and that it is in good working order. Also be absolutely sure that the AVC resistor has not drifted. A high value resistor will cause instability in the RF section.
Thomas