I am restoring a Zenith L505 portable (AC or Battery). Nagging problem ... No gain when turned on -- if I zap it from a sig gen with 455kHz (400Hz mod), it starts and continues to work. Turn set off, wait 10 minutes, and back on, radio needs another zap to get started. I suspect a problem with the mica cap in one of the two IF cans, but am not sure. I have given it a whack with a small mallet to see if it is a mechanical contact problem, and this does not fix problem. Ideas?

Where are you injecting the 455kc?
My shooting from the hip guess would be to see if the local oscillator is running. Just put your scope pobe there..
Maybe the 455kc sig is mixing and helping the osc to beging running.
Check or change the mixer tube and replace all the caps in the oscilator section and check the resistor values.

Or the easy way is to take some duct tape and tape the signal generator to the top or side of the radio ... whereever it looks nice. Then tape the two power switches together... keep the 455kc signal wired in... and .....your problem is solved...

:Where are you injecting the 455kc?
:My shooting from the hip guess would be to see if the local oscillator is running. Just put your scope pobe there..
:Maybe the 455kc sig is mixing and helping the osc to beging running.
:Check or change the mixer tube and replace all the caps in the oscilator section and check the resistor values.
:

Could be a weak 1R5, check the filament voltage it could be low. It should have the caps and lytics replaced before you do any trouble shooting.

Dave
:Or the easy way is to take some duct tape and tape the signal generator to the top or side of the radio ... whereever it looks nice. Then tape the two power switches together... keep the 455kc signal wired in... and .....your problem is solved...

The mixer tube suggestion by all is a great one, and is a common one for me as well. Definitely obtain a replacement. I have been able to at least temporarily fix these tubes by running a lot of voltage through them. I use an EICO 625 tester. The #4 Selector setting puts out 200 volts at the strongest current. With all of the grids and the plate connected together, the control grid will be 200 volts positive, and will be right next to the filament. Test at regular filament voltage or at 2.0 volts. Let some sparks fly while depressing the merit button for brief moments. Hold for longer if it doesn't seem like severe damage will occur. Try tube again. It should work well for at least a week. If you can get it to work for a really long time, congratulations. I have repeatedly performed this on one tube. I finally replaced the tube because I got sick of "fixing" it. I could still fix it again, though. You may even try holding down the button for 5 to 15 minutes if no damage is apparent. There is a risk of damaging the tube, but otherwise this just shakes impurities off of the cathode.

Thomas