Symptums and Solutions: Philco 48-460

[Tubejunke]

I have this radio that I brought back to in what looks to be a very short lived life. Meaning it was a completely dead set and bore an all original compliment of Philco loctal tubes and has a chassis one can eat off of. The on/off switch was all that was wrong with it and I think it could have been a factory defect.

At any rate, I had it playing and it was great. SUPER sensitive to stations across the dial where most radios I have only seem to pull in the stronger stations. After a while I noticed it went silent and I thought to myself there went a cap or whatnot. I turned the tuner and what I got was the whirling radio sounds that remind me more of shortwave than broadcast. The sounds were everywhere there would be a station, yet no station would tune.

I am sure that this is directly connected to a specific section of a AA5 superhetrodyne receiver, but I must admit that exactly which eludes me. Yeah, I could do what I might normally do and willy nilly search under the chassis for something that looks not quite right and most often will be successful. The very unscientific approach is getting old after many years of restoring/repairing old radios. I am more interested in knowing each section )beyond the power supply) and what the outward symptom would be to go along with a failure within that section. I would like to use test equipment that I have to do this at some point. All I have is an oscilloscope, VTVMs' and a signal generator with AF output, but that is the biggest part of what I need.

I'm thinking that perhaps a resistor is changing value over play time and the reason is because I allowed the set to cool and powered it back up and it was normal. One must always consider caps in an unrestored unit as well. Perhaps not as scientific as I hoped, but knowing the section that matches the symptom and starting there would be a more professional approach at radio repair/restoration.

[old_coot88]

Sounds like the local oscillator (LO) is not running.
Try wiggling the tubes, particularly the mixer/osc. tube. Loctal tubes are particularly prone to flaky connection in their sockets.

[Reece]

Yes, wiggle tubes, clean tube pins and sockets. Remember to tilt loctal tubes towards the bump on their base to remove from socket. If this fails to clear the fault, check if oscillator is working. Place a good working radio tuned to about 1450 next to your Philco. Tune the Philco up and down around 1000 and you should hear a sweep or squeal in the other radio if your oscillator is working.

[Tubejunke]

Originally Posted by Reece

Place a good working radio tuned to about 1450 next to your Philco. Tune the Philco up and down around 1000 and you should hear a sweep or squeal in the other radio if your oscillator is working.

It did turn out to be I suppose a dirty/loose pin. I had cleaned the pins and sockets, but I guess not good enough. This is a neat trick quoted above, but couldn't I use my RF signal generator or my signal tracer (tracer I just got) to do this?

I got all of this matching 50s Heathkit test equipment and I want to put it to good use. So far I only use the "magic eye" condenser tester which is a wonderful tool that everyone in the electronics trade/hobby should have. I have a few extra in the classifieds to let go of if anyone wants one.

I want to learn to use the other devices for ligament and troubleshooting. I have never done alignment before. I have all the tools, so there is no excuse anymore.

[Reece]

Good that you got it going. The easiest way to see if an oscillator is working is how I described. The oscillator is a mini transmitter and you're picking it up on the other radio. You are actually doing two tests in one: first, it proves the oscillator is working, and second, with the Philco on 1000 and the second set picks it up around 1450, it proves that the IF is reasonably close to its proper 455.

You could use a VTVM or DVM to see if there's negative voltage on the oscillator grid, but there's nothing more positive and simple than the second radio trick. Imagine that you proved the oscillator was working but the set still didn't play. Now you could use the signal tracer in the IF stages and follow towards the audio section to see where you lose the signal. Or use the AF output of the signal generator in the audio section to prove it's OK and then switch to modulated 455 RF output ahead of the detector and into the IF stages. It would be good to read up on the uses of this equipment and then try it out on a working radio.

[old_coot88]

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