Help with Arvin 442 - 4 tube radio

[ibm3270]

Greetings, I have restored a non working Arvin 442 radio and the issue is it only detects one radio station part of the time on the wrong spot on the dial. For example channel 660 comes in around 1020. The only other noise it makes us a loud whoop whoop or click in the high channel range above 1000 kHz. Anything under that is dead silent not even a slight hum. I have gone over the schematic like a fine tooth comb. All voltages are correct. The plate voltages are correct. All the tubes test fine and I've even swapped out the rectifier, the heptode and the double diode/triode tube, still the same thing. I'm thinking something is wrong in the IF stage but the IF coil has acceptable voltage and does not test with a break using an ohm meter. The amp stage is beautiful and sounds amazing when I put an in series capacitor to the grid of the triode. The antenna coil is brand new and a ferrite upgrade. I'm stumped. What else could it be?

 

[Dave451]

First, you might want to use a small amount of contact cleaner on the wiper(s) of the tuning air variable capacitor and work it back and forth through its range several times. Use compressed air or canned air to remove any excess and don't get it in the plates of the cap.

Second, have you aligned the radio? With repairs and new tubes you'll probably need to.
Good luck with it!
Dave

 

[Dave451]

Also, that tuning cap like love looks pretty crusty. The plates can't be shorted by dirt or corrosion. You may want to gently blow out anything between the plates. They can't touch, either.
Dave

 

[ibm3270]

I used a soft bristle paint brush and cleaned the tuning cap. It has some yellowing which is common for antique aluminum. No bent plated or shorted plates. The stater was oiled and the tuning cap moves freely and easy. I don't think a tuning cap would cause this issue. I worked on alot of old radios and still in my experience this radio would have normal AM static if everything was working. It doesn't even have that. It's dead silent, then when tuned above 1000 it starts chirping and oscillating in sounds. If played with some more then it will detect 660 which is a very strong AM station here in Pittsburgh area. (However 660 tunes in on about 1060 on the dial) Then when still tuned higher the chirping gets painly loud and then tapers off the higher tuning kHz go up. So it's detecting, sort of, which is why I think it has to be in the IF stage. If the IF stage was working then I should hear more than one station or at least typical AM static. As far as alignment of the unit. I need to detect a weak station to run a proper alignment. Since it only detects one strong station an alignment is pointless. I dont have a signal generator to align without a working detecting radio... but if it is wrong in the IF stage it won't align anyway. IF stage according to schematic is 455khz.

 

[gadget73]

bad padder cap for the oscillator trimmer perhaps? Or maybe component placement causing oscillation. I've had trouble with very basic sets being extremely picky about where parts sit. If its oscillating, it will also tend to swamp the AVC line so it has squat for sensitivity.

Try moving parts with a suitable insulated pokentool, especially any parts you have changed or moved. I have a little Westinghouse that gave me similar problems, finally chased it to one capacitor that had to be moved just a little bit in order for it to behave.

 

[toxcrusadr]

It doesn't have caps built into the IF cans does it? Like those silver mica ones that get the disease?

I had (have) a radio that I recapped that just howled afterwards. Never did figure out what caused it. But I had put new filter caps under the chassis (space was tight) and I always figured it may have been feedback or stray EMF from having those so close to other circuitry.

An RF sig gen with audio modulation is a great help for alignment. An older restoration expert sold me one for $25 when I was getting started. You should get one.