11-07-2019, 08:46 AM
Morning All!!
Last night I whipped a 40-120 that was being a pain. It needed a replacement RF transformer, which I got from Ed Locker.
It came from a 40-130. I had to change the swamping resistor to match the 40-120 circuit, otherwise it worked great.
What was a FIGHT was the oscillations in the set. No matter what I did, we had a case of the "whistles". If you left the IF's sufficiently detuned, the radio would sort of work. Kinda like an early Crosley. 4 or 5 stations and that wasn't good. So I went back through the circuit with a .001 cap hung on a clip lead to find the cause and found that when I touched it to the second grid connection for the 7B7 tube, the problem went away. So I dug deeper. Seems this chassis was built missing part numbers 17 and 25. So I put the capacitor into the set that goes from that second grid to the cathode of the 7B7 and everybody was happy. Cool!! So I tried installing the 110 pf cap across the 7C6 detector tube and that killed the set altogether. So this set is fixed. What puzzles me it how that radio left the factory. They must have detuned the thing so that it would work and kicked it out the door. What's even better is the parts radio I bought (same model) is missing the same two capacitors. So I made some notes in the Philco book so I don't have to replow this ground.
Just as observation: When this little box is tuned up right, it's really sensitive. It has an RF amp that works pretty good for an untuned RF amp. Drags stuff in from all over the place. So now we'll let it play for a while, before we put it back together with its second audio input.
Last night I whipped a 40-120 that was being a pain. It needed a replacement RF transformer, which I got from Ed Locker.
It came from a 40-130. I had to change the swamping resistor to match the 40-120 circuit, otherwise it worked great.
What was a FIGHT was the oscillations in the set. No matter what I did, we had a case of the "whistles". If you left the IF's sufficiently detuned, the radio would sort of work. Kinda like an early Crosley. 4 or 5 stations and that wasn't good. So I went back through the circuit with a .001 cap hung on a clip lead to find the cause and found that when I touched it to the second grid connection for the 7B7 tube, the problem went away. So I dug deeper. Seems this chassis was built missing part numbers 17 and 25. So I put the capacitor into the set that goes from that second grid to the cathode of the 7B7 and everybody was happy. Cool!! So I tried installing the 110 pf cap across the 7C6 detector tube and that killed the set altogether. So this set is fixed. What puzzles me it how that radio left the factory. They must have detuned the thing so that it would work and kicked it out the door. What's even better is the parts radio I bought (same model) is missing the same two capacitors. So I made some notes in the Philco book so I don't have to replow this ground.
Just as observation: When this little box is tuned up right, it's really sensitive. It has an RF amp that works pretty good for an untuned RF amp. Drags stuff in from all over the place. So now we'll let it play for a while, before we put it back together with its second audio input.
Kim Herron W8ZV
w8zv at goldenradioservice.com
1-616-677-3706
