01-21-2011, 04:19 PM
I finished removing the old mica capacitors in the 1st IF transformer and the RF transformer. From their looks it was needed. The the silver oxide looks like it migrated all the way to the transformer case! The RF transformer is the second photo with only one tunning slug.
[Image: http://www-rohan.sdsu.edu/~nash/philco/1st%20IF.png] [Image: http://www-rohan.sdsu.edu/~nash/philco/RF.png]
I pre-aligned the 1st IF before re-installing it, using 120pf capacitors. What a difference! I easily hear twice the number of stations now. The IF transformers now have a very noticeable peak when checking the alignment.
I really appreciate the comments on the value of the RF cap. I would not have guessed the RF transformer capacitor to be only 10pf. A 33pf is used for now. Even with the 33pf, the RF coil alignment now peaks easily in less than a 1/4 turn. There are some spurious noises (oscillation?) in a couple places on the dial when the tuning capacitor RF trimmer is aligned for max sensitivity. I think that may go away when the 33pf is replaced with a 10pf.
Now the only issue is what I would call a RF buzz between stations and on weaker stations (these could not even be heard before fixing the IF and RF transformers). Tuning in most stations, the audio is great, no hum or buzz. The antenna coil is not open, and the plug is making contact. The buzz varies with the volume control. Oddly, If I touch the external antenna screw with my test meter, the buzz is greatly reduced. I have a 0.01uf X1Y2 250VAC cap across the AC line, in the original location of C15. C15 was a 0.047uf originally. I don't think the value of C15 is the issue as when I unplug the radio, the buzz continues for a few seconds until the radio quits.
[Image: http://www-rohan.sdsu.edu/~nash/philco/1st%20IF.png] [Image: http://www-rohan.sdsu.edu/~nash/philco/RF.png]
I pre-aligned the 1st IF before re-installing it, using 120pf capacitors. What a difference! I easily hear twice the number of stations now. The IF transformers now have a very noticeable peak when checking the alignment.
I really appreciate the comments on the value of the RF cap. I would not have guessed the RF transformer capacitor to be only 10pf. A 33pf is used for now. Even with the 33pf, the RF coil alignment now peaks easily in less than a 1/4 turn. There are some spurious noises (oscillation?) in a couple places on the dial when the tuning capacitor RF trimmer is aligned for max sensitivity. I think that may go away when the 33pf is replaced with a 10pf.
Now the only issue is what I would call a RF buzz between stations and on weaker stations (these could not even be heard before fixing the IF and RF transformers). Tuning in most stations, the audio is great, no hum or buzz. The antenna coil is not open, and the plug is making contact. The buzz varies with the volume control. Oddly, If I touch the external antenna screw with my test meter, the buzz is greatly reduced. I have a 0.01uf X1Y2 250VAC cap across the AC line, in the original location of C15. C15 was a 0.047uf originally. I don't think the value of C15 is the issue as when I unplug the radio, the buzz continues for a few seconds until the radio quits.
