I recently did a full restoration on a Philco model 20 chassis (Schematic). For the filter rebuild I used a 1uf @ 630V for the filter before the choke and a .15uf @ 630V for the final one off the choke. I used a 10uf @ 450V to replace the 1.5 audio filter, watching my polarity. The .13 across the choke, I replaced with a .1uf @ 630V. All of the bakelite blocks have been rebuilt with the proper value of NOS 630V capacitors. All of the resistors were way out of tolerance so they were replaced with NOS 1W carbon comp resistors. The interstage transformer had one half of the secondary open so it was replaced with a NOS Stancor A-53C. The radio came without a speaker so I purchased one of the same size, a Jensen D-16 with a 4K field coil and a NOS SP/PP output transformer strapped for the PP 71A's. All tubes have been tested in two different testers, one a standard emission type and the other a transconductance. The original 71A's and the 80 rectifier were replaced with good used tubes. The radio picks up all the stations with plenty of volume. PROBLEM: 
When turned on the radio immediately starts to hum to where the volume has to be too loud for comfort. I have 120VAC input from the mains and the filaments are all well withing spec. With all tubes installed, The B+ off the output of the choke reads 295VDC. The B+ on the plates of the 71A's is 295VDC and their grid bias from either side of the filament reads -55V. The only way to kill the hum entirely is to pull the 227 AF amplifier. Since I have found another 227 that did lower the hum somewhat. I have checked and rechecked all of the wiring and had another person check it over and nothing is wired incorrectly. I have never had a radio stump me in my 52 years of repairing them. Any suggestions would be greatly appreciated. Incidentally I have changed out all of the tubes too and that didn't help. Many Thanks
Another question is how do you keep the tunning capacitor from slightly slipping when tuning the stations in the middle of the dial or when the tuning capacitor is nearly full open? Do you take the tuning shaft out and stretch the spring to make it hold the dial tighter? The tuning dial has been thourghly cleaned and just a minimal amount of heavy grease applied to where the rotor shaft rides on the capacitor frame. This stopped the scratching noise when tuning stations. Any ideas?
When turned on the radio immediately starts to hum to where the volume has to be too loud for comfort. I have 120VAC input from the mains and the filaments are all well withing spec. With all tubes installed, The B+ off the output of the choke reads 295VDC. The B+ on the plates of the 71A's is 295VDC and their grid bias from either side of the filament reads -55V. The only way to kill the hum entirely is to pull the 227 AF amplifier. Since I have found another 227 that did lower the hum somewhat. I have checked and rechecked all of the wiring and had another person check it over and nothing is wired incorrectly. I have never had a radio stump me in my 52 years of repairing them. Any suggestions would be greatly appreciated. Incidentally I have changed out all of the tubes too and that didn't help. Many ThanksAnother question is how do you keep the tunning capacitor from slightly slipping when tuning the stations in the middle of the dial or when the tuning capacitor is nearly full open? Do you take the tuning shaft out and stretch the spring to make it hold the dial tighter? The tuning dial has been thourghly cleaned and just a minimal amount of heavy grease applied to where the rotor shaft rides on the capacitor frame. This stopped the scratching noise when tuning stations. Any ideas?
