The wiring from the tuning condenser terminals to the various components under the chassis of Philco Models 20, 21, 70 and 90 can sometimes go bad, causing the set to exhibit noisy and/or intermittent reception as the set is tuned across the band.
There are actually two causes of noisy reception; besides the wires, the screws which hold the stators (and the terminal lugs) in place on the tuning condenser assembly have a habit of working loose over the years. This problem seems to be rather common in Model 70 receivers, although I’ve seen it in Model 20 and 90 sets, also.
To take care of this problem:
Make notes of how the tuning condenser is hooked up under the chassis.
Unsolder the tuning condenser leads.
Remove the dial scale, then the dial lamp assembly, from the tuning condenser.
Remove the three mounting bolts under the chassis which holds the tuning condenser in place, and carefully remove the tuning condenser assembly.
While it is off the radio and on the bench, examine it carefully, making sure the rotor and stator plates are aligned properly with an even gap between the plates, and that the rotor and stator plates do not touch each other as the condenser is rotated back and forth.
Tighten the stator mounting screws on both sides of the tuning condenser assembly. Recheck the rotor and stator plates to be sure the alignment of the plates did not change, and that the plates still do not rub each other.
Connect new wires to the tuning condenser’s terminal lugs, cut to the same length as the originals.
Reinstall the tuning condenser, using new rubber grommets. New grommets which are similar to the original are available from Don Patterson or from Renovated Radios. (This does not apply to Model 20 as the tuning condensers on these sets are mounted directly to the chassis.)
Connect the new wires to the same places that the old wires were connected to (you didn’t lose your notes, did you?). Reinstall the dial lamp assembly and the dial scale. Try the set out; the noisy reception should be gone and the condenser should sit firmer on the chassis with its new rubber mountings (again, Model 20 sets did not use the rubber mounts).