I've got a model 65 receiving and playing wonderfully. I'd like to reduce the 60 cycle hum which seems to be a little louder than it should. It does not increase with the volume control. It's constant and at the same level as long as the set is turned on. The set has a x2 safety cap across the ac supply to reduce line noise from the house. It seems like this hum is coming from the transformer. Any ideas?
Thanks, Dave
I've seen (heard) this once in a while. Sometimes it is just that the mounting bolts for the transformer are loose, and , of course transformers do hum. So this can be fixed by tightening them up.
Other times, expecially if the transformer has overheated, the wax or tar melts away, eliminating the damping factor, so the whole assembly hums. The fix is to remove the bells, and tighten everything up with some filler and padding. There are as many ways to do this as there are members of this forum. More exotic methods are removing the transformer entirely, soaking the core and windings in varnish for a long time, and baking it for several hours, and letting it sit for a couple of weeks.
I've never done that, but I have glued the whole thing up with gobs of Elmer's Glue and set the core and windings atop the oil burner for about a week. Others dab on some epoxy to keep the winding stationary on the core.
The least and fastest, and most reversable method is to put some foam rubber in the space between the windings and the bells, such that when tightening up the bolts, the windings will not be able to move.
Having said that, I assume you have ruled out any electrical reason for the hum, as in bad filter caps, tubes with cathode to heater shorts, gassy output tube, etc... Let us know more!
The hum seems to be present in the speaker. I have recapped the radio. Replaced the second filter choke. Replaced three bad tubes. The hum was there before I replaced three tubes. It's still there and is unaffected by the volume control. It receives well and sounds great. It's grounded and has a long wire antennae. Thanks for your suggestions.
Dave
The 24s or the 27 may have heater to cathode leakage causing residual hum. I have seen many 27s that develop H-K leakage.
Even on a good day putting your ear next to an EM speaker will result is some very slight hum, unless the field coil is powered with pure D.C. Since the field coil is usually the power supply filter choke, there's really no way around a slight residual hum real close up.
Also be sure no autio tube grid wires were moved or re-routed during restoration. Grid wires anywhere near an filament AC wire will pick it up too.
Chuck