In over forty years of "dinking and dorking" around with old radios I've had my share of trying to solve "hum" problems. In fact right now I'm working on a Airline chairside with a apparent 60 Hz. hum in the audio. The hum is more pronounced after the chassis is installed in the cabinet -- the wood cabinet affords a significant bass boost and GREATLY accentuates the problem!
I thought I would take the time to share some hum trouble shooting hints with my fellow radio collectors and restorers. I've loosely classified the hum into two categories; 60 Hz problems and 120 Hz problems (or CPS for you 'ole guys!)
60 Hertz Hum
120 Hertz Hum
- The detector tube is too close to 60 Hz source (power transformer, power switch or AC line). Use a shield around tube and/or power switch.
- Ground bonding problems; This is where ground current is carried through the rivets of tube sockets and terminal strips. Millivolts of 60 Hz voltage drop, due to loose and corroded rivets and screws, can cause substantial hum. Run a separate ground wire for the heater circuit -- and don't forget the pilot lamps. Also, you can move ground connections away from the detector and first audio tubes to a grounding at common locations where the circuit is naturally referenced to ground such as a tube cathode or cathode bias resistor.
- In an AA5 set, the 12SQ7 should be the last tube in the series string with one end of its heater at the same potential as the cathode.
- Be sure that the center tap of the power transformer's filament winding is at a "good" ground.
- Remember if its 60 Hz hum, then its probably heater voltage hum. Double check this by cutting off the heater 6.3 volt supply with the power on. If the hum immediately goes away before the tubes cool off you've found it!
- If it buzzes (60 Hz but lots of harmonics) then its probably some RF interference coming through the antenna or nearby fluorescent lights, etc.
- Tubes with heater/cathode leakage -- substitute to isolate.
- Pull tubes one at a time working from the RF, Osc, IF, Detector, 1st. audio to assist in isolating the source.
- Also, remember that many times there may be multiple causes of hum.
- Try .02-.05 mfd caps on the AC input side to the power transformer. Tie them to ground. Put bypass caps to ground on the filament string.
- Try grounding the power transformer.
- Try shielding all or various tubes.
That's about all ideas I've got for now. I want to thank George Gonzalez and Henry van Cleef, contributors to rec.antiques.radio+phono, the antique radio newsgroup, for unknowingly allowing me to use some of their posted info.
- Poor power supply filtering. Time for that "recap" job in the filter network. The capacitance of 20-40 mfd is usually OK, But watch that DC working voltage -- keep it at 450 v. AC/DC sets use higher mfd's, but a lower DC working voltage. Keep it at 200-250 WVDC.
- Capacitor installation wiring errors are classic, like when one negative leg of a paired filter cap does not go to ground. Use a schematic and don't assume the last guy did it right! Some older radios generated negative bias by using negative lead filtering with the speaker field coil connected between chassis ground and power transformer B+ winding center tap. These various circuits require two separate or individual filter caps.
- Try replacing the rectifier tube (Substitution)
- Use a 'scope to check the AC ripple voltage at various points in the power supply system. Add more filter capacitance and study the ripple reduction affect. Don't add too much capacitance, especially before the choke (In-rush current considerations). Also some circuits are sensitive to too high capacitor value -- adding some unwanted low-frequency gain that may push the circuit into motor-boating or oscillation. That will do a number on your ear drums!
- Some radio circuit designs and component/tube layouts require the use of metal tubes. Possibly true if the audio and rectifier tubes are too close. Watch for glass substitutions as a cause for hum.
- The electrodynamic speaker hum bucking coil leads may be reversed. Try them the opposite way. See which way produces less hummmm. Does the hum vary with the volume control audio output setting - or is it constant? There could be important clues here!
- As with 60 Hz.hum, look for poor grounding of the power supply components, You don't want no ground loops nohow.
- Go around with a sharp test probe who's other end is attached to a "GOOD" ground. Check for hum (both 50 R 120 Hz) reduction when checking various grounding lugs and ground tie points.
Bahhhh Hummmmm Bug!