Re: {Collins} Kws-1 hum

----- Original Message ----- From: <>
To: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Cc: "Collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
Sent: Monday, July 01, 2013 11:05 AM
Subject: {Collins} Kws-1 hum

Hi All, had the same problem , the dynamic microphone was acting as a flux detector and picking up the magnetic field of the power supply. This is why Art encased the high power supply assemblies in iron. Changed to D104 crystal and problem reduced but not totally removed.

the mike plug was incorrectly wired, but the system limped along to near satisfactory results which in turn directed your problem solving focus elsewhere.
Cheerio from Steve VK4VN

Sometimes a dynamic or ribbon microphone can pick up hum from a magnetic field but you can tell by moving the mic around. Bill Carns alrady suggested this. I am more suspicious of this hum coming from a bad ground somewhere. If possible connect the transmitter to a dummy load and listen to it on a nearby receiver. Usually there is enough leakage to be picked up. Use headphones to eliminate acoustical feedback. Try the mic and see if moving it changes the hum. You should be able to tell if the hum is from induction pickup or from a bad ground. I don't think this is from the HV cable since it is carrying DC. You can also put a resistor or just a short across the mic terminals and key the transmitter. This will tell you if the hum is coming from something in the circuit. Usually power supply hum is 120 hz and line or heater hum is 60 hz. Magnetic induction hum from a transformer will also be 60 hz. There are many KWS-1s being used with dynamic mics. The hum pickup varies a lot with the microphone. I find Electro-Voice mics are usually pretty well shielded but some others are very vulnerable to induced hum fields. If you find the hum is internal and the power supply is OK the most likely cause is a hummy mic pre-amp tube. Even a little heater to cathode leakage will make the tube hum but of course also check that all the grounds are OK.

Richard Knoppow
Los Angeles

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