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![]() "Antonio Vernucci" wrote in message ... Its MAGNETIC induction from the power transformer into the audio transformer. -Al In the HRO, the power trasformer is maybe 20 inches away from the audio transformer. My problem is called "tuneable hum". It has nothing to do with filter capacitors, transformers coupling, cathode to filament leakage, etc. It only shows up on certain stations. It depends on the path followed by RF signal (e.g. if it passes through the rectifiers). Typical cures a - bypass capacitor across the power supply rectifiers - bypasscapacitor between mains and ground. But there still are some stations affected by a considerable hum. I was seeking some suggestions on what to do more. 73 Tony I0JX Tunable hum from the power supply where the filtering is good is found mainly in AC/DC receivers. There are other sources, mainly lack of bypassing especially of the screens of tubes with AVC on them and heater to cathode leakage anywhere in the RF path. In either case the tube or tubes affected can modulate hum onto signals. Since AVC tubes are operating with a non-linear characteristic they can be pretty good modulators. If you have access to a good tube checker, one which will indicate shorts and leakage, test the tubes with it. A tube may test fine for emmision and transconductance but still be leaky. Bypass condensers can sometimes be checked by simply paralleling the cap with a good one but, if its got low enough series resistance this test may not work and only substituting another, known good, cap will do. You are working on a receiver of first class design which did not have this problem designed into it so it must be coming from a defective component. The tubes and by-pass or decoupling capacitors are the most likely. However, I would also do a routine check for correct tube socket voltages and resistance values. These can somtime give you a good clue as to what is wrong. Note that where tubes on AVC are concerned tunable hum can vary with the strength of the signal and with the setting of the RF gain control. This may be another clue. You have not answered my question about getting the same hum on other radios at the same location. There are conditions where the actual signal can be modulated by somthing often high voltage power lines nearby. This is an effect familiar to those with auto radios. -- --- Richard Knoppow Los Angeles, CA, USA |
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