"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