"Antonio Vernucci" wrote in message
...
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.

Hi Dick,

thanks for useful info.

I have to carry more tests on my HRO, including that of
connecting my Yagi antenna to it instead of a short indoor
piece of wire.

I must also try to connect that same piece of wire to
another receiver, to see whether there is any difference
(actually I have a second HRO to compare).

The hum heavily depends on signal strength and on the
particular station (there are some with no hum at all), so
I would tend not to attribute the cause to internal tube
leakage.

Your supposition that poor screen / AVC bypassing could be
the cause looks interesting and I will do some tests at
that regard.

However I did not fully understand your argument that
putting a capacitor in parallel to an existing one may not
a good way to do the test. If I use a low-ESR capacitor,
why is the existing one not out of the game?

Thanks and 73

Tony I0JX
First of all heater to cathode leakage may cause hum
modulation which is dependant on the AVC bias voltage so it
should not be discounted.
While leakage of capacitors is often called equivalent
series resistance (ESR) it can also be parallel resistance.
For instance its pretty common for bad electrolytic caps to
look like dead shorts. The same can happen to paper caps. If
the parallel resistance of a capacitor is low paralleling
another cap across it will simply put the low resistance
across both. So, if one is checking for bad caps its best to
actually substitute the cap otherwise you can be mislead.
If you don't have a tube checker you can also check by
substitution. You can switch the tubes around since the HRO
uses the same types in a couple of places. Moving a bad tube
should change the symptoms. Since you have another receiver
you can do more complete subsitution checking. Check tubes
with AVC on them since the varying bias will change the way
the tube modulates the signal, assuming its bad.
BTW, have you checked to see if the hum changes when you
go to manual RF gain control? There may be a clue there if
it does.


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Richard Knoppow
Los Angeles, CA, USA