If you pull the detector tube out, is there hum? How about the last IF
tube? How close to the front end can you get before there is still hum
after the tube is pulled out?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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

"Antonio Vernucci" wrote in message news:47bf2ba9$0$36443$
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.

Tony I0JX


Tony

Have you tried using an external balanced antenna? Is this problem occuring
with a random length antenna wire that is using the AC lines, etc. as the
counterpoise for the antenna system? Some more information on the antenna
might provide some clues. Tuneable hum shouldn't be a problem when
using an external balanced antenna.

Pete k1zjh

"Antonio Vernucci" wrote in message
...
Most of the strong AM signals received on my National HRO
are affected by hum.

I put a ceramic capacitor in parallel to each of the
80-type vacuum diodes with no noticeable improvement. Does
anyone remember which other cure was proposed to solve the
problem?

73

Tony, I0JX

Tunable hum is sometimes caused by bad bypass caps, its
worth checking. This could also be intermodulation from
something in the neighborhood especially if these are strong
signals. Can be checked by using another receiver,
preferably a battery receiver. Someone else mentioned hummy
tubes, also worth checking.
Putting a ceramic in parrallel with the filter caps may
help but I think its more likely a decoupling or bypass cap
has gone bad.


--
---
Richard Knoppow
Los Angeles, CA, USA

"Antonio Vernucci" wrote in message
...
Most of the strong AM signals received on my National HRO are affected by
hum.

I put a ceramic capacitor in parallel to each of the 80-type vacuum diodes
with no noticeable improvement. Does anyone remember which other cure was
proposed to solve the problem?

73

Tony, I0JX

Tony

I'm assuming you're talking about tuneable hum, where the AC line frequency
modulates the received carrier.

Does the receiver have AC line bypass caps from each leg of the AC power
to the chassis? Have you tried grounding the chassis to a good earth ground
to see if the hum modulation level is reduce? If grounding helps, you
should
check to see that the bypass caps are installed. Also adding ferrite snap on
cores on the power may help along with the ground connection.

pete k1zjh

Its MAGNETIC induction from the power transformer into the audio
transformer.
-Al




Antonio Vernucci wrote:
Most of the strong AM signals received on my National HRO are affected
by hum.

I put a ceramic capacitor in parallel to each of the 80-type vacuum
diodes with no noticeable improvement. Does anyone remember which other
cure was proposed to solve the problem?

73

Tony, I0JX

Al Schapira wrote:
Its MAGNETIC induction from the power transformer into the audio
transformer.

If this is the case, putting a piece of thick sheet steel between the
two transformers will change the noise, for diagnostic purposes. Then
you can try and reorient the transformers to reduce coupling.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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

"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

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

"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.


--
---
Richard Knoppow
Los Angeles, CA, USA