"Al Schapira" wrote in message
...
I think its MAGNETIC coupling from the power transformer to
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

If its an older HRO, and it has to be with a type 80
rectifier, the power supply is on a separate chassis with no
way that hum could be induced into the output transformer.
In any case, this would be quite low level hum. Much more
likely to be hum modulated onto the carriers due to poor
grounding, bypassing, etc. The HRO was always a well
designed and well made receiver and should not have this
problem.


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

On Feb 21, 6:28*pm, "Richard Knoppow" wrote:
"Al Schapira" wrote in message

...



I think its MAGNETIC coupling from the power transformer to
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

* * If its an older HRO, and it has to be with a type 80
rectifier, the power supply is on a separate chassis with no
way that hum could be induced into the output transformer.
In any case, this would be quite low level hum. Much more
likely to be hum modulated onto the carriers due to poor
grounding, bypassing, etc. The HRO was always a well
designed and well made receiver and should not have this
problem.

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


While I am not familiar with this particular rcvr, if the pwr supply
is on a separate chassis and is connected to the main chassis with a
cable, would a bad ground connection in the cable or plugs (if any)
cause this? Maybe try running a temporary wire from chassis to chassis
to eliminate the possibility. 73 Mike KF6KXG

"tchrme" wrote in message
...
On Feb 21, 6:28 pm, "Richard Knoppow"
wrote:
"Al Schapira" wrote in
message

...



I think its MAGNETIC coupling from the power transformer
to
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

If its an older HRO, and it has to be with a type 80
rectifier, the power supply is on a separate chassis with
no
way that hum could be induced into the output transformer.
In any case, this would be quite low level hum. Much more
likely to be hum modulated onto the carriers due to poor
grounding, bypassing, etc. The HRO was always a well
designed and well made receiver and should not have this
problem.

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


While I am not familiar with this particular rcvr, if the
pwr supply
is on a separate chassis and is connected to the main
chassis with a
cable, would a bad ground connection in the cable or plugs
(if any)
cause this? Maybe try running a temporary wire from chassis
to chassis
to eliminate the possibility. 73 Mike KF6KXG

Its certainly worth a try. I am not sure from the
original poster's question if the hum is constant or is
present on some stations. Both conditions can be caused by
poor filter caps but the second is more likely to come from
something else. Strong RF pickup from a poorly grounded
supply would be one of the causes.
If these are broadcast stations (would mean he has one
or more accessory coil sets) another tool would be a simple
loop antenna, perhaps two turns of wire around a shoebox
sized form. This will give low enough signal strength from
local stations to avoid front end overload and will also
help to track down modulation coming from sources external
to the receiver. And front end overload is certainly another
possible cause.


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

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."

Are you sure the hum isn't real? A lot of solid state
devices trash up the power line. Rectifiers are a big
source, lamp dimmers and fluorescent lamps are another.

Barring that, look for heater cathode shorts from the
detector tube back to the power amplifier tube.

-Chuck

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

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

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 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
...
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