"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

On 2/22/08 12:07 PM, in article
, "Antonio Vernucci"
wrote:

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


What happens when you reduce the RF Gain? How about in CW mode?

Antonio Vernucci wrote:
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.

Tunable hum is easy: most of the time it is caused by either heater-cathode
leakage on the AGC, or pentodes in the IF amplifier, or bypass capacitors on
the AGC, or IF tubes.

Tunable hum is a modulation issue, the small amount of ripple that comes from
the heater-cathode leakage is amplified greatly by the AGC circuitry, and
AM modulates the IF signal with power-line hum.

A little tube swapping very often finds the problem. Start with the AGC/detector
tube, and then go to the tubes that are controlled by the AGC line.

Make sure that you put any tube that isn't bad back in its original position.

If you look at the power supply, you will probably see a multi section electrolytic
capacitor that has resistors between the sections. Usually, the AGC tube is powered
from the most filtered section of the power supply filter, so if the hum is coming
from there, it is usually everywhere.

-Chuck

"Antonio Vernucci" wrote in message
...
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).

Tony

You have been offered several suggestions, and I offered some advice
and never heard a reply back to some questions I asked regarding the
antenna system and grounding.

Tuneable hum can indeed be caused by poor cathode RF bypass caps
in the RF and IF stages. The lack of adequate bypass capacitors would
allow any RF/IF stage with cathode/filament leakage to be modulated
by the AC signal on the filament. No signal, no hum.

Pete k1zjh

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.

Using my HF Yagi antenna instead of the indoor wire all problems completely
disappeared and there is no hum at all on any station. The problem however is
not due to interfering sources (e.g. lamps, power supplies, etc.) in my house,
but to the fact that the RF signal of certain stations (not all of them) follows
a path where it gets modulated by alternating current. As a matter of fact,
using a simple wire, the ground counterpoise is represented by the house
electrical wiring

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.

Still, I am unable to follow your argument. Assume that a screen bypass
capacitor is leaky. Its DC resistance typically is in the range of several
hundreds kohm (should it be lower, the screen DC voltage would be greatly
affected). Having a resistance in parallel to a good capacitor does not impair
its bypassing function. Even a low resistance value would not harm (apart from
its effect on the DC voltage). In the limit, a zero resistance would turn into a
perfect bypass (again, apart from the DC voltage).

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.

The hum-to-sound ratio was almost independent of the RF gain control setting (a
potentiometer of the tubes cathode).

73

Tony I0JX

Antonio Vernucci wrote:
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.

Using my HF Yagi antenna instead of the indoor wire all problems
completely disappeared and there is no hum at all on any station.
....

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.

The hum-to-sound ratio was almost independent of the RF gain control
setting (a potentiometer of the tubes cathode).


Both of the above statements indicate that the hum was caused by emissions
from your receiver, or your shack.

Classically, on a solid state rectified device, the rectifiers don't switch
instantly, and hand up for an instant at every zero crossing of the AC waveform.
That hang is a 1 usec long short circuit, and emits 60Hz glitches at every crossing.

It will radiate 60 Hz modulated broadband hash over a frequency range that includes
the AM broadcast band.

When you switched to an external antenna, you did two things, one, you got the
antenna away from the interfering source, and two, you increased the signal
strength of the received stations.

Probable sources of such hash are compact fluorescent lamps, regular fluorescent
lamps, the power supplies in any test equipment, or soldering irons in your shop,
and the radio itself.

-Chuck

On Feb 24, 6:36 am, Chuck Harris wrote:
Antonio Vernucci wrote:
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.

Using my HF Yagi antenna instead of the indoor wire all problems
completely disappeared and there is no hum at all on any station.
...

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.

The hum-to-sound ratio was almost independent of the RF gain control
setting (a potentiometer of the tubes cathode).

Both of the above statements indicate that the hum was caused by emissions
from your receiver, or your shack.

Classically, on a solid state rectified device, the rectifiers don't switch
instantly, and hand up for an instant at every zero crossing of the AC waveform.
That hang is a 1 usec long short circuit, and emits 60Hz glitches at every crossing.

It will radiate 60 Hz modulated broadband hash over a frequency range that includes
the AM broadcast band.

When you switched to an external antenna, you did two things, one, you got the
antenna away from the interfering source, and two, you increased the signal
strength of the received stations.

Probable sources of such hash are compact fluorescent lamps, regular fluorescent
lamps, the power supplies in any test equipment, or soldering irons in your shop,
and the radio itself.

-Chuck

Tony et al,

I have followed this thread with interest as I had a similar problem
on a Hammarlund SP-200. Pulling tubes I isolated to the first or
second audio or detector. It was only when I measured voltages for
each pin on each tube that I began to see the problem. On the first
audio I measured the grid and found it to be a bit high