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

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?

Before someone gives me a simple answer, I confirm that the electrolytic
capacitors were checked to be good and the DC HV is perfectly filtered. No hum
in absence of signal.

73

Tony I0JX

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?

You sure the power supply filter caps are good?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

You sure the power supply filter caps are good?

Please read my self-reply.

Tony I0JX

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

Antonio Vernucci wrote:
You sure the power supply filter caps are good?

Please read my self-reply.

Tony I0JX

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

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

Antonio Vernucci wrote:
You sure the power supply filter caps are good?

Please read my self-reply.

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."