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

"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:
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."

"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

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