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