Hum when using headphone
 

I have a backgound hum when listening to short wave on my RS66
radio with headphones. Is there a modification I can make to the
headphone jack to eliminate the hum? The headphones do not have the
hum when I use them on my other radios. Is this an impedance matching
problem?

I have a backgound hum when listening to short wave on my RS66 radio with
headphones.

I'm not familiar with that radio, but is the hum heard while on battery power?

My DX392 eats batteries but, when using the wall thingie, it has hum on CW/SSB
and hum bars on FAX. So, I built a regulated power supply for it using R.S.
parts.

I'll try this coming year to get all the info about the the power supply posted
as a web page. The supply is usable for any of the portable SW radios, by
adjusting the output voltage and/or changing the power connector wiring.

Bill, K5BY

The radio in question is a Radio Shack DX66 from around 1986.
It does not have the hum when operating on battery power.
Can the power supply in the radio be modified to eliminate the hum?
How do I do it?


WShoots1 wrote in article
...
I have a backgound hum when listening to short wave on my RS66 radio
with
headphones.

I'm not familiar with that radio, but is the hum heard while on battery
power?

My DX392 eats batteries but, when using the wall thingie, it has hum on
CW/SSB
and hum bars on FAX. So, I built a regulated power supply for it using
R.S.
parts.

I'll try this coming year to get all the info about the the power supply
posted
as a web page. The supply is usable for any of the portable SW radios, by
adjusting the output voltage and/or changing the power connector wiring.

Bill, K5BY

"author" pondered the futility of human existence in
an uncaring universe and yet found the courage to write:

The radio in question is a Radio Shack DX66 from around 1986.
It does not have the hum when operating on battery power.

Bingo, the AC supply.

I don't know anything about the radio in question. Does it use the
typical "wall wart" power supply? These often are poorly regulated so
it's not surprising if you get a hum when using it.

If you are using a wall wart there are various things you can do to
reduce hum. The simplest is to try connecting a Great Big Honking
Capacitor (tm) across the leads (being careful to observe proper
polarity). If the AC to DC conversion is internal to the radio,
things get a little more complicated.


--
hambu n hambu hodo

Charles Hawtrey wrote:
"author" pondered the futility of human existence in
an uncaring universe and yet found the courage to write:


The radio in question is a Radio Shack DX66 from around 1986.
It does not have the hum when operating on battery power.


Bingo, the AC supply.

I don't know anything about the radio in question. Does it use the
typical "wall wart" power supply? These often are poorly regulated so
it's not surprising if you get a hum when using it.

If you are using a wall wart there are various things you can do to
reduce hum. The simplest is to try connecting a Great Big Honking
Capacitor (tm) across the leads (being careful to observe proper
polarity). If the AC to DC conversion is internal to the radio,
things get a little more complicated.


--
hambu n hambu hodo

Besides AC hum from a poorly filtered power supply, an AC adapter can
bring in RFI from various noisy gizmos around the house - this is solved
by putting a ferrite bead on the power lead.

What Charles said, "author." The "wall wart" was the problem I had with my
DX392, so I built a regulated supply.

What's the supply voltage for that DX66?

Bill, K5BY

The DX66 takes 4 type C batteries so the supply voltage must be 6V.
I should point out that there is a hum without the headphones but it is
less notisable when using the speaker. I became aware of the hum
after using headphones.

The schematic shows each end of transformer secondary connected to
a diode ...a filter cap across it ...and a transistor with a zener
diode on the base at the output of the power supply.

Can I modify this with something to get rid of the hum?

WShoots1 wrote in article
...
What Charles said, "author." The "wall wart" was the problem I had with
my
DX392, so I built a regulated supply.

What's the supply voltage for that DX66?

Bill, K5BY

"author" pondered the futility of human existence in
an uncaring universe and yet found the courage to write:

The DX66 takes 4 type C batteries so the supply voltage must be 6V.
I should point out that there is a hum without the headphones but it is
less notisable when using the speaker. I became aware of the hum
after using headphones.

The schematic shows each end of transformer secondary connected to
a diode ...a filter cap across it ...and a transistor with a zener
diode on the base at the output of the power supply.

Can I modify this with something to get rid of the hum?

Rather than internally modifying the unit, it would be simpler to buy
or build a 6V regulated supply and connect it to the battery
terminals.

I don't know how experienced you are with a soldering iron but with
modern integrated-circuit regulators it is not at all hard to build a
decent supply. An easy approach is to start with a 12V wall wart (new
or salvaged from some other appliance) and use a LM317 regulator. You
need the higher voltage wall wart because the LM317 drops about 3V.
Google for "LM317" to get manufacturer's data sheets which contain
example circuits, or email me and I'll send them to you.


--
hambu n hambu hodo

The schematic shows each end of transformer secondary connected to a diode
....a filter cap across it ...and a transistor with a zener diode on the base at
the output of the power supply. Can I modify this with something to get rid of
the hum?

Hmm... Possibly a capacitor across the output might help. Or even replacing the
existing capacitor. It may have derated with age or, because there's a zener
involved that may keep the ripple down, even opened.

Any corroboration from others? G

Bill, K5BY

I wrote: Possibly a capacitor across the output might help. Or even
replacing the
existing capacitor.

Oops! Do NOT put a capacitor across the output. Without surge suppresion, the
surge of that capacitor charging at turn on can damage the transistor.

DO, though, try replacing the existing filter capacitor. If that doesn't do the
trick, then the supply's design is probably deficient.

To consider other possible cures gets too involved for any brief comment in
this forum.

cc: via e-mail

Bill, K5BY