[Hallicrafters] SX 101A hum

[Glen Zook]

Are you using high impedance headphones or low
impedance (i.e. 8 ohm) type?

The receiver was designed for high impedance
"communications" quality (not "hi-fi") headphone
operation which do not have good low frequency
response.  Many of the older receivers will have "hum"
when using modern low impedance hi-fi headphones
because those types have a much better low frequency
response.

All of my Collins receivers do exhibit some "hum" when
I use hi-fi type headphones yet there is none when I
use older high impedance types.  It is the same for
most of my "boat anchor" receivers.  Adding
capacitance to the B+ filtering and other things may,
or may not, help reduce this "hum".  Some receivers
seem to be better or worse in how much "hum" there is
when using headphones.  Generally, the speaker is
pretty much "hum" free.

Since I normally use the "ole tyme" high impedance
headphones I don't "worry" about the "hum" when using
low impedance hi-fi headphones.

Now one thing that may help is to tighten all of the
machine screws and nuts that go through the chassis. 
These work loose and/or corrode over the years and all
of the grounds in the receiver are made through these.
 By getting these connectors "tight" again all sorts
of "screwy" problems are eliminated.

If the tube sockets, etc., are held in with rivits
instead of machine screws you are going to have to use
some sort of punch and a hammer to get the connectors
"tight" again.

Glen, K9STH


--- Andy Bullington <abullington at comcast.net> wrote:

It's winter project time so I've decided to revisit my
SX 101A. I've realigned it with good results but on
using headphones I'm noticing an annoying hum. It
doesn't seem to be a tube , there is a 60 cycle hum
even in standby mode. Using a speaker the hum isn't as
noticeable but still there. The overall audio sound is
very smooth and better than my NC303 but the NC 303 is
hum free. I've long ago replaced all the paper caps
and electrolytics and also some out of spec resistors.
At the same time I did all that work I replaced the
power cord with a 3 prong. I'm wondering if I did that
wrong. Any help for a poor technically challenged
musician would be much appreciated. Here's where I
stand: I get hum even in standby or with the audio
turned all the way down or the audio tubes removed.
It's not run you out of the room hum, but it shouldn't
be there. Is it something in the power supply? Thanks
in advance.









Glen, K9STH

Website:  http://k9sth.com


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