[Hallicrafters] SX 101A hum

[David Franklin]

Andy,

When you added the three wire line cord did you tie the ground lead to the 
chassis?  If so I do not see how it is causing a problem.  To be sure, use 
an isolation plug and see if it helps. Since you have replaced all of the 
capacitors, here are a few other things to try:

1. Replace the .01 mfd capacitor from one side of the line to the chassis if 
you haven't already.  (C92)

2. Pull all of the tubes and see if you still hear the hum.  Sometimes a 
filament can be shorted to the cathode inside the tube.  The tube may work 
fine but it will cause hum. If the hum goes away replace the tubes one at a 
time starting with the rectifier.

3. If you still hear hum with all of the tubes removed you may have a bad 
ground somewhere around the audio output.  Check all connections to the 
chassis especially the tube sockets to make sure there is good contact.

73,
Dave
W4DEF

----- Original Message ----- 
From: "Andy Bullington" <abullington at comcast.net>
To: <hallicrafters at mailman.qth.net>
Sent: Tuesday, February 05, 2008 10:18 AM
Subject: [Hallicrafters] SX 101A hum


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.
                     Andy W1AWB