[Hallicrafters] SX 101A hum

[Kenneth G. Gordon]

On 5 Feb 2008 at 10:18, Andy Bullington 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.
>                      Andy W1AWB 

Andy:

Sometimes this sort of hum can be difficult to track down. It can 
be as simple as a tube, somewhere, with either a cathode-to-
filament short or high cathode-to-filament leakage, or a low-level 
audio stage interconnect lead running too close to a filament wire.

You might try, as an experiment, to temporarily use DC on the 
entire filament string. That should confirm or eliminate that as the 
source of hum.

If the filaments are all 6.3 VAC, use a 6 VDC car or motorcycle 
battery with an appropriately sized fuse in series, after 
disconnecting the filament transformer leads.

You can't simply rectify the filament transformer connections 
since the output voltage with any decent filter would be nearly 9 
VDC.

As far as a three-wire cord is concerned, the green wire goes to 
the chassis, the black wire goes to the switch or the fuse holder 
hot side, whichever is first, and the white wire goes to transformer 
common.

Lastly, you COULD have a ground-loop.

Ken Gordon W7EKB