[Hallicrafters] SX-111 Receiver

[Mark Shaum]

Bill,

I have a couple dozen vintage tube receivers and several of them have
some residual hum (either 60 or 120hz) that is generally barely
detectable on the speaker with AF gain down. This is usually more
detectable with wide-bandwidth headphones in the phones jack.  The level
is low enough that even the slightest advance of the AF gain will
generally cover the hum with receiver internal noise or atmospherics,
depending on circumstances.  This can be cleaned up but can involve
considerable effort, especially if it is 60hz and a result of magnetic
coupling into the AF stage output transformer or some such situation
where you need to invest in magnetic shielding or rotating transformers
to minimize coupling, etc.. usually not worth the effort.  If the hum is
120 hz, "permanent" hum can be addressed with more efficient
filtering/bypassing, grounding, etc.

Since your hum starts an hour or so after warmup, and if it is 60hz, I
suspect a tube may have excessive heater to cathode leakage developing
(filaments are at 60hz).  A tube swap would help pinpoint this. There
might also be a capacitor or two that gets a little off-base when warmed
up, temperature wise.  I'd concentrate on components around the af
preamp triode and the power amp.

73! - Mark K9TR

----- Original Message -----
From: "Bill Gerhold" <wpgerhold at elongo.com>


> I have an excellent SX-111 receiver that receives very well and is in
> almost
> Pristine condition.  However, using it recently, I noticed after about
1
> hour
> Warmup, and the volume all the way down, a slight 60hz hum was coming
> From the speaker.  Is this normal?  Or is it time for the PS caps to
go?
>