I recapped my Swan 270B about 21 years ago and its been my most modern
HF rig since. When I use a nice old set of headphones, there is no hum.
But a modern set of high fidelity phones introduces enough hum to
notice but not to impair operation.
So I just use the older phones and assume that it might be possible to
find the source of the hum, but maybe not.
73,
Maynard
W6PAP
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On 2/18/25 14:03, Jim Whartenby via groups.io wrote:
Tom and Jacques
You are overlooking an alternative, that nothing is actually wrong.
AFAIK. the radio works well but there is a noticeable hum.? Hum is
subjective, if you have listened to solid state audio most of your life
you expect to hear no background audio.? If you have listened to tube
audio all of your life, you expect some hum.
The question about what type of speaker is used with the SX-28 has been
asked twice now but there has been no answer.? Newer speakers have a
very wide dynamic range, both in frequency and volume.? Hum in tube
powered radios is common, actually it is part of the charm.? If an older
midrange speaker is connected to the SX-28, what then?
The Bass IN/OUT thing could be attributed to the change in gain of the
1st audio amp.? Figure 11, the audio filter curve, shows that the gain
is 100+X higher when the bass switch is in the IN position at 60
cycles.? I would not say that the hum is gone in the OUT position, but I
would say that it is not easily heard in the speaker.? If you put your
ear next to the speaker, is there a weak hum present?
If the control grids of both 6V6 tubes are grounded, then no hum is
heard.? What about grounding the two control grids of the 6SC7?? With
this done, the two 6V6 tubes should have proper self bias and the 6V6
control grids should not have any audio input applied.? In this
condition the quietest 6V6 tubes can be selected assuming that several
are available to pick from.
It just occurred to me that if C49 is defective, or not actually 30uF,
then the plates of the 6V6 would have a high ripple AC on them.? Do you
have another 30 uF 450 volt electrolytic to temporarily bridge C49 with?
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
On Tuesday, February 18, 2025 at 01:44:16 PM CST, Jacques_VE2JFE via
groups.io <jacques.f@...> wrote:
*Let¡¯s resume the case?:*
Tom pulled the 6SC7 and the ¡°hum¡± remained.
He shorted the two 6V6 grids to GND and the hum disappeared.
No hum when the ¡°bass¡± switch is in the IN position (both plate loads of
the 6SC7 equal and CH2 + C43 shorted).
Hum present when the ¡°bass¡± sw in OUT position.
The ¡°hum¡± is at 60Hz¡
Which suspects remains ??
_Does the C44 is ¡°grounded¡± at the same place than the filaments of the
6V6s and this connection is resistive ??_
_Clue: when the driver stage is ¡°balanced¡± plates loads wise, no hum._
_When it is not: hum !_
Note here that the 4H. inductor reactance at 60Hz is only 1500 ohms¡ and
that the C43 is practically an open circuit at 60 Hz.
*Could it be that the 60Hz injection point is the C44 grounding lug ?*
My two SX-28As are too far to be quickly reached.
But If I manage to dig the ¡°never touched¡± one out of the storage during
the next weekend, I can check how it is wired.
*Keep thinking¡.*
And about the meaning of the ¡°Bass¡± switch: when it is IN, the bass is
ENABLED (literally, ¡°bass¡± is in).
When out: bass is out (DISABLED).
CH2 ¨C C43 forms a resonant circuit at ~ 1.1kHz.
The obvious intent is to ¡°tailor¡± an audio bandpass response for voice
frequencies (300Hz ¨C 3kHz)
*73, Jacques, VE2JFE in Montreal***
Gents, dunno about all that, but why the no hum in the In position? And
when base is boosted?
don??? va3drl
