Tom Didn't you report that you replaced the 4H choke on Monday and that there was still hum?? What changed?
Looking at your videos, I notice that the frequency readout on the o'scope does not agree with the displayed waveform.? The evanescent waveform looks to me like an oscillation.? There is a reduction in amplitude in several peaks then it returns to the initial amplitude.? It looks like an analog counting circuit.? Strange.
If the hum returns, I would check all of the circuit ground connections and solder them to the chassis.? I agree that there might be a strange feedback path causing the oscillation.? I had a Gonset G-63 receiver that I recapped but could not do a successful alignment.? I soldered all of the tube sockets and other terminals used for grounds and that cured the problem. Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Ok folks, I found the culprit – that dog gone choke! I replaced it with a 4hy one from an R390A AF deck and the hum is gone. I laid it in the chassis, insulated from the circuits and wired it in with short leads. Actually the hum is barely there, you have to put your ear up to the speaker to get a hint of it, but this is the level I’d expect from an old radio like this. ? There is a small difference in chokes in that the original is, I believe, 220 ohms and the Collins one is 110 ohms. I’ll have to try to locate one that fits the mounting holes. ? For now, I think we can put this head scratcher to rest. ? I appreciate all of the thought and input from everyone, I definitely learned something about this circuit which is always a good thing. Problem is will I remember it next week…………. ? ? Thank you all, ? Tom W3TA ?
From: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 6:08 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum ? Jacques, ? I “may” guilty of not returning caps to the same ground as original………… I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ? ? 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 – C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to “tailor” an audio bandpass response for voice frequencies (300Hz – 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
|
I was thinking about magnetic coupling through the air. Also about
just electric coupling due to currents in the chassis. Has to be coming
from someplace. I had a very puzzling hum problem in an R-388. 60Hz hum
there all the time. Pulled out the rectifier, still hum. I found that
one of the heater strings was completed through a ground connection via
the frame of the headphone jack on the front panel. Did not make good
contact due to paint. I scraped it and that fixed the hum. Put a star
washer on the jack to be sure. I had been thinking of magnetic coupling
via a choke or something. Realized it would have to be 120 Hz. This was
evidently coupled right into the speaker leads via the jack.
Probably something different in the SX-28 but I am curious about
grounding of the choke frame. Since there is 60Hz current through the
chassis perhaps a high resistance connection to the choke frame could
induce current. At least worth looking at since the choke is going to be
removed anyway. I suggest that before swapping it that Tom try grounding
it thoroughly to the chassis. I don't know how its fastened but if its
just sitting on the chassis and screwed down try putting star washers on
the screws so that the frame is thoroughly connected to the chassis. I
am VERY curious about this.
I used to lust after an SX-28, I think mainly because they are sexy
looking. Too heavy these days.
Final answer: Hums because it doesn't know the lyrics. Oh, dear.
toggle quoted message
Show quoted text
On 2/18/2025 7:26 PM, don Root wrote: Richard I just replied about that now. Your wording might be better than
mine.. well not quite , you propose magnetic coupling thru the air?, I
was thinking the steel chassis might couple depending on the mounting
“濒别驳蝉”.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Hi, Lately I’ve been waxing nostalgic about the old Hallicrafters transceivers. ?A friend loaned me his SR-160 for several months back in 1971 while I was in Senior in High School and I used it for that summer before heading off to the Vietnam War. ? It was a wonderful transceiver! ?
The premium Hurricane, Cyclone, and Tornado have fascinated me for many years but I never pursued them during my collecting years. ?The collecting years are long over and I’ve sold 99% of my collection with only a few left, but I’m still intrigued by those premium Halli transceivers.?
Maybe I need another one just to enjoy the nostalgia it gives.?I’m thinking of buying a Tornado for my one last remaining SSB tube transceiver. The Hurricane and Cyclone scare me in terms of big size and complexity of repair. ?What about the Tornado? ?Do you Tornado owners find them easy or hard to repair? ?Are final amplifier tubes available or do you have to rewire the socket for something else? ?
What do you Tornado owners think of them? ? Do you know of any working one’s for sale?
Thanks. ?73, Scott WA9WFA
|
I wonder about that. R-38 is shown in all three versions of the
handbook I have. In the civilian S-28 and S-28A its shown as a 47K or
50K half or third watt. In both drawings there is a straight wire from
the junction of R-38 to C-44 to the bass switch. I suspect the copy of
TM11-274 on BAMA was touched up, the resistor values are in a font that
does not match anything else. I will see if I can find another version
of this book. The slight differences in the value of the resistor shown
are due to the standardized RMA values being changed. The old value of
50K changed to 47K. R-38 is identified in the SX-28A manual as the
plate decoupling resistor for V-12 as I stated earlier.
toggle quoted message
Show quoted text
On 2/18/2025 6:29 PM, Jacques_VE2JFE via groups.io wrote: Hi Don,
Yeah, there is a misprint in the TM11-874 schematic.
The 180M (180K) is the value of R39 (see parts list), but there is
another “270M” in excess just below it….
*73, Jacques, VE2JFE in Montreal***
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Richard I just replied about that now. Your wording might be better than mine.. well not quite , you propose magnetic coupling thru the air?, I was thinking the steel chassis might couple depending on the mounting “濒别驳蝉”. ?
toggle quoted message
Show quoted text
From: HallicraftersRadios@groups.io [mailto:HallicraftersRadios@groups.io] On Behalf Of Richard Knoppow via groups.io
Sent: Tuesday, February 18, 2025 9:54 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? Once the choke is removed from the chassis can you investigate if
its position makes any difference. I mean is the hum the result of an
induced field from something? Also, does it make any difference if the
case or frame is grounded to the chassis?
I agree about the level of hum one would expect, audible only if
you put your ear the speaker.
On 2/18/2025 5:57 PM, thoyer via groups.io wrote: Ok folks, I found the culprit – that dog gone choke! I replaced it with
a 4hy one from an R390A AF deck and the hum is gone. I laid it in the
chassis, insulated from the circuits and wired it in with short leads.
Actually the hum is barely there, you have to put your ear up to the
speaker to get a hint of it, but this is the level I’d expect from an
old radio like this.
There is a small difference in chokes in that the original is, I
believe, 220 ohms and the Collins one is 110 ohms. I’ll have to try to
locate one that fits the mounting holes. --
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998 ? -- don??? va3drl
|
Tom, that is good, but ?that choke is always zipping up and down wrt ground.? Is it possible that the mounting to the chassis makes the difference. As in magnetic coupling to the power transformer? At the moment I don’t see just how that would do it, but I wonder is the original is partly isolated, would it be better. Or how is that choke failing? Insulation to the core and ground. ?
toggle quoted message
Show quoted text
From: HallicraftersRadios@groups.io [mailto:HallicraftersRadios@groups.io] On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 8:57 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? Ok folks, I found the culprit – that dog gone choke! I replaced it with a 4hy one from an R390A AF deck and the hum is gone. I laid it in the chassis, insulated from the circuits and wired it in with short leads. Actually the hum is barely there, you have to put your ear up to the speaker to get a hint of it, but this is the level I’d expect from an old radio like this. ? There is a small difference in chokes in that the original is, I believe, 220 ohms and the Collins one is 110 ohms. I’ll have to try to locate one that fits the mounting holes. ? For now, I think we can put this head scratcher to rest. +++++++++ ? -- don??? va3drl
|
Hi Mike, then the question becomes which loop?? They use/assume ?the chassis as if it has zero resistance and zero self inductance and zero mutual inductance. We just hope there was some method hidden in the madness when they designed it so critical areas are mostly isolated. It must be full of UNMARKED small loops It is a mystery to me how you don’t run into bad coupling. And like it was said I think, take a heater to a ground point, add a signal ground to the same point, and wait for corrosion to add some common resistance et voila a hum generator in the grid circuit. Don’t know if that is a real loop, but you can predict that those can go bad, and you can usually see them as possible hot spots.? Probably Jacques is talking about that sort of thing, but I have not gotten that far yet. ???? ?
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From: HallicraftersRadios@groups.io [mailto:HallicraftersRadios@groups.io] On Behalf Of Mike Langner via groups.io
Sent: Tuesday, February 18, 2025 8:56 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? I received a direct email from a member of this reflector asking what my comments about ground loops in an amplifier chassis had to do with the SX-28 hum discussion.
Sorry, I wasn’t specific enough.
?++++++++++++++++++++ -- don??? va3drl
|
Once the choke is removed from the chassis can you investigate if
its position makes any difference. I mean is the hum the result of an
induced field from something? Also, does it make any difference if the
case or frame is grounded to the chassis?
I agree about the level of hum one would expect, audible only if
you put your ear the speaker.
toggle quoted message
Show quoted text
On 2/18/2025 5:57 PM, thoyer via groups.io wrote: Ok folks, I found the culprit – that dog gone choke! I replaced it with
a 4hy one from an R390A AF deck and the hum is gone. I laid it in the
chassis, insulated from the circuits and wired it in with short leads.
Actually the hum is barely there, you have to put your ear up to the
speaker to get a hint of it, but this is the level I’d expect from an
old radio like this.
There is a small difference in chokes in that the original is, I
believe, 220 ohms and the Collins one is 110 ohms. I’ll have to try to
locate one that fits the mounting holes.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Dang ! Could you measure a leakage between the winding of the “original” choke and it’s core ?? Happy that this “mystery” is solved ! ? 73, Jacques, VE2JFE in Montreal ? De?: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> De la part de thoyer via groups.io
贰苍惫辞测é?: 18 février 2025 20:57
??: HallicraftersRadios@groups.io
Objet?: Re: [HallicraftersRadios] SX-28A Hum ? Ok folks, I found the culprit – that dog gone choke! I replaced it with a 4hy one from an R390A AF deck and the hum is gone. I laid it in the chassis, insulated from the circuits and wired it in with short leads. Actually the hum is barely there, you have to put your ear up to the speaker to get a hint of it, but this is the level I’d expect from an old radio like this. ? There is a small difference in chokes in that the original is, I believe, 220 ohms and the Collins one is 110 ohms. I’ll have to try to locate one that fits the mounting holes. ? For now, I think we can put this head scratcher to rest. ? I appreciate all of the thought and input from everyone, I definitely learned something about this circuit which is always a good thing. Problem is will I remember it next week…………. ? ? Thank you all, ? Tom W3TA ?
toggle quoted message
Show quoted text
From: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 6:08 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? Jacques, ? I “may” guilty of not returning caps to the same ground as original………… I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ? ? 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 – C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to “tailor” an audio bandpass response for voice frequencies (300Hz – 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
|
Hi Don, ? Yeah, there is a misprint in the TM11-874 schematic. The 180M (180K) is the value of R39 (see parts list), but there is another “270M” in excess just below it…. ? 73, Jacques, VE2JFE in Montreal ? De?: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> De la part de don Root
贰苍惫辞测é?: 18 février 2025 17:17
??: HallicraftersRadios@groups.io
Objet?: Re: [HallicraftersRadios] SX-28A Hum ? Jacques, I was going thru your recent post and started turning into a pretzel;? ?adding and then removing a resistor. See my “side-by-each” . ????180 “M?” but no squiggle ?
|
Hi Tom. Very clean SX-28A specimen ! Better shape than my two samples, actually… And an original SX-42 (R-42) speaker ! I have the radio, but never found the matching speaker… I have a PM-23 one, however (late war production, with no embedded “H” in the front grill). ? Agree with you that the “hum” is annoying and should not be there. What surprised me also is the shape (on your scope) of the 60 Hz signal. Really looks like a hi-current load passing thru a non-linear resistor and something in the 6SC7 stages seems to be referenced to. Can be the GND spot for the C44 capacitor, as I mentioned previously. And your third video tell me exactly the same thing: see how the 60Hz signal is distorted right on the plate (pin 2) when you put the bass sw “out”. And there is already a small 60Hz that can be seen even when the SW is “in” !!! It should be flat as a rule there, with the volume control at zero. My guess is that the + side of C44 is “moving” at 60 Hz, and this should not be. Can you probe it directly ? Where this cap is physically grounded ?? ? BTW, the schematic I post from the beginning comes from the TM11-874 and it indicates pin 5 for the plate of the 6SC7 related to the SW. Yours (from the 3rd video) is identified as pin 2 WHY ?? ? 73, Jacques, VE2JFE in Montreal ? De?: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> De la part de thoyer via groups.io
贰苍惫辞测é?: 18 février 2025 18:05
??: HallicraftersRadios@groups.io
Objet?: Re: [HallicraftersRadios] SX-28A Hum ? Ok, made a couple videos, excuse the camera work and narrating………. ?
?
?
? Tom W3TA
|
Ok folks, I found the culprit – that dog gone choke! I replaced it with a 4hy one from an R390A AF deck and the hum is gone. I laid it in the chassis, insulated from the circuits and wired it in with short leads. Actually the hum is barely there, you have to put your ear up to the speaker to get a hint of it, but this is the level I’d expect from an old radio like this. ? There is a small difference in chokes in that the original is, I believe, 220 ohms and the Collins one is 110 ohms. I’ll have to try to locate one that fits the mounting holes. ? For now, I think we can put this head scratcher to rest. ? I appreciate all of the thought and input from everyone, I definitely learned something about this circuit which is always a good thing. Problem is will I remember it next week…………. ? ? Thank you all, ? Tom W3TA ?
toggle quoted message
Show quoted text
From: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 6:08 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? Jacques, ? I “may” guilty of not returning caps to the same ground as original………… I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ? ? 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 – C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to “tailor” an audio bandpass response for voice frequencies (300Hz – 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
|
I received a direct email from a member of this reflector asking what my comments about ground loops in an amplifier chassis had to do with the SX-28 hum discussion.
Sorry, I wasn’t specific enough.
If along the way someone has moved a ground from some source of hum to a more convenient location during a repair, requiring the AC to return through a path via the chassis that also carries an audio signal return can and will (to a greater or lesser degree) impress hum into the audio. If 60 Hz, it can be a filament connection.? If 120 Hz it can be a B+ filter return.
That’s all !
Mike/
K5MGR
________________________ ? Mike Langner
929 Alameda Road NW
Albuquerque, NM 87114-1901
(505) 898-3212 home/home office
(505) 238-8810 cell
mlangner@...
|
Richard, ? Did you see my response with links to videos of the hum? ? Tom ?
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From: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> On Behalf Of Richard Knoppow via groups.io
Sent: Tuesday, February 18, 2025 7:30 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? A couple of things: Its quite possible the SX-28 has some residual
hum. I don't have one and little experience with them. The hum in the
original complaint has not been described in level. Is it barely audible
or is it very obvious? Makes a difference in the possible explanation.
The circuit with C-44 and R-38 is obviously a decoupling network and
additional B+ filter. It is the source for the plates of both halves of
the 6SC7, a fairly high gain tube. It is there all the time regardless
of the position of the BASS switch. I think it has nothing to do with
mid-range gain.
In the late 1930s speakers had pretty good good bass response.
Many modern speakers take advantage of the relationship between
efficiency and low frequency response. For a direct radiator speaker
extending the bass means reducing mid-range efficiency. The extended
bass is obtained by lowering the free air cone resonance of the speaker
and increasing its mid range damping to reduce the mid range efficiency
to match some arbitrary bass cut off. That results in reducing the
overall efficiency and as a result requiring more power to drive them.
The overall efficiency for a given bass cut off can be improved by
departing from the direct radiator type and going to either a resonant
matching network, i.e. a bass-reflex or acoustical labrynth encloser.
Another method is to horn load the speaker, which acts as an acoustical
impedance transformer. Both require larger and heavier, and more
complex, enclosures, so the direct radiator speaker, in a closed box,
with lots of excess amplifier power is the most common method. The
typical speaker for a communications receiver was an open back small
box. These are reasonable with a fairly large speaker but have the
disadvantage of an "organ pipe" resonance. Often putting the speaker in
a larger, fully closed box, will improve its sound surprisingly.
I don't know why Hallicrafters decided to put a substantial amount
of bass boost in the receiver but can think of a couple of reasons. One
was to give some sense of false bass for small speakers, another was to
provide equalization for certain kinds of phonograph pickups. The SX-28
has a phono plug on the back. The amplifier would have been considered
good quality in the pre-Hi-Fi days so having a phonograph attachment
would have resulted in a sort of home entertainment center with perhaps
a bit better sound than a typical portable record player. Crystal
pickups can be made self equalizing but magnetic pickups can't so
require compensation for the bass roll off built into phonograph records.
I am getting far off the track. The fact is I don't understand what
is causing the hum. The original poster has written that he has tried a
couple of different tubes with no results and, at this point, I am not
sure what to try next.
On 2/18/2025 2:03 PM, 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
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
A couple of things: Its quite possible the SX-28 has some residual
hum. I don't have one and little experience with them. The hum in the
original complaint has not been described in level. Is it barely audible
or is it very obvious? Makes a difference in the possible explanation.
The circuit with C-44 and R-38 is obviously a decoupling network and
additional B+ filter. It is the source for the plates of both halves of
the 6SC7, a fairly high gain tube. It is there all the time regardless
of the position of the BASS switch. I think it has nothing to do with
mid-range gain.
In the late 1930s speakers had pretty good good bass response.
Many modern speakers take advantage of the relationship between
efficiency and low frequency response. For a direct radiator speaker
extending the bass means reducing mid-range efficiency. The extended
bass is obtained by lowering the free air cone resonance of the speaker
and increasing its mid range damping to reduce the mid range efficiency
to match some arbitrary bass cut off. That results in reducing the
overall efficiency and as a result requiring more power to drive them.
The overall efficiency for a given bass cut off can be improved by
departing from the direct radiator type and going to either a resonant
matching network, i.e. a bass-reflex or acoustical labrynth encloser.
Another method is to horn load the speaker, which acts as an acoustical
impedance transformer. Both require larger and heavier, and more
complex, enclosures, so the direct radiator speaker, in a closed box,
with lots of excess amplifier power is the most common method. The
typical speaker for a communications receiver was an open back small
box. These are reasonable with a fairly large speaker but have the
disadvantage of an "organ pipe" resonance. Often putting the speaker in
a larger, fully closed box, will improve its sound surprisingly.
I don't know why Hallicrafters decided to put a substantial amount
of bass boost in the receiver but can think of a couple of reasons. One
was to give some sense of false bass for small speakers, another was to
provide equalization for certain kinds of phonograph pickups. The SX-28
has a phono plug on the back. The amplifier would have been considered
good quality in the pre-Hi-Fi days so having a phonograph attachment
would have resulted in a sort of home entertainment center with perhaps
a bit better sound than a typical portable record player. Crystal
pickups can be made self equalizing but magnetic pickups can't so
require compensation for the bass roll off built into phonograph records.
I am getting far off the track. The fact is I don't understand what
is causing the hum. The original poster has written that he has tried a
couple of different tubes with no results and, at this point, I am not
sure what to try next.
toggle quoted message
Show quoted text
On 2/18/2025 2:03 PM, 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
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
With the switch in the “out” position, no bass enhancement, the hum is present. ? With the bass set to the “in” position, there is bass enhancement but no hum. ? Tom ?
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From: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> On Behalf Of don Root
Sent: Tuesday, February 18, 2025 6:10 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? Jim ? Re:: “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.?“ ?? But ?post #1 says ?“When I switch the Bass in, the hum goes away.” , from witch I gather there IS a HUM problem in “OUT” where the 60Hz gain is lower ,, ???? I was doing mental cartwheels about this and my head is still sore. ? ? ? 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.?
--
don??? va3drl
|
Years ago, a PA system amplifier that had been donated to us for hamfest use had an annoying hum when we checked it out before its first hamfest use.
Nothing I tried would “fix” the hum.
Until ---
(Trumpets, please)
I thought about a possible ground loop within the amplifier’s chassis.
Bingo!
There was a “single point” bare ground buss wire that ran from one end of the chassis to the other.
But --
(More trumpets) --
It, the “single point ground,” was connected to the chassis at both ends.
I snipped one end of the buss wire, hanging it in the air.
No more hum!
I wonder how many hundreds of these PA amplifiers were out in the field, merrily humming away!
All for the lack of snipping one wire!
Yes, ground loops creating hum within chassis are very real!
Mike/
K5MGR
_________________________________________ ? Mike Langner
929 Alameda Road NW
Albuquerque, NM 87114-1901
(505) 898-3212 home/home office
(505) 238-8810 cell
mlangner@...
?
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From: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 4:08 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? Jacques, ? I “may” guilty of not returning caps to the same ground as original………… I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ? ? 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 – C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to “tailor” an audio bandpass response for voice frequencies (300Hz – 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
|
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 – C43 forms a resonant circuit at ~ 1.1kHz.
The obvious intent is to “tailor” an audio bandpass response for voice
frequencies (300Hz – 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
|
Jim ? Re:: “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.?“ ?? But ?post #1 says ?“When I switch the Bass in, the hum goes away.” , from witch I gather there IS a HUM problem in “OUT” where the 60Hz gain is lower ,, ???? I was doing mental cartwheels about this and my head is still sore. ? ?
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From: HallicraftersRadios@groups.io [mailto:HallicraftersRadios@groups.io] On Behalf Of Jim Whartenby via groups.io
Sent: Tuesday, February 18, 2025 5:03 PM
To: hallicraftersradios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? 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.? -- don??? va3drl
|
Jacques, ? I “may” guilty of not returning caps to the same ground as original………… I need to look into this. I took lots of before and after pics so I should be able to confirm ? Tom ?
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From: HallicraftersRadios@groups.io <HallicraftersRadios@groups.io> On Behalf Of Jacques_VE2JFE via groups.io
Sent: Tuesday, February 18, 2025 2:44 PM
To: HallicraftersRadios@groups.io
Subject: Re: [HallicraftersRadios] SX-28A Hum? 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 – C43 forms a resonant circuit at ~ 1.1kHz. The obvious intent is to “tailor” an audio bandpass response for voice frequencies (300Hz – 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
|
Jim Whartenby
don Root
Jacques_VE2JFE
Mike Langner