|
Agree Jacques, I did some poking around as well and found what you described.
Thanks,? Tom
Hello Tom, I believe that the “bubbles” in the original choke encapsulation just came from when the coal tar was poured in during manufacture. There is no possibility that the part “overheated” IMHO. For the suitability of the replacements: as the 4Hy choke from the R-390A Audio Deck you used only have 110 ohms of internal resistance, it will be 100% OK to add a 110 ohms resistor in series with it to “emulate” the 220 ohms of the original part. I also checked if any recent manufacture Hammond choke can be used, but no luck there: the smallest 4Hy one have a 300 ohms internal resistance, and that value cannot be “reduced” in any way. The other 4Hy parts are really too big for the task, not speaking of the cost… ? 73, Jacques, VE2JFE in Montreal ? Well that's not what I wanted to hear, but I understand. I'm actively looking for a suitable replacement choke. The on I substituted in was all I had laying around and figured it would be good for a test. The original choke is encapsulated and the bottom side shows bubbles in that encapsulation. I have designed products that have used encapsulation in the past and have seen these bubbles before, they are usually caused by the part overheating. Hopefully the attached image shows.
|
Hello Tom, I believe that the “bubbles” in the original choke encapsulation just came from when the coal tar was poured in during manufacture. There is no possibility that the part “overheated” IMHO. For the suitability of the replacements: as the 4Hy choke from the R-390A Audio Deck you used only have 110 ohms of internal resistance, it will be 100% OK to add a 110 ohms resistor in series with it to “emulate” the 220 ohms of the original part. I also checked if any recent manufacture Hammond choke can be used, but no luck there: the smallest 4Hy one have a 300 ohms internal resistance, and that value cannot be “reduced” in any way. The other 4Hy parts are really too big for the task, not speaking of the cost… ? 73, Jacques, VE2JFE in Montreal ? Well that's not what I wanted to hear, but I understand. I'm actively looking for a suitable replacement choke. The on I substituted in was all I had laying around and figured it would be good for a test. The original choke is encapsulated and the bottom side shows bubbles in that encapsulation. I have designed products that have used encapsulation in the past and have seen these bubbles before, they are usually caused by the part overheating. Hopefully the attached image shows.
|
Jim, that waveform is what I previously commented on. Looks like a base frequency of 60 with third harmonic. ?the 3rd harmonic is generated by the PT magnetizing current and it ?along with the 60 HZ coupled ?to wires in the choke circuit somehow; wiring being most likely. The location of the choke ?makes it unlikely that it is directly from the transformer. ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Wednesday, February 19, 2025 10:14 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? I am not sure that the problem is resolved.? CH2 was replaced with a similar value choke with less winding resistance so it will have higher Q.? Higher Q will affect the audio filter curve so there will most likely be more ripple in the audio bandpass in the bass out switch position.? This may not be that noticeable to the ear but the scope will see it. In Tom's first video, at about the 45 second mark, the scope shows a strange waveform.? The main peaks are of a 60 cycle waveform.? The center peak is at 120 cycle but it's peak is negative, the two remaining peaks are at 240 hertz and positive.? There must be some nonlinearity to cause these harmonics to be generated.? This nonlinearity is acting like a diode and causing the second and forth harmonics of the 60 cycle waveform. -- don??? va3drl
|
Hi Jim,?
Well that's not what I wanted to hear, but I understand. I'm actively looking for a suitable replacement choke. The on I substituted in was all I had laying around and figured it would be good for a test.
The original choke is encapsulated and the bottom side shows bubbles in that encapsulation. I have designed products that have used encapsulation in the past and have seen these bubbles before, they are usually caused by the part overheating.
Hopefully the attached image shows.
Tom
I am not sure that the problem is resolved.? CH2 was replaced with a similar value choke with less winding resistance so it will have higher Q.? Higher Q will affect the audio filter curve so there will most likely be more ripple in the audio bandpass in the bass out switch position.? This may not be that noticeable to the ear but the scope will see it.
In Tom's first video, at about the 45 second mark, the scope shows a strange waveform.? The main peaks are of a 60 cycle waveform.? The center peak is at 120 cycle but it's peak is negative, the two remaining peaks are at 240 hertz and positive.? There must be some nonlinearity to cause these harmonics to be generated.? This nonlinearity is acting like a diode and causing the second and forth harmonics of the 60 cycle waveform. Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Thank you. Since its a different choke it doesn't explain why the
original was humming. Doesn't matter if the problem is solved but would
be interesting to know. Its possible there is a leak between the winding
and core or frame in the original. A sensitive ohm meter would probably
tell. I had thought of suggesting putting the old choke on an insulator
(just a sheet of paper) temporarily to see if the hum stopped. Not worth
taking the new choke out to try.
The SX-28 is an interesting receiver, an attempt by Hallicrafters
at advanced engineering that didn't quite come off. The Lamb noise
blanker was a very good idea but was not well implemented. Eventually,
Drake and others, did a better job with it. This type of blanker works
well on pulse type noise, like ignition noise, but hardly at all on the
more common (now) power line noise. Hallicrafters eventually published a
modification (on BAMA) to disable it and replace it with a conventional
series limiter. I hope not many SX-28s were mutilated this way.
On 2/19/2025 11:57 AM, thoyer via groups.io wrote: Yes, I replaced the original CH2 with another 4hy one I had from an
R390A AF module.
On Wednesday, February 19, 2025 at 02:43:41 PM EST, Richard Knoppow via
groups.io <1oldlens1@...> wrote:
Please clarify, do you mean you replaced the original choke and the
problem went away? If so it could confirm my idea of filament current
somehow coupling through the chassis.
-- Richard Knoppow Los Angeles WB6KBL SKCC 19998
|
That's what I wanted to know.
toggle quoted message
Show quoted text
On 2/19/2025 5:57 PM, thoyer via groups.io wrote: In one test, I removed the original choke from the chassis, still wired
in place though and still had the hum so connection to the chassis makes
no difference.
Tom
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
I am not sure that the problem is resolved.? CH2 was replaced with a similar value choke with less winding resistance so it will have higher Q.? Higher Q will affect the audio filter curve so there will most likely be more ripple in the audio bandpass in the bass out switch position.? This may not be that noticeable to the ear but the scope will see it.
In Tom's first video, at about the 45 second mark, the scope shows a strange waveform.? The main peaks are of a 60 cycle waveform.? The center peak is at 120 cycle but it's peak is negative, the two remaining peaks are at 240 hertz and positive.? There must be some nonlinearity to cause these harmonics to be generated.? This nonlinearity is acting like a diode and causing the second and forth harmonics of the 60 cycle waveform. Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Thank you. Since its a different choke it doesn't explain why the
original was humming. Doesn't matter if the problem is solved but would
be interesting to know. Its possible there is a leak between the winding
and core or frame in the original. A sensitive ohm meter would probably
tell. I had thought of suggesting putting the old choke on an insulator
(just a sheet of paper) temporarily to see if the hum stopped. Not worth
taking the new choke out to try.
The SX-28 is an interesting receiver, an attempt by Hallicrafters
at advanced engineering that didn't quite come off. The Lamb noise
blanker was a very good idea but was not well implemented. Eventually,
Drake and others, did a better job with it. This type of blanker works
well on pulse type noise, like ignition noise, but hardly at all on the
more common (now) power line noise. Hallicrafters eventually published a
modification (on BAMA) to disable it and replace it with a conventional
series limiter. I hope not many SX-28s were mutilated this way.
On 2/19/2025 11:57 AM, thoyer via groups.io wrote: Yes, I replaced the original CH2 with another 4hy one I had from an
R390A AF module.
On Wednesday, February 19, 2025 at 02:43:41 PM EST, Richard Knoppow via
groups.io <1oldlens1@...> wrote:
Please clarify, do you mean you replaced the original choke and the
problem went away? If so it could confirm my idea of filament current
somehow coupling through the chassis.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
In one test, I removed the original choke from the chassis, still wired in place though and still had the hum so connection to the chassis makes no difference. ? Tom ?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of Richard Knoppow via groups.io
Sent: Wednesday, February 19, 2025 3:14 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Thank you. Since its a different choke it doesn't explain why the
original was humming. Doesn't matter if the problem is solved but would
be interesting to know. Its possible there is a leak between the winding
and core or frame in the original. A sensitive ohm meter would probably
tell. I had thought of suggesting putting the old choke on an insulator
(just a sheet of paper) temporarily to see if the hum stopped. Not worth
taking the new choke out to try.
The SX-28 is an interesting receiver, an attempt by Hallicrafters
at advanced engineering that didn't quite come off. The Lamb noise
blanker was a very good idea but was not well implemented. Eventually,
Drake and others, did a better job with it. This type of blanker works
well on pulse type noise, like ignition noise, but hardly at all on the
more common (now) power line noise. Hallicrafters eventually published a
modification (on BAMA) to disable it and replace it with a conventional
series limiter. I hope not many SX-28s were mutilated this way.
On 2/19/2025 11:57 AM, thoyer via groups.io wrote: Yes, I replaced the original CH2 with another 4hy one I had from an
R390A AF module.
On Wednesday, February 19, 2025 at 02:43:41 PM EST, Richard Knoppow via
groups.io <1oldlens1@...> wrote:
Please clarify, do you mean you replaced the original choke and the
problem went away? If so it could confirm my idea of filament current
somehow coupling through the chassis.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Thanks Jacques, well that seems to pour cold water on the magnetic coupling theory. If the trace I copied is typical {I don’t know}… ie; assuming? that hum is 60 cps with third harmonics at /I n /close to the choke, there can be electrostatic coupling by close parallel wires from AC incoming wires being too close to the? choke ??wires. ??? Moving stuff might mean moving wires and coupling. I worry more about long wires running to the switch. ?Another long shot.? … but the old stories of just moving stuff raises its head. I’m dried out. ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Wednesday, February 19, 2025 3:04 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? In a SX-28A, the CH2 is as far as it can be from the power transformer: ? ? ? The black “core” at the left is the CH2. You can see the bottom of the power transformer and the filtering choke at right. The closest transformer to the CH2 is the audio output. ? ? 73, Jacques, VE2JFE in Montreal ? ? Tom and/or anybody still tuned in. Still looking for what really failed and how. For anyone wondering about the details and possible source, I copied a bit of your first Video so I could take a closer look. I see 60 Hz and strong third harmonics, and no visible high harmonics. It reminds me of core saturation. Power transformer magnetics and primary is the only source I can think of. Any comments?
--
don??? va3drl
-- don??? va3drl
|
Thank you. Since its a different choke it doesn't explain why the
original was humming. Doesn't matter if the problem is solved but would
be interesting to know. Its possible there is a leak between the winding
and core or frame in the original. A sensitive ohm meter would probably
tell. I had thought of suggesting putting the old choke on an insulator
(just a sheet of paper) temporarily to see if the hum stopped. Not worth
taking the new choke out to try.
The SX-28 is an interesting receiver, an attempt by Hallicrafters
at advanced engineering that didn't quite come off. The Lamb noise
blanker was a very good idea but was not well implemented. Eventually,
Drake and others, did a better job with it. This type of blanker works
well on pulse type noise, like ignition noise, but hardly at all on the
more common (now) power line noise. Hallicrafters eventually published a
modification (on BAMA) to disable it and replace it with a conventional
series limiter. I hope not many SX-28s were mutilated this way.
toggle quoted message
Show quoted text
On 2/19/2025 11:57 AM, thoyer via groups.io wrote: Yes, I replaced the original CH2 with another 4hy one I had from an
R390A AF module.
On Wednesday, February 19, 2025 at 02:43:41 PM EST, Richard Knoppow via
groups.io <1oldlens1@...> wrote:
Please clarify, do you mean you replaced the original choke and the
problem went away? If so it could confirm my idea of filament current
somehow coupling through the chassis.
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
In a SX-28A, the CH2 is as far as it can be from the power transformer: ?
? The black “core” at the left is the CH2. You can see the bottom of the power transformer and the filtering choke at right. The closest transformer to the CH2 is the audio output. ? ? 73, Jacques, VE2JFE in Montreal ? ? Tom and/or anybody still tuned in. Still looking for what really failed and how. For anyone wondering about the details and possible source, I copied a bit of your first Video so I could take a closer look. I see 60 Hz and strong third harmonics, and no visible high harmonics. It reminds me of core saturation. Power transformer magnetics and primary is the only source I can think of. Any comments?
--
don??? va3drl
|
Yes, I replaced the original CH2 with another 4hy one I had from an R390A AF module.
Please clarify, do you mean you replaced the original choke and the
problem went away? If so it could confirm my idea of filament current
somehow coupling through the chassis.
On 2/19/2025 9:52 AM, thoyer via groups.io wrote: Interesting Don........
When I replaced the choke, CH2, this all went away so I don't know how
it would be related to the power xfmr?
Tom
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Please clarify, do you mean you replaced the original choke and the
problem went away? If so it could confirm my idea of filament current
somehow coupling through the chassis.
toggle quoted message
Show quoted text
On 2/19/2025 9:52 AM, thoyer via groups.io wrote: Interesting Don........
When I replaced the choke, CH2, this all went away so I don't know how
it would be related to the power xfmr?
Tom
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Tom, if I recall it was not fastened to the chassis… at least in the same way. ?If the power transformer is near by the choke, it’s flux can/might? use a bit of the chassis and the choke could pick it up from the flux in the chassis. For that I would try removing steel fastening on one hold down point or both and grounding the core for safety at one point only or separately.? You could Put a little space under the choke as a test? ? But it is also possible that wires from the on/of switch run too close to the BASE ?switch. ? If you want more stable scope traces sync to the “line” or whatever it might be called now. ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of thoyer via groups.io
Sent: Wednesday, February 19, 2025 12:52 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? When I replaced the choke, CH2, this all went away so I don't know how it would be related to the power xfmr? On Wednesday, February 19, 2025 at 12:31:39 PM EST, don Root <drootofallevil@...> wrote: Tom and/or anybody still tuned in. Still looking for what really failed and how. For anyone wondering about the details and possible source, I copied a bit of your first Video so I could take a closer look. I see 60 Hz and strong third harmonics, and no visible high harmonics. It reminds me of core saturation. Power transformer magnetics and primary is the only source I can think of. Any comments?  --
don??? va3drl ??
-- don??? va3drl
|
Interesting Don........
When I replaced the choke, CH2, this all went away so I don't know how it would be related to the power xfmr?
Tom
Tom and/or anybody still tuned in. Still looking for what really failed and how. For anyone wondering about the details and possible source, I copied a bit of your first Video so I could take a closer look. I see 60 Hz and strong third harmonics, and no visible high harmonics. It reminds me of core saturation. Power transformer magnetics and primary is the only source I can think of. Any comments?
--
don??? va3drl
|
Tom and/or anybody still tuned in. Still looking for what really failed and how. For anyone wondering about the details and possible source, I copied a bit of your first Video so I could take a closer look. I see 60 Hz and strong third harmonics, and no visible high harmonics. It reminds me of core saturation. Power transformer magnetics and primary is the only source I can think of. Any comments? -- don??? va3drl
|
Excerpt from?.? Might want to read the entire article.
There is also an underlying very low-level
hum present which is totally independent of audio gain level. I did some
research on the matter ... i.e. I googled 'SX-28 hum'. And, it turns
out its a thing! There are more than a few discussions on-line regarding
the issue. Investigating the issue, I concluded that the hum was indeed
mains-hum ... i.e. 50Hz (UK Mains). Now, since the 5Z3 is a full wave
rectifier, any AC ripple on the B+ line would be 100Hz. So that rules
out an issue with the smoothing pack. Several of the on-line discussions
come to the same conclusion, that the hum is not electrical pick-up but
actually electro-magnetically induced, with the massive mains
transformer being the source. Because of its proximity to the mains
transformer, the audio transformer actually picks up the induced field,
albeit small and transfers it to the loudspeaker or headphones ... and
since it is post AF-Gain control, it is a constant level. This also
explains why when you switch on your SX-28, you get a loud hum from the
speaker almost immediately, before the valves have heated up. This then
fades away before coming back as a very low level hum. Note that in the
SX-28, the mains transformer, smoothing choke and audio transformer are
all aligned on the same plane, so it isn't surprising that there is some
electromagnetic coupling.
This helps with your situation.? Still looking to find my answer to different problem.
Bill
On Wednesday, February 19, 2025 at 07:24:42 AM EST, thoyer via groups.io <thoyer1@...> wrote:
Richard,?
The choke is held to the chassis with two screws that have internal tooth lockwashers on both sides - one under the screw head, one under the nut. That is how it was when I removed it and that is how I put it back in.?
That being said, the choke frame is painted all around including the bottom side of the flanges that come in contact to the chassis. I think I'll remove the paint from those contact surfaces and reinstall it to see if there is any change.
Tom
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.
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
|
Jim,?
You are correct. I had replaced the choke but with 12" leads outside the chassis. There was a reduction in hum but it was still there. This time I installed it in the same location as the original with short leads. Could be the longer leads as well as the routing of them made a difference with the initial try?
I also thought the waveform was "funky" but chalked it up to the 60hz summing with other asynchronous noise peaks.
Tom
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: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 6:08 PM
To: [email protected]
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
|
Richard,?
The choke is held to the chassis with two screws that have internal tooth lockwashers on both sides - one under the screw head, one under the nut. That is how it was when I removed it and that is how I put it back in.?
That being said, the choke frame is painted all around including the bottom side of the flanges that come in contact to the chassis. I think I'll remove the paint from those contact surfaces and reinstall it to see if there is any change.
Tom
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.
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
|
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: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 6:08 PM
To: [email protected]
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
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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: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Tuesday, February 18, 2025 6:08 PM
To: [email protected]
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
|
Jacques_VE2JFE
don Root
Jim Whartenby