I suppose I have to reply to each comment. The plate load is the
impedance in the plate circuit between supply voltage and plate, that
develops the signal output. In the half of the 6SC7 that the bass switch
is in it is either the resonant choke or R-37 depending on the position
of the switch. R-38 is not part of the plate load because there is no AC
on it. It is bypassed to ground via C-44. C-44 is a 10uF electrolytic
but is probably effective over the entire audio range. If electrolytic
caps were as bad as you indicate no bypass cap could work. The voltage
at the junction of C-44 and R-38 is well filtered DC.
If you look up the circuit for a "floating paraphase" phase splitter
you will find exactly what is in the SX-28.
The term "load" may be confusing, I mean the impedance in the plate
circuit. In this circuit it is coupled to the grid of one of the 6V6
tubes via a capacitor C-45 which is connected to the grid resistor of
one 6V6 R-41. The actual load on the 6SC7 is the combination of the
plate load and the following grid load.
I still want to know what is actually in these receivers. The
presentation of the bass tone switch is exactly the same in both the
SX-28 and SX-28A and in the military manuals. I have seen cases of other
errors carried over from generation to generation of instruction manuals
or even text books but a real answer would be to look at an actual
receiver and see how its wired. From the curves in the books the
labeling means that the boost is ON when the switch is IN.
To clarify a previous remark, at the time lots of bass was
considered desirable for entertainment audio. Not necessarily good bass,
just lots of it. This was how juke boxes were designed. Probably also
intended for use with the phonograph input.
This still does not address the hum problem but I strongly suspect
that the increased low end gain is exaggerating a hum problem in the tube.
toggle quoted message
Show quoted text
On 2/17/2025 7:56 AM, Jim Whartenby via groups.io wrote: Richard
Comments inserted in the text below.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
On Monday, February 17, 2025 at 03:09:24 AM CST, Richard Knoppow via
groups.io <1oldlens1@...> wrote:
Ch2 and C-43 form a parallel resonant circuit, a tank, if you will.
It is in the plate lead of the pre amplifier half of the 6SC7. When the
switch is in the position marked IN on the diagram it is shorted out by
the switch. The plate load of the tube is then R-37.
No, the plate load for the 1st triode section is R37 and R38.? The
voltage gain of the 1st audio stage is approximately the plate load
resistance divided by the cathode resistance or 147k / 1k? or 147 when
SW10 is in the IN position. All of those bypass capacitors that are
sprinkled throughout the audio amplifier cause a fast roll off of the
midrange audio so that the bass frequencies are emphasized because of
the higher gain.
When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out.
What about R38?? In the out position, the voltage gain of the 1st triode
is approximately 47k / 1k or 47.? Figure 11 shows this gain reduction
when SW10 is in the OUT position so Figure 11 actually agrees with the
schematic.
Note that C-44 along with R-48 is a bypass filter for the B+ going
to both 6SC7 plates. Since the plate load with the switch in the IN
position is resistive there should be no frequency discrimination. In
OUT the plate load is a resonant choke (about 1100 Hz).
Again, what about R38?? It is still in series with the parallel
combination of CH2 and C43.? In the OUT position, the frequency response
of the 1st Audio is now relatively flat so there is no base boost
compared to what you see in the IN position curve.
I was surprised
the frequency is not lower but calculated it a couple of times. I have
not looked up the plate resistance of the tube. R-36 and R-37 are 100K.
C-44 is 10uF and R-48 is also 100K so there should be no audio at that
point even down to quite low frequencies.
You are considering the electrolytic capacitors to be ideal.? The
components in the late 1930s and early 1940s were far from that.? The
ESL and ESR were not controlled back then.? Modern components are much
closer to the ideal but still aluminum electrolytic capacitors start to
become inductive at midrange audio frequencies.? This is why they are
not recommended as coupling & bypass capacitors for high end audio.
They cause distortion.
As far as fidelity the SX-28 was intended to be a good fidelity
receiver for AM broadcasts. Like the Super-Pro, it is designed to have a
wide IF and, for the time, a relatively high quality output amplifier.
This begs the question, what is the speaker that is used with this
SX-28?? Could it simply be the lower gain of the 1st audio amplifier
when SW10 is in the OUT position that causes the normal 120 cycle hum to
disappear into the mud?? If so, then there is actually no problem in the
SX-28 audio amplifier.? We are chasing our tail.
While broadcast stations in the old days were required to have good
performance to about 10Khz (double the standard now) few receivers could
recover much beyond perhaps 4 or 5 Khz. Hallicrafters offered a "High
Fidelity" speaker for use with the SX-28, a bass-reflex made by Jensen.
The bass boost offered is, IMO extreme according to the response chart.
There is a crude high frequency control, all roll off, probably to
reduce the effect of static and other noise. A HF boost would have
required another stage of amplification. They knew how to do it, see the
old (third edition) of the RDH for some circuits.
I think the labeling of the BASS control in the handbook is an
error even though it got continued in the 28A.
If you consider that BASS really means BASS BOOST then what is happening
will make more sense.
Someone with an actual
receiver can determine it for us, I am just guessing from what's on the
schematic.
I am strongly suspicious of the tube, easy to prove by substitutingit.
On 2/16/2025 11:11 PM, Jim Whartenby via groups.io wrote:
So the hum in question might just be 60 cycle leakage instead of the
originally stated 120 cycle?? I guess that this is possible but what is
the explanation for the loss of the hum when SW10 is in the Bass
(boost)
IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in
the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 &
R38
make up the plate resistance.? In the Bass (boost) OUT condition, CH2
and C43 are in circuit but only R38 is used as the plate resistance.? A
lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are
believable
since the contribution at 1kc of CH2 and C43 are, as previously stated,
clearly evident in the Bass (boost) OUT curve.? What the
contribution of
R35 for the midrange frequencies in either curve is not mentioned but
clearly, the lower the resistance of R35, the lower the available
midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before
the high fidelity craze of the 1950s.? Today, over all tone is
considered to be composed of bass, midrange and treble frequencies.
Treble frequencies for the most part are not available in the SX-28
since the high audio frequencies are limited to perhaps 3 to 4 kc.
Midrange frequencies are fixed by C40 in the phase splitter and by C42
and the R35 pot in the 1st audio triode section of the 6SC7.? Since the
SX-28 is, in the end, a communications receiver, the loss of treble
frequency control is not an issue.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
_._,_._,_
------------------------------------------------------------------------
Groups.io Links:
You receive all messages sent to this group.
View/Reply Online (#31789) </g/HallicraftersRadios/
message/31789> | Reply to Group <mailto:[email protected]?
subject=Re:%20Re%3A%20%5BHallicraftersRadios%5D%20SX-28A%20Hum> | Mute
This Topic </mt/111174219/518617> | New Topic <https://
groups.io/g/HallicraftersRadios/post>
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Jacques,? yes, but it just makes things harder to understand.? ?I was originally focused on when and how each L3 coil shorts to ground or doesn¡¯t, and quickly decided them must be using the mutual conductance. Physically those coils are as close together as possible [ Emanuele¡¯s picture and words].? So more ways to go wrong.? I am hoping we will see sweeps of the mixer tanks so we can see if they track with the RF tanks, or what. No matter the case what is to blame for the apparent low frequencies and very bad tracking with the LO ?? Since the trimmers are accessible, I just suggested lifting a trimmer to see what happens to the frequency at the top of the dial mostly. And Re the mixer tanks, it is hard to know what is wrong when you don¡¯t know? ¡°what should be right¡±. ???Pictures show a few new resistors; are they connect properly? I continue to be haunted by the common mode: all 4 tanks apparently tuning too Low. What {reasonable?}? condition would make that happen?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Sunday, February 16, 2025 10:11 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!? Don, OK, understand what you mean by ¡°L3 serve both bands¡± but that cannot explain why either coil cannot be tuned properly to the low frequency alignment value. ? 73, Jacques, VE2JFE in Montreal -- don??? va3drl
|
Richard Comments inserted in the text below. Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Ch2 and C-43 form a parallel resonant circuit, a tank, if you will.
It is in the plate lead of the pre amplifier half of the 6SC7. When the
switch is in the position marked IN on the diagram it is shorted out by
the switch. The plate load of the tube is then R-37. No, the plate load for the 1st triode section is R37 and R38.? The voltage gain of the 1st audio stage is approximately the plate load resistance divided by the cathode resistance or 147k / 1k? or 147 when SW10 is in the IN position.??All of those bypass capacitors that are sprinkled throughout the audio amplifier cause a fast roll off of the midrange audio so that the bass frequencies are emphasized because of the higher gain. When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out. What about R38?? In the out position, the voltage gain of the 1st triode is approximately 47k / 1k or 47.? Figure 11 shows this gain reduction when SW10 is in the OUT position so Figure 11 actually agrees with the schematic. Note that C-44 along with R-48 is a bypass filter for the B+ going
to both 6SC7 plates. Since the plate load with the switch in the IN
position is resistive there should be no frequency discrimination. In
OUT the plate load is a resonant choke (about 1100 Hz). Again, what about R38?? It is still in series with the parallel combination of CH2 and C43.? In the OUT position, the frequency response of the 1st Audio is now relatively flat so there is no base boost compared to what you see in the IN position curve.?? I was surprised
the frequency is not lower but calculated it a couple of times. I have
not looked up the plate resistance of the tube. R-36 and R-37 are 100K.
C-44 is 10uF and R-48 is also 100K so there should be no audio at that
point even down to quite low frequencies. You are considering the electrolytic capacitors to be ideal.? The components in the late 1930s and early 1940s were far from that.? The ESL and ESR were not controlled back then.? Modern components are much closer to the ideal but still aluminum electrolytic capacitors start to become inductive at midrange audio frequencies.? This is why they are not recommended as coupling & bypass capacitors for high end audio.? They cause distortion.
As far as fidelity the SX-28 was intended to be a good fidelity
receiver for AM broadcasts. Like the Super-Pro, it is designed to have a
wide IF and, for the time, a relatively high quality output amplifier.
This begs the question, what is the speaker that is used with this SX-28?? Could it simply be the lower gain of the 1st audio amplifier when SW10 is in the OUT position that causes the normal 120 cycle hum to disappear into the mud?? If so, then there is actually no problem in the SX-28 audio amplifier.? We are chasing our tail. While broadcast stations in the old days were required to have good
performance to about 10Khz (double the standard now) few receivers could
recover much beyond perhaps 4 or 5 Khz. Hallicrafters offered a "High
Fidelity" speaker for use with the SX-28, a bass-reflex made by Jensen.
The bass boost offered is, IMO extreme according to the response chart.
There is a crude high frequency control, all roll off, probably to
reduce the effect of static and other noise. A HF boost would have
required another stage of amplification. They knew how to do it, see the
old (third edition) of the RDH for some circuits.
I think the labeling of the BASS control in the handbook is an
error even though it got continued in the 28A. If you consider that BASS really means BASS BOOST then what is happening will make more sense.? Someone with an actual
receiver can determine it for us, I am just guessing from what's on the
schematic.
I am strongly suspicious of the tube, easy to prove by substituting?it.
On 2/16/2025 11:11 PM, Jim Whartenby via groups.io wrote: So the hum in question might just be 60 cycle leakage instead of the
originally stated 120 cycle?? I guess that this is possible but what is
the explanation for the loss of the hum when SW10 is in the Bass (boost)
IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in
the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38
make up the plate resistance.? In the Bass (boost) OUT condition, CH2
and C43 are in circuit but only R38 is used as the plate resistance.? A
lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are believable
since the contribution at 1kc of CH2 and C43 are, as previously stated,
clearly evident in the Bass (boost) OUT curve.? What the contribution of
R35 for the midrange frequencies in either curve is not mentioned but
clearly, the lower the resistance of R35, the lower the available
midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before
the high fidelity craze of the 1950s.? Today, over all tone is
considered to be composed of bass, midrange and treble frequencies.
Treble frequencies for the most part are not available in the SX-28
since the high audio frequencies are limited to perhaps 3 to 4 kc.
Midrange frequencies are fixed by C40 in the phase splitter and by C42
and the R35 pot in the 1st audio triode section of the 6SC7.? Since the
SX-28 is, in the end, a communications receiver, the loss of treble
frequency control is not an issue.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Re: For Sale - Hallicrafters R-274/FRR with speaker
Oops, I overlooked the price. Sounds great.
Tom
|
Re: For Sale - Hallicrafters R-274/FRR with speaker
Hi Dwight,
I have an observatory in Pie Town, New Mexico and plan a trip there as soon as it warms up a bit. I¡¯m now sure exactly where you live but I would like to buy your receiver. What are you asking for it. I could pay you now with a US postal money order and pick it up later.
?
Thanks,
Tom N5AMA
|
Last night I was able to find another 4hy choke. I knew I had one around just couldn't put my finger on it. It was salvaged from an old R390A AF module.?Dawned on me last night at 8:30 to go check the shed. Bundled up headed out into the 50mph wind storm we were having (dog didn't even want to follow me out). Found the choke, swapped it into the radio - hum still there..........
That tube is looking more and more suspect............
Tom w3TA
Could be....?
One other tid bit, whenever I switch between in/out there is a loud thump in the speaker - annoying to say the least.
So the hum in question might just be 60 cycle leakage instead of the originally stated 120 cycle?? I guess that this is possible but what is the explanation for the loss of the hum when SW10 is in the Bass (boost) IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38 make up the plate resistance.? In the Bass (boost) OUT condition, CH2 and C43 are in circuit but only R38 is used as the plate resistance.? A lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are believable since the contribution at 1kc of CH2 and C43 are, as previously stated, clearly evident in the Bass (boost) OUT curve.? What the contribution of R35 for the midrange frequencies in either curve is not mentioned but clearly, the lower the resistance of R35, the lower the available midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before the high fidelity craze of the 1950s.? Today, over all tone is considered to be composed of bass, midrange and treble frequencies.? Treble frequencies for the most part are not available in the SX-28 since the high audio frequencies are limited to perhaps 3 to 4 kc.? Midrange frequencies are fixed by C40 in the phase splitter and by C42 and the R35 pot in the 1st audio triode section of the 6SC7.? Since the SX-28 is, in the end, a communications receiver, the loss of treble frequency control is not an issue. Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote: 6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
-- Richard Knoppow Los Angeles WB6KBL SKCC 19998
|
Could be....?
One other tid bit, whenever I switch between in/out there is a loud thump in the speaker - annoying to say the least.
So the hum in question might just be 60 cycle leakage instead of the originally stated 120 cycle?? I guess that this is possible but what is the explanation for the loss of the hum when SW10 is in the Bass (boost) IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38 make up the plate resistance.? In the Bass (boost) OUT condition, CH2 and C43 are in circuit but only R38 is used as the plate resistance.? A lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are believable since the contribution at 1kc of CH2 and C43 are, as previously stated, clearly evident in the Bass (boost) OUT curve.? What the contribution of R35 for the midrange frequencies in either curve is not mentioned but clearly, the lower the resistance of R35, the lower the available midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before the high fidelity craze of the 1950s.? Today, over all tone is considered to be composed of bass, midrange and treble frequencies.? Treble frequencies for the most part are not available in the SX-28 since the high audio frequencies are limited to perhaps 3 to 4 kc.? Midrange frequencies are fixed by C40 in the phase splitter and by C42 and the R35 pot in the 1st audio triode section of the 6SC7.? Since the SX-28 is, in the end, a communications receiver, the loss of treble frequency control is not an issue. Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote: 6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
-- Richard Knoppow Los Angeles WB6KBL SKCC 19998
|
Another coincidence - I just happen to have that book at the office! I quickly looked it up and there it is. Hopefully today is a slow day and I can spend some time reading. Also saw in the index a section on "bass boost" which might be worth a look see.
Thanks,? Tom W3TA
The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote: 6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Thanks - will try a couple more. I have tapped on it a few times with no change, but have not tried a different one - yet....
6SC7 tubes are renown for developing heater-cathode leakage and for developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
____________________________________________ ? Mike Langner
929 Alameda Road NW
Albuquerque, NM 87114-1901
(505) 898-3212 home/home office
(505) 238-8810 cell
mlangner@...
?
From: [email protected] <[email protected]> On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 6:32 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum ? Try another tube. Maybe heater to cathode leakage -------- Original message -------- Date: 2/16/25 5:07 PM (GMT-08:00) Subject: Re: [HallicraftersRadios] SX-28A Hum Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ? ? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
|
Funny, as I was driving into work I thought the same thing, now I have to wait until tonight to try it..........
Will update later
Try another tube. Maybe heater to cathode leakage
-------- Original message -------- From: "thoyer via groups.io" <thoyer1@...> Date: 2/16/25 5:07 PM (GMT-08:00) Subject: Re: [HallicraftersRadios] SX-28A Hum
Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ? ? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
|
FWIW, I think the best manual for the SX-28A to be found on-line is
the military manual TM11-874, which may be found on BAMA at:
<>
toggle quoted message
Show quoted text
On 2/16/2025 1:59 AM, Richard Knoppow via groups.io wrote:
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Ch2 and C-43 form a parallel resonant circuit, a tank, if you will.
It is in the plate lead of the pre amplifier half of the 6SC7. When the
switch is in the position marked IN on the diagram it is shorted out by
the switch. The plate load of the tube is then R-37. When the switch is
in the OUT condition, the tank forms the plate load and R-37 is shorted
out. Note that C-44 along with R-48 is a bypass filter for the B+ going
to both 6SC7 plates. Since the plate load with the switch in the IN
position is resistive there should be no frequency discrimination. In
OUT the plate load is a resonant choke (about 1100 Hz). I was surprised
the frequency is not lower but calculated it a couple of times. I have
not looked up the plate resistance of the tube. R-36 and R-37 are 100K.
C-44 is 10uF and R-48 is also 100K so there should be no audio at that
point even down to quite low frequencies.
As far as fidelity the SX-28 was intended to be a good fidelity
receiver for AM broadcasts. Like the Super-Pro, it is designed to have a
wide IF and, for the time, a relatively high quality output amplifier.
While broadcast stations in the old days were required to have good
performance to about 10Khz (double the standard now) few receivers could
recover much beyond perhaps 4 or 5 Khz. Hallicrafters offered a "High
Fidelity" speaker for use with the SX-28, a bass-reflex made by Jensen.
The bass boost offered is, IMO extreme according to the response chart.
There is a crude high frequency control, all roll off, probably to
reduce the effect of static and other noise. A HF boost would have
required another stage of amplification. They knew how to do it, see the
old (third edition) of the RDH for some circuits.
I think the labeling of the BASS control in the handbook is an
error even though it got continued in the 28A. Someone with an actual
receiver can determine it for us, I am just guessing from what's on the
schematic.
I am strongly suspicious of the tube, easy to prove by substituting
it.
toggle quoted message
Show quoted text
On 2/16/2025 11:11 PM, Jim Whartenby via groups.io wrote: So the hum in question might just be 60 cycle leakage instead of the
originally stated 120 cycle?? I guess that this is possible but what is
the explanation for the loss of the hum when SW10 is in the Bass (boost)
IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in
the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38
make up the plate resistance.? In the Bass (boost) OUT condition, CH2
and C43 are in circuit but only R38 is used as the plate resistance.? A
lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are believable
since the contribution at 1kc of CH2 and C43 are, as previously stated,
clearly evident in the Bass (boost) OUT curve.? What the contribution of
R35 for the midrange frequencies in either curve is not mentioned but
clearly, the lower the resistance of R35, the lower the available
midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before
the high fidelity craze of the 1950s.? Today, over all tone is
considered to be composed of bass, midrange and treble frequencies.
Treble frequencies for the most part are not available in the SX-28
since the high audio frequencies are limited to perhaps 3 to 4 kc.
Midrange frequencies are fixed by C40 in the phase splitter and by C42
and the R35 pot in the 1st audio triode section of the 6SC7.? Since the
SX-28 is, in the end, a communications receiver, the loss of treble
frequency control is not an issue.
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.
Murphy
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
So the hum in question might just be 60 cycle leakage instead of the originally stated 120 cycle?? I guess that this is possible but what is the explanation for the loss of the hum when SW10 is in the Bass (boost) IN position?
I believe that the SX-28 schematic is correct after all.? With SW10 in the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38 make up the plate resistance.? In the Bass (boost) OUT condition, CH2 and C43 are in circuit but only R38 is used as the plate resistance.? A lot of control is accomplished by a simple SPDT switch.
It seems to me that the curves in Figure 11 of the manual are believable since the contribution at 1kc of CH2 and C43 are, as previously stated, clearly evident in the Bass (boost) OUT curve.? What the contribution of R35 for the midrange frequencies in either curve is not mentioned but clearly, the lower the resistance of R35, the lower the available midrange audio frequencies.
Perhaps part of the problem is that the SX-28 was made a decade before the high fidelity craze of the 1950s.? Today, over all tone is considered to be composed of bass, midrange and treble frequencies.? Treble frequencies for the most part are not available in the SX-28 since the high audio frequencies are limited to perhaps 3 to 4 kc.? Midrange frequencies are fixed by C40 in the phase splitter and by C42 and the R35 pot in the 1st audio triode section of the 6SC7.? Since the SX-28 is, in the end, a communications receiver, the loss of treble frequency control is not an issue. Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote: 6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Not the right coils used initially, or was it made on a Friday 13 ? ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de don Root
·¡²Ô±¹´Ç²â¨¦?: 16 f¨¦vrier 2025 18:11
??: [email protected]
Objet?: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak! ? Jacques, ??yes or a mysterious additional capacitance.? But it is fishy that both L3 and L6 exhibit the same thing and on both bands. Out the door Friday 5pm ?before Christmas?? ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Sunday, February 16, 2025 5:29 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!? IF the variable capacitor is the original one, I cannot see else than the antenna coils that have too much inductance. That will screw both low and hi end alignment. And it is exactly what happen. ? Emanuele, where are you in Italy ?? ? 73, Jacques, VE2JFE in Montreal
--
don??? va3drl
|
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Don, OK, understand what you mean by ¡°L3 serve both bands¡± but that cannot explain why either coil cannot be tuned properly to the low frequency alignment value. ? 73, Jacques, VE2JFE in Montreal ? De?: [email protected] <[email protected]> De la part de don Root
·¡²Ô±¹´Ç²â¨¦?: 16 f¨¦vrier 2025 17:47
??: [email protected]
Objet?: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak! ? Jacques Are you sure you are looking at an S-85?? I see no indication of cores in the RF tuning ?for bands 1,2 ?. Note Bands go 1,2,3,4? up in frequency, but coil numbers go the other way.. gets us all!? ? Re ???Is it just me, or the two lower band inductors are connected in reverse (one for each other) in the S85 schematic ??? Do you mean L3? which serves both bands ? ?IF so we have gone on wild trips about that. I finally drew the switch wafer in all positions, It is somewhere. It is too hard to spell it out, without a picture /g/HallicraftersRadios/message/31639?? ?I drew this just to find out the ground switch stuff while the you need to keep the coils in hand 2. I could not rotate this switch in my head. ?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Sunday, February 16, 2025 4:57 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Importance: High? Emanuele, Fantastic pictures, but I do not believe that the C7A tuning cap can be at fault. Obviously the coil have too much inductance, so it¡¯s core have to be moved OUT of the coil (to the top, or to the bottom, I do not know which one will be best). Normally, the trimmer cap should be set at it¡¯s center setting, then the core of the coil have to be adjusted to obtain a peak at the low frequency specified in the alignment instructions, then the response should be peaked with the trimmer at the high frequency specified in the alignment instructions. Then go back to the low frequency with the core, then to the high frequency with the trimmer, and doing this several times until no improvement is measured. ? But Ah¡ I understand: no alignment instructions in the manual for the CORES of the coils, like if nobody had touched those since the day of the manufacture !!! AARGH ! ? Find a plastic driver and MOVE those cores while observing what happen with your Spectrum Analyser, PLEASE ! BC band: low side at 600kHz (L3 core ??), High side at 1400 kHz (C4 trimmer), then loop again. ? Is it just me, or the two lower band inductors are connected in reverse (one for each other) in the S85 schematic ??? ? ? ? 73, Jacques, VE2JFE in Montreal
--
don??? va3drl
|
Re: S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!
Emanuele ??? Re ??Here is what I get:? Band 1: main tune dial at 0.54MHz - the bottom of band 1 (C7A fully meshed - maximum capacity) ref only.I shriveled it..don The peak is a little below 500KHz (in fact not visible) - C4 has no visible effects. Turning the main dial toward higher frequencies, I see the peak moving right. At the end: Good you did all this work. ?I guess you did not verify 600Kcs? but used 540 kcs on the dial and here it is actually peaking a 500 or just under. That is 8 % off, and ?? that is a very sharp? peak so RF tuning alignment need to track exactly¡? and does not .. a confirmation of what you said all along the trimmer will be dwarfed by the Main cap so you will be stuck with what you got [for now] I have assumed you know all about calculating capacitors in parallel etc, but you have said little about that. If for some reason L3 is too high [ not that simple but..] by that 8% ?and manage to lower it, then at 1400? [actually 1200?? Forget now] things should shift up by 8% but that isn¡¯t ??near enough. The trimmer seems to work ?but it is hard to know, what range to expect. ?I guess you have not sweeped the mixer tank coil ?L6? when set ?Hi , Low at 600 and 1400??? ? to see if it is much the same ?? ? At some point maybe you can lift the trimmer cap C4 ?? ?up just to verify that it really reaches minimum¡ or doesn¡¯t [ simpler than the coil?} ? Now, because of the complexity of the L3 ?coils and the band switch, it is too tough to analyze, ?? Here is a bit more on how I see the first tank, but it needs some reviews, and some clarification but it is a start ??? It would seem that the coil windings and the trimmer should interact with those of the other band ,,, strange stuff I¡¯m shot, good luck
?
toggle quoted message
Show quoted text
From: [email protected] [mailto:[email protected]] On Behalf Of Emanuele Girlando via groups.io
Sent: Sunday, February 16, 2025 4:12 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] S-85 refurbish: I've got a problem during the RF alignment of bands 1 and 2 - trimcaps don't peak!? I think it's time to narrow? the investigation down and tackle one problem at a time. As the "v1 / V2 coupling" and the "not C62" problems are really intriguing but seem loosely related to the "trimcaps don't peak" initial problem, I would put them temporarily a part. In order to reduce complexity, I would also remove from the table the fact that the problem occurs in both the antenna circuits and the mixer circuits. I would start the analysis again considering the antenna circuit only. There will be no shortage of surprises though. Since the circuit is difficult to understand, and in fact it seems it shouldn't even work as it is, let's temporarily consider it as a black box and examine its operation as if it were a pure unknown DUT. The receiver is turned OFF. Band switch on band 1. BS dial fully CW (-> C5-A fully open that is at minimum capacity. Tracking generator output set at -40dBm. Absolute vertical readings are only qualitative measurements. Relative vertical readings can make some sense. START 500KHz / STOP 1800KHz - SPAN=1300KHz - 130KHZ/DIV. Band 1: main tune dial at 0.54MHz - the bottom of band 1 (C7A fully meshed - maximum capacity) The peak is a little below 500KHz (in fact not visible) - C4 has no visible effects. Turning the main dial toward higher frequencies, I see the peak moving right. Band 1: main tune at 1.6MHz - the top of band 1? (C7A fully open - minimum capacity) - C4 fully close (max capacitance) I expected to see the peak to move up to 1.6MHz. It doesn't happen! It stops a 960KHz (!!) without even reaching the alignment frequency of 1400KHz. Band 1: main tune at 1.6MHz - top of band 1 (C7A fully open - minimum capacity) - C4 fully open (min capacitance) So, turning C4 fully open the peak moved right by about 180KHz reaching 1140KHz, still far away from 1.6MHz (and even below 1400KHz!). At 1400KHz (the alignment frequency) I cannot never ever have a peak. - the peak, while correctly starting from the bottom of the band, never ever reaches the top, even when the mail dial is at 1.6MHz; it doesn't even reach the alignment frequency of 1400KHz. This is the problem.
- C4 (that I confirm is part no. 44-191) moves the peak left and right of about 180KHz; it would be good if the peak would be within a 180KHz span from 1400MHz but it is not.
- Given this data C4 will never ever "peak" at 1400KHz as required by the alignment procedure.
If the peak went up to 1.6MHz, then I would be able to get the peak around 1400KHz as per service procedure and carefully looking for it tweaking C4. Another possibility is: the span C4 provides is not sufficient. It should be in the order of 450KHz, too much in my opinion. Changing figures, the same happens on band 2, 3 and 4 (the lower peak is at the very bottom of the band while the top peak never exceeds 2/3 of the band. So the question is: why does the peak stop at 1140KHz without ever reaching the top of the band?
In my opinion the only possible reasons are: "C7-A defective: too high capacitance" or "a coil is seriously defective". Another alternative: C4 is faulty not allowing the due frequency span,
This is why I started thinking I have to disassemble the ANT unit and check the wiring and all the components one by one. C7 is my preferred suspect. If for some reason it has too much capacitance, this would affect all bands the way we see. How can I check C7 without destroying it? May be grease deposited on the plates over time (at eye I don't see anything)? May be it needs cleaning? can I use IPA? Oil? gasoline? -- don??? va3drl
|
The 6SC7 is used as a "floating paraphase" phase splitter. At least
in the fixed bias version it is very sensitive to hum in the bias
supply. Maybe not in the self bias version as used here. However, I am
very suspicious of this tube. Easy to prove it by changing tubes.
There is some information about the floating paraphase in the
Radiotron 4th edition. Very widely used circuit with several variations.
The illustration in the RDH is almost identical to what is used in the
SX-28.
toggle quoted message
Show quoted text
On 2/16/2025 5:37 PM, Mike Langner via groups.io wrote: 6SC7 tubes are renown for developing heater-cathode leakage and for
developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever
rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often
vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility
maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a
continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
6SC7 tubes are renown for developing heater-cathode leakage and for developing inter-element shorts and leakage as well.
Often, if you shake one near your ear, you can hear loose whatever rattling around inside the tube.
In addition, thumping a 6SC7 while it¡¯s operating in a circuit can often vary the hum and noise produced by the stage.
I spent over 60 years in broadcast engineering and facility maintenance.? 6SC7 tubes in phono preamps (remember LP¡¯s?) was a continuing headache.
May or may not be relevant in this case.
Mike/
K5MGR
____________________________________________ ? Mike Langner
929 Alameda Road NW
Albuquerque, NM 87114-1901
(505) 898-3212 home/home office
(505) 238-8810 cell
mlangner@...
?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of Richard Knoppow via groups.io
Sent: Sunday, February 16, 2025 6:32 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Try another tube. Maybe heater to cathode leakage -------- Original message -------- Date: 2/16/25 5:07 PM (GMT-08:00) Subject: Re: [HallicraftersRadios] SX-28A Hum Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ? ? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
|
Try another tube. Maybe heater to cathode leakage
toggle quoted message
Show quoted text
-------- Original message -------- From: "thoyer via groups.io" <thoyer1@...> Date: 2/16/25 5:07 PM (GMT-08:00) Subject: Re: [HallicraftersRadios] SX-28A Hum
Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ? ? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
|
Hi Jacques, ? Took the grids to ground, no hum so it looks like it is coming from the 6SC7 area. ? Narrowing it down¡¡¡. ? I appreciate all of the discussion going on ¨C thank you everyone. We¡¯ll get to the bottom of this one way or another. ? Tom W3TA ?
toggle quoted message
Show quoted text
From: [email protected] <[email protected]> On Behalf Of Jacques_VE2JFE via groups.io
Sent: Sunday, February 16, 2025 3:17 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Hi again Tom, Maybe some will find the following approach brutal, but here is what I will try with that SX-28A. Solder-tack two pieces of wire from the 6V6 pins 5 to GND: connect GND where both C47 and R42 connects to the chassis. Power-up the set again. If the 120Hz hum is gone, the problem is really around the 6SC7 stage(s). ? BUT, if it is still there, it means that plate currents for the two 6V6 in the output transformer primaries are not balanced. Many causes for this: one side of the primary is open, or have developed a high resistance value from a failing connection within. OR there is shorted turns in the winding of the primary on one side. OR the two 6V6 are very different to each other in DC, one passing way more plate current than the other. ? I just hope that this makes sense¡ ? ? ? ?
? 73, Jacques, VE2JFE in Montreal ? ? Yes, hum is present ¨C at the same level ¨C regardless of the AF gain position. It does not change with AF gain adjustment so the issue is after the AF gain pot. ? CH1 and CH2 are not near each other CH2 is on the front of the chassis and CH1 is in the rear corner. ? Tom
|
don Root
Jim Whartenby
Jacques_VE2JFE
Mike Langner