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I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA
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I should clarify, Whether the bass is in or out, the ripple remains at the same amplitude. I just don’t hear it with the bass “in” ? Tom W3TA ?
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From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Thursday, February 13, 2025 7:22 PM
To: [email protected]
Subject: [HallicraftersRadios] SX-28A Hum? I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA
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Tom Good for you Presumably, the base “In” position filters out the hum as expected… And the hum comes from upstream, maybe the AF gain? Pot area or a bad contact in the phones switch, or even before that. ?? ?
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From: [email protected] [mailto:[email protected]] On Behalf Of thoyer via groups.io
Sent: Thursday, February 13, 2025 7:22 PM
To: [email protected]
Subject: [HallicraftersRadios] SX-28A Hum? I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA -- don??? va3drl
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Check whether the windings of the audio output transformer are really symmetrical. This is how the two anodes of the 6v6 are fed and the remaining hum in the end circuit is eliminated. The hum is eliminated by the two (opposing) windings. ? ? I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA
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Good idea, but I pulled V12, 6SC7, and the hum remains. It is definitely in the output stages Tom
Tom Good for you Presumably, the base “In” position filters out the hum as expected… And the hum comes from upstream, maybe the AF gain? Pot area or a bad contact in the phones switch, or even before that. ?? ?
From: [email protected] [mailto:[email protected]] On Behalf Of thoyer via groups.io
Sent: Thursday, February 13, 2025 7:22 PM
To: [email protected]
Subject: [HallicraftersRadios] SX-28A Hum ? I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA
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don??? va3drl
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Interesting, I'll look into this.
Thanks,? Tom
Check whether the windings of the audio output transformer are really symmetrical. This is how the two anodes of the 6v6 are fed and the remaining hum in the end circuit is eliminated. The hum is eliminated by the two (opposing) windings. ? ? I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA
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Look at the audio response curve in the manual.? With the BASS switch IN, the low frequency audio is emphasized, not attenuated.??
The BASS switch in the IN position shorts out two components, CH2 and C43, I would look there first.? Perhaps there is AC being coupled into the audio circuit via CH2?? Check that the mounting screws for CH2 are making good contact with the chassis,? Corrosion between the screw head and chassis could be the problem so loosen and then tighten them.? Might as well do the same for any screw that supplies a ground or chassis connection for the audio circuit in that area of the chassis. Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Good idea, but I pulled V12, 6SC7, and the hum remains. It is definitely in the output stages Tom
Tom Good for you Presumably, the base “In” position filters out the hum as expected… And the hum comes from upstream, maybe the AF gain? Pot area or a bad contact in the phones switch, or even before that. ?? ?
From: [email protected] [mailto:[email protected]] On Behalf Of thoyer via groups.io
Sent: Thursday, February 13, 2025 7:22 PM
To: [email protected]
Subject: [HallicraftersRadios] SX-28A Hum ? I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA
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don??? va3drl
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Thanks Jim, will look into those areas.
Tom
Look at the audio response curve in the manual.? With the BASS switch IN, the low frequency audio is emphasized, not attenuated.??
The BASS switch in the IN position shorts out two components, CH2 and C43, I would look there first.? Perhaps there is AC being coupled into the audio circuit via CH2?? Check that the mounting screws for CH2 are making good contact with the chassis,? Corrosion between the screw head and chassis could be the problem so loosen and then tighten them.? Might as well do the same for any screw that supplies a ground or chassis connection for the audio circuit in that area of the chassis. Regards, Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Good idea, but I pulled V12, 6SC7, and the hum remains. It is definitely in the output stages Tom
Tom Good for you Presumably, the base “In” position filters out the hum as expected… And the hum comes from upstream, maybe the AF gain? Pot area or a bad contact in the phones switch, or even before that. ?? ?
From: [email protected] [mailto:[email protected]] On Behalf Of thoyer via groups.io
Sent: Thursday, February 13, 2025 7:22 PM
To: [email protected]
Subject: [HallicraftersRadios] SX-28A Hum ? I just finished pretty much a complete overhaul of a nice condition (physically) SX-28A. Replaced just about every resistor and all caps. ? Radio is working well except for a 120hz hum. When I switch the Bass in, the hum goes away. ? Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that is there independent of the bass switch position. The output side of the HV filter is clean, no ripple. I paralleled another 47uf 450v cap across the choke input cap and there was no difference. ? I double checked the wiring against the schematic along with component values. Is the schematic correct in this area? I found once schematic error maybe there is another? ? Thoughts? ? Tom W3TA
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don??? va3drl
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Jim, it seems like switching makes the big difference. Presumably there are 3 longer wires running a distance to the switch. Might one of them be getting to close to the B+ wires, or something like that. Although It hardly matters, the switch is for base “boost” I think, but the boost seems to be bypassed while in the ‘IN” position, and in use while in the “out position”.. confusing to me so far. ?? -- don??? va3drl
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Don More likely that the filter inductor and the tone inductor are talking to each other.? I don't know exactly where they are located on the chassis but I have noticed that high power Peavey 8 ohm to 70 volt line transformers talk to each other when co-located.??
Lots of strange things happen when there is some corrosion between the mounting screw and the chassis.? Loss of a "common" connection or some weird ohmic connection plays havoc with any circuit. Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Jim, it seems like switching makes the big difference. Presumably there are 3 longer wires running a distance to the switch. Might one of them be getting to close to the B+ wires, or something like that. Although It hardly matters, the switch is for base “boost” I think, but the boost seems to be bypassed while in the ‘IN” position, and in use while in the “out position”.. confusing to me so far. ??
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don??? va3drl
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Hi Jim Re paragraph 1:? ?I would not be surprised about the talking, but can that cause 120 cycle hum? Re paragraph 2: ?I agree, and there must be lots of return currents from B+ running to and thru the chassis, and chassis connections are rarely soldered, and to top it off, the schematic gives no hint as to ?the common wiring running to a chassis connection, so if one chassis connection opens a bit, what all lifts above the chassis common. After 70 years, there is lots of opportunity for corrosion. Thinking out loud about the original complaint, hum would not seem to be from the B+ into the plates,? since there is no hum in one switch position, but it might be getting into 1 or both 6V6 grids, but it would seem to be both. When it comes to the switch area, that schematic is spinning my head. ?I would probably try to poke around with a scope and a high impedance probe. So often hum comes when there is an open connection from a lower impedance source signal wire. With so many schematic connections to the plate and choke etc it is hard to speculate on the real wiring. This is likely no help at all, but I tried. ?
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From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Saturday, February 15, 2025 1:29 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? More likely that the filter inductor and the tone inductor are talking to each other.? I don't know exactly where they are located on the chassis but I have noticed that high power Peavey 8 ohm to 70 volt line transformers talk to each other when co-located.?? Lots of strange things happen when there is some corrosion between the mounting screw and the chassis.? Loss of a "common" connection or some weird ohmic connection plays havoc with any circuit. -- don??? va3drl
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“When it comes to the switch area, that schematic is spinning my head” ? Glad it is not just me…… ? ? I have not had time to get back to this, maybe tonight. ? I believe it is in the 6V6 area because when I pull the 6SA7 the symptom remains. Maybe the choke has some leakage to ground? I was going to pull it last night and check it on my Sencore inductance tester. Got side tracked and didn’t get there. I’ll try tonight. ? Tom W3TA ?
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From: [email protected] <[email protected]> On Behalf Of don Root
Sent: Saturday, February 15, 2025 5:14 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Hi Jim Re paragraph 1:? ?I would not be surprised about the talking, but can that cause 120 cycle hum? Re paragraph 2: ?I agree, and there must be lots of return currents from B+ running to and thru the chassis, and chassis connections are rarely soldered, and to top it off, the schematic gives no hint as to ?the common wiring running to a chassis connection, so if one chassis connection opens a bit, what all lifts above the chassis common. After 70 years, there is lots of opportunity for corrosion. Thinking out loud about the original complaint, hum would not seem to be from the B+ into the plates,? since there is no hum in one switch position, but it might be getting into 1 or both 6V6 grids, but it would seem to be both. When it comes to the switch area, that schematic is spinning my head. ?I would probably try to poke around with a scope and a high impedance probe. So often hum comes when there is an open connection from a lower impedance source signal wire. With so many schematic connections to the plate and choke etc it is hard to speculate on the real wiring. This is likely no help at all, but I tried. ? ? More likely that the filter inductor and the tone inductor are talking to each other.? I don't know exactly where they are located on the chassis but I have noticed that high power Peavey 8 ohm to 70 volt line transformers talk to each other when co-located.?? Lots of strange things happen when there is some corrosion between the mounting screw and the chassis.? Loss of a "common" connection or some weird ohmic connection plays havoc with any circuit.
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don??? va3drl
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My two cents: If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal
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Tom I’m glad I have company on that. Because the switch makes the difference and is before the 6V6s, I tend to think it is up there. Perhaps put a sizable cap on each grid to see if the switch affects the hum. Do you have a scope? Nasty little problems keep us busy on the forum. ?
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From: [email protected] [mailto:[email protected]] On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 5:34 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? “When it comes to the switch area, that schematic is spinning my head” ? Glad it is not just me…… ? ? I have not had time to get back to this, maybe tonight. ? I believe it is in the 6V6 area because when I pull the 6SA7 the symptom remains. Maybe the choke has some leakage to ground? I was going to pull it last night and check it on my Sencore inductance tester. Got side tracked and didn’t get there. I’ll try tonight. ? Tom W3TA -- don??? va3drl
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Hi Jacques, I just received 1.5 cents from you. Your proposal may well be, but how do you explain the difference ?the switch setting makes?
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From: [email protected] [mailto:[email protected]] On Behalf Of Jacques_VE2JFE via groups.io
Sent: Saturday, February 15, 2025 5:57 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? My two cents: If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal ? -- don??? va3drl
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I have not read all the posts on this thread so may be repeating
something. Tom, I do not have an SX-28, In normal operation what does
the bass switch do? Does the bass increase or decrease when the switch
is in the IN position? The handbook is confusing. The schematic suggests
the BASS switch boosts the bass by adding a resonant choke on the output
of the first audio amplifier. The schematic shows the switch cutting off
the choke in ON, that looks backward to me. The additional TONE control
is just a conventional high roll off. It appears that both of these tone
controls are independent, is that correct.
It seems to me the bass boost should INCREASE hum.
The plate transformer of the output stage connects to the input of
the B+ filter. This is not raw AC since there is a fairly large cap
across it. The balanced circuit should remove any residual hum. The
rest of the amplifier is fed by filtered B+ and seems unlikely to have
significant hum on it. Try shorting out the tone control choke, if its
getting induced hum that should eliminate it.
Since you practically rebuilt the receiver perhaps some wiring
error was made or some new part is actually bad. Worth going over again.
It is always frustrating to me to try to trouble shoot problems
where I can't just jump in an make measurements.
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On 2/13/2025 4:21 PM, thoyer via groups.io wrote: I just finished pretty much a complete overhaul of a nice condition
(physically) SX-28A. Replaced just about every resistor and all caps.
Radio is working well except for a 120hz hum. When I switch the Bass in,
the hum goes away.
Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that
is there independent of the bass switch position. The output side of the
HV filter is clean, no ripple. I paralleled another 47uf 450v cap across
the choke input cap and there was no difference.
I double checked the wiring against the schematic along with component
values. Is the schematic correct in this area? I found once schematic
error maybe there is another?
Thoughts?
Tom
W3TA
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
Ok, tried two different 6V6’s, no change. ? On the plates of the 6V6’s there is 6.8Vrms of 120hz ripple. If I remove the 6v6’s it drops to 4.7Vrms and the hum is no longer audible (as expected). ? With the 6V6’s back in (Russian tubes BTW), switching the bass “IN”, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple. ? These measurements are made with my o’scope. ? I did some poking through my “inventory” of parts and do not have a suitable choke to swap in for CH2 – still thinking there may be some leakage going on there……? ? Fun stuff huh? ? Tom W3TA ?
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From: [email protected] <[email protected]> On Behalf Of don Root
Sent: Saturday, February 15, 2025 6:19 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Hi Jacques, I just received 1.5 cents from you. Your proposal may well be, but how do you explain the difference ?the switch setting makes? ? My two cents: If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal ?
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don??? va3drl
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I unscrewed CH2 from the chassis and let it float, no change. ?
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From: [email protected] <[email protected]> On Behalf Of thoyer via groups.io
Sent: Saturday, February 15, 2025 8:07 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? Ok, tried two different 6V6’s, no change. ? On the plates of the 6V6’s there is 6.8Vrms of 120hz ripple. If I remove the 6v6’s it drops to 4.7Vrms and the hum is no longer audible (as expected). ? With the 6V6’s back in (Russian tubes BTW), switching the bass “IN”, which shorts CH2 and C43, the hum goes away but has no impact on the measured ripple. ? These measurements are made with my o’scope. ? I did some poking through my “inventory” of parts and do not have a suitable choke to swap in for CH2 – still thinking there may be some leakage going on there……? ? Fun stuff huh? ? Tom W3TA ? ? Hi Jacques, I just received 1.5 cents from you. Your proposal may well be, but how do you explain the difference ?the switch setting makes? ? My two cents: If the 120Hz “hum” is still heard when the 6SC7 tube is removed, that could be that the output stage is not balanced current wise. Meaning: if one of the 6V6 is way less polarized (more weak) than the other, that could be the cause. ? 73, Jacques, VE2JFE in Montreal ?
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don??? va3drl
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Richard, ? When the bass switch is “in” it boosts the bass. Going by the schematic it shows that it shorts the choke / cap. Audibly it definitely boosts the bass, I need to confirm the wiring of the switch vs the schematic. Wouldn’t be the first error I’ve found. ? Yes, the tone adj and the bass boost are independent. ? You suggested shorting the choke – but isn’t that effectively what the switch is doing? ? And I agree, I had most of this radio torn apart so it is a good possibility there is a miswire somewhere in that circuit – wouldn’t be the first time……….. ? Thanks, Tom W3TA ?
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From: [email protected] <[email protected]> On Behalf Of Richard Knoppow via groups.io
Sent: Saturday, February 15, 2025 7:15 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] SX-28A Hum? I have not read all the posts on this thread so may be repeating
something. Tom, I do not have an SX-28, In normal operation what does
the bass switch do? Does the bass increase or decrease when the switch
is in the IN position? The handbook is confusing. The schematic suggests
the BASS switch boosts the bass by adding a resonant choke on the output
of the first audio amplifier. The schematic shows the switch cutting off
the choke in ON, that looks backward to me. The additional TONE control
is just a conventional high roll off. It appears that both of these tone
controls are independent, is that correct.
It seems to me the bass boost should INCREASE hum.
The plate transformer of the output stage connects to the input of
the B+ filter. This is not raw AC since there is a fairly large cap
across it. The balanced circuit should remove any residual hum. The
rest of the amplifier is fed by filtered B+ and seems unlikely to have
significant hum on it. Try shorting out the tone control choke, if its
getting induced hum that should eliminate it.
Since you practically rebuilt the receiver perhaps some wiring
error was made or some new part is actually bad. Worth going over again.
It is always frustrating to me to try to trouble shoot problems
where I can't just jump in an make measurements.
On 2/13/2025 4:21 PM, thoyer via groups.io wrote: I just finished pretty much a complete overhaul of a nice condition
(physically) SX-28A. Replaced just about every resistor and all caps.
Radio is working well except for a 120hz hum. When I switch the Bass in,
the hum goes away.
Yes, there is ripple on the plates of the 6V6’s, about 6vrms, but that
is there independent of the bass switch position. The output side of the
HV filter is clean, no ripple. I paralleled another 47uf 450v cap across
the choke input cap and there was no difference.
I double checked the wiring against the schematic along with component
values. Is the schematic correct in this area? I found once schematic
error maybe there is another?
Thoughts?
Tom
W3TA
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
|
It seems that the way to look at this switch is that in one position the plate load is provided by the choke while in the other the load is switched to R37, and as an aside the choke gets shorted ? ??Any votes on this? ?just for fun try the thumbs ?voting machine, I won’t look. ? -- don??? va3drl
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don Root
Jim Whartenby
Jacques_VE2JFE