webcor B-124

[MarkS]

This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams circuit has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!

[Peter Wieck]

In rank order of non-invasive options - most of which I expect you have done already:

a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.


Best of luck with it!

Peter Wieck
Melrose Park, PA

[t***@gmail.com]

Post by Peter Wieck
a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.
Best of luck with it!
Peter Wieck
Melrose Park, PA

pickup wires connected the wrong way round? Or passing too close to mains.
Original OP transformer replaced with a non-humbucking one?
Deck not connected to chassis (safely/indirectly!)


NT

[MarkS]

Post by t***@gmail.com

Post by Peter Wieck
a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.
Best of luck with it!
Peter Wieck
Melrose Park, PA

pickup wires connected the wrong way round? Or passing too close to mains.
Original OP transformer replaced with a non-humbucking one?
Deck not connected to chassis (safely/indirectly!)

Seems all is in order. All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap. Reversed the cartridge leads, no change, and the lead integrity is uncompromised right to the RCA phono plug. I believe the OPT original as it still has rivets holding it to the chassis. I did replace the cartridge with one that's close to the original but not identical. Wondering if the output of this cart isn't as high as the original meaning could it be the original's output put it above the amps SN level?

[MarkS]

Post by MarkS

Post by t***@gmail.com

Post by Peter Wieck
a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.
Best of luck with it!
Peter Wieck
Melrose Park, PA

pickup wires connected the wrong way round? Or passing too close to mains.
Original OP transformer replaced with a non-humbucking one?
Deck not connected to chassis (safely/indirectly!)

Seems all is in order. All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap. Reversed the cartridge leads, no change, and the lead integrity is uncompromised right to the RCA phono plug. I believe the OPT original as it still has rivets holding it to the chassis. I did replace the cartridge with one that's close to the original but not identical. Wondering if the output of this cart isn't as high as the original meaning could it be the original's output put it above the amps SN level?

Looking at the Sams, there is a circuit version at has a 12SJ7 pentode connected in place of the 12SQ7, higher gain for sure. Was it there for a lower output cartridge perhaps?

[Peter Wieck]

What happens if you short the cartridge inputs?
What is the voltage across the filaments? |
And, have you checked the rectifier on a tester (if tube), or measured AC over DC on the output(s)?

Peter Wieck
Melrose Park, PA

[t***@gmail.com]

Post by MarkS

Post by t***@gmail.com

Post by Peter Wieck
a) Try reversing the plug. Every so often, the layout of the circuit is such that this simple expedient works.
b) And, if you have a bad rectifier, or one on its way out, it could pass more AC than it should.
c) Try dressing the wires under the chassis closer to the original pathway(s). Anyone who has mucked about with portable SW radios will understand this right away! Yes, 'just that little bit' can make a difference.
d) Make absolutely sure that the cartridge is installed properly, and that the leads are not compromised.
e) Just for giggles - make sure that none of the caps are reversed, and MAKE SURE that if there are multiple filter caps that all the negative leads are installed correctly. Zenith, as an example, was infamous for having a filter cap on their pre-war radios that was entirely above ground. The typical careless restorer would put all three caps at ground - and wonder why there was so much hum.
Best of luck with it!
Peter Wieck
Melrose Park, PA

pickup wires connected the wrong way round? Or passing too close to mains.
Original OP transformer replaced with a non-humbucking one?
Deck not connected to chassis (safely/indirectly!)

Seems all is in order. All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap. Reversed the cartridge leads, no change, and the lead integrity is uncompromised right to the RCA phono plug. I believe the OPT original as it still has rivets holding it to the chassis. I did replace the cartridge with one that's close to the original but not identical. Wondering if the output of this cart isn't as high as the original meaning could it be the original's output put it above the amps SN level?

Show us the circuit, then we can see more precisely where potential issues lie & what to test. There were some amps where the main reservoir cap did not return to chassis.


NT

[Peter Wieck]

All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap.

Yabbut.... are you sure that there is only one (1) B-? Please check the schematic very carefully, as quite often, one-or-more filter caps is 'above ground', and connecting it/them to chassis B- will cause hum. And, as it happens, more often than not, that particular hum will be VC -dependent.

Peter Wieck
Melrose Park, PA

[MarkS]

Post by MarkS
All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap.
Yabbut.... are you sure that there is only one (1) B-? Please check the schematic very carefully, as quite often, one-or-more filter caps is 'above ground', and connecting it/them to chassis B- will cause hum. And, as it happens, more often than not, that particular hum will be VC -dependent.
Peter Wieck
Melrose Park, PA

They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?

[John Robertson]

Post by MarkS

Post by MarkS
All the filter cap negs are connected to B-; the chassis connected to B- via .047 cap.
Yabbut.... are you sure that there is only one (1) B-? Please check the schematic very carefully, as quite often, one-or-more filter caps is 'above ground', and connecting it/them to chassis B- will cause hum. And, as it happens, more often than not, that particular hum will be VC -dependent.
Peter Wieck
Melrose Park, PA

They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?

I don't know about the old binary usenet lists - if they still work, but
probably best to put it on a free host that doesn't want emails, etc for
harvesting...and give us the link.

John :-#)#

[Michael_A_Terrell]

Post by MarkS
They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?

Create a free Dropbox account. Post it there as a public file, then
post a link here.

[MarkS]

Post by Michael_A_Terrell

Post by MarkS
They are all to the same B -, I know the practice of lifting one cap above B- to generate bias voltage. I would like to post the schematic, how does one do that on usenet these days?

Create a free Dropbox account. Post it there as a public file, then
post a link here.
--
Never piss off an Engineer!
They don't get mad.
They don't get even.
They go for over unity! ;-)

https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0

OK thanks! see if it works. I should tell you 1) The OPT is different (as stated before), the model I have mostly resembles the 12SQ7 version. I'm thinking mfg was done of the fly depending on available components. I want to get this damn thing off my bench so I can dig into an Ampex F44. I've never had something so simple give me so much grief (doing this for a friend of course). Been around tube electronics for >44 years. Restored many radios; built many amps. I have a decent scope if need be.

[Fox's Mercantile]

Post by MarkS
OK thanks! see if it works.

I'm almost tempted to suggest a heater cathode short.

[Michael_A_Terrell]

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is different (as stated before), the model I have mostly resembles the 12SQ7 version. I'm thinking mfg was done of the fly depending on available components. I want to get this damn thing off my bench so I can dig into an Ampex F44. I've never had something so simple give me so much grief (doing this for a friend of course). Been around tube electronics for >44 years. Restored many radios; built many amps. I have a decent scope if need be.

What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its heater
84 ohms at 2Watts would give a 12.6 drop in the heater string. 82 is the
closest standard value. That, or use a Variac set to about 105 volts and
short the two heater pins with the tube removed. One other possibility
is a damaged tube socket that is leaking AC to the grid.

http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

[John Robertson]

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is different
(as stated before), the model I have mostly resembles the 12SQ7
version. I'm thinking mfg was done of the fly depending on available
components. I want to get this damn thing off my bench so I can dig
into an Ampex F44. I've never had something so simple give me so much
grief (doing this for a friend of course). Been around tube
electronics for >44 years. Restored many radios; built many amps. I
have a decent scope if need be.

   What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its heater
84 ohms at 2Watts would give a 12.6 drop in the heater string. 82 is the
closest standard value. That, or use a Variac set to about 105 volts and
short the two heater pins with the tube removed. One other possibility
is a damaged tube socket that is leaking AC to the grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then you
have something sneaking in via C5, R3 or C4.

Or the wiring for same components.

Can't see the 12SQ7 as the cause as it is after the pot.

Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

John :-#)#

[Michael_A_Terrell]

Post by John Robertson

Post by Michael_A_Terrell

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is
different (as stated before), the model I have mostly resembles
the 12SQ7 version. I'm thinking mfg was done of the fly depending
on available components. I want to get this damn thing off my bench
so I can dig into an Ampex F44. I've never had something so simple
give me so much grief (doing this for a friend of course). Been
around tube electronics for >44 years. Restored many radios; built
many amps. I have a decent scope if need be.

Webcor built some models for several years, so ECOs would occur.

Post by John Robertson

Post by Michael_A_Terrell
What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its
heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
82 is the closest standard value. That, or use a Variac set to about
105 volts and short the two heater pins with the tube removed. One
other possibility is a damaged tube socket that is leaking AC to the
grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then
you have something sneaking in via C5, R3 or C4.
Or the wiring for same components.
Can't see the 12SQ7 as the cause as it is after the pot.

He said he had tried multiple tubes. A bad pot can cause problems,
as well. Some need the case soldered to ground, because the thing isn't
built on a metal chassis. The tip about eliminating a tube for a test is
a general suggestion for troubleshooting. If it uses wafer sockets,
leakage is a real possibility.

Post by John Robertson
Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

Is the shield intact? I replaced the lead on more than one turntable
because the repeated flexing had worn through the braid.

[John Robertson]

Post by John Robertson

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is
different  (as stated before), the model I have mostly resembles
the 12SQ7 version. I'm thinking mfg was done of the fly depending
on available components. I want to get this damn thing off my bench
so I can dig into an Ampex F44. I've never had something so simple
give me so much grief (doing this for a friend of course). Been
around tube electronics for >44 years. Restored many radios; built
many amps. I have a decent scope if need be.

   Webcor built some models for several years, so ECOs would occur.

Post by John Robertson

    What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its
heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
82 is the closest standard value. That, or use a Variac set to about
105 volts and short the two heater pins with the tube removed. One
other possibility is a damaged tube socket that is leaking AC to the
grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then
you have something sneaking in via C5, R3 or C4.
Or the wiring for same components.
Can't see the 12SQ7 as the cause as it is after the pot.

   He said he had tried multiple tubes. A bad pot can cause problems,
as well. Some need the case soldered to ground, because the thing isn't
built on a metal chassis. The tip about eliminating a tube for a test is
a general suggestion for troubleshooting. If it uses wafer sockets,
leakage is a real possibility.

Post by John Robertson
Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

   Is the shield intact? I replaced the lead on more than one turntable
because the repeated flexing had worn through the braid.

Good point, that fits equally well with my thought the ground connection
at the RCA jack is possibly compromised.

So, Mark, what is the word?

John :-#)#

[MarkS]

Post by John Robertson

Post by Michael_A_Terrell

Post by John Robertson

Post by Michael_A_Terrell

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is
different (as stated before), the model I have mostly resembles
the 12SQ7 version. I'm thinking mfg was done of the fly depending
on available components. I want to get this damn thing off my bench
so I can dig into an Ampex F44. I've never had something so simple
give me so much grief (doing this for a friend of course). Been
around tube electronics for >44 years. Restored many radios; built
many amps. I have a decent scope if need be.

Webcor built some models for several years, so ECOs would occur.

Post by John Robertson

Post by Michael_A_Terrell
What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its
heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
82 is the closest standard value. That, or use a Variac set to about
105 volts and short the two heater pins with the tube removed. One
other possibility is a damaged tube socket that is leaking AC to the
grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then
you have something sneaking in via C5, R3 or C4.
Or the wiring for same components.
Can't see the 12SQ7 as the cause as it is after the pot.

He said he had tried multiple tubes. A bad pot can cause problems,
as well. Some need the case soldered to ground, because the thing isn't
built on a metal chassis. The tip about eliminating a tube for a test is
a general suggestion for troubleshooting. If it uses wafer sockets,
leakage is a real possibility.

Post by John Robertson
Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

Is the shield intact? I replaced the lead on more than one turntable
because the repeated flexing had worn through the braid.

Good point, that fits equally well with my thought the ground connection
at the RCA jack is possibly compromised.
So, Mark, what is the word?
John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
[phone] (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

[MarkS]

Post by MarkS

Post by John Robertson

Post by Michael_A_Terrell

Post by John Robertson

Post by Michael_A_Terrell

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is
different (as stated before), the model I have mostly resembles
the 12SQ7 version. I'm thinking mfg was done of the fly depending
on available components. I want to get this damn thing off my bench
so I can dig into an Ampex F44. I've never had something so simple
give me so much grief (doing this for a friend of course). Been
around tube electronics for >44 years. Restored many radios; built
many amps. I have a decent scope if need be.

Webcor built some models for several years, so ECOs would occur.

Post by John Robertson

Post by Michael_A_Terrell
What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its
heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
82 is the closest standard value. That, or use a Variac set to about
105 volts and short the two heater pins with the tube removed. One
other possibility is a damaged tube socket that is leaking AC to the
grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then
you have something sneaking in via C5, R3 or C4.
Or the wiring for same components.
Can't see the 12SQ7 as the cause as it is after the pot.

He said he had tried multiple tubes. A bad pot can cause problems,
as well. Some need the case soldered to ground, because the thing isn't
built on a metal chassis. The tip about eliminating a tube for a test is
a general suggestion for troubleshooting. If it uses wafer sockets,
leakage is a real possibility.

Post by John Robertson
Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

Is the shield intact? I replaced the lead on more than one turntable
because the repeated flexing had worn through the braid.

Good point, that fits equally well with my thought the ground connection
at the RCA jack is possibly compromised.
So, Mark, what is the word?
John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
[phone] (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .0022 cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp like this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.

[John Robertson]

Post by MarkS

Post by MarkS

Post by John Robertson

Post by Michael_A_Terrell

Post by John Robertson

Post by Michael_A_Terrell

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is
different (as stated before), the model I have mostly resembles
the 12SQ7 version. I'm thinking mfg was done of the fly depending
on available components. I want to get this damn thing off my bench
so I can dig into an Ampex F44. I've never had something so simple
give me so much grief (doing this for a friend of course). Been
around tube electronics for >44 years. Restored many radios; built
many amps. I have a decent scope if need be.

Webcor built some models for several years, so ECOs would occur.

Post by John Robertson

Post by Michael_A_Terrell
What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its
heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
82 is the closest standard value. That, or use a Variac set to about
105 volts and short the two heater pins with the tube removed. One
other possibility is a damaged tube socket that is leaking AC to the
grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then
you have something sneaking in via C5, R3 or C4.
Or the wiring for same components.
Can't see the 12SQ7 as the cause as it is after the pot.

He said he had tried multiple tubes. A bad pot can cause problems,
as well. Some need the case soldered to ground, because the thing isn't
built on a metal chassis. The tip about eliminating a tube for a test is
a general suggestion for troubleshooting. If it uses wafer sockets,
leakage is a real possibility.

Post by John Robertson
Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

Is the shield intact? I replaced the lead on more than one turntable
because the repeated flexing had worn through the braid.

Good point, that fits equally well with my thought the ground connection
at the RCA jack is possibly compromised.
So, Mark, what is the word?
John :-#)#
--

Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .0022 cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp like this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.

Is the Volume Control (VC) floating from the chassis? Perhaps the hum is
coming in via either the Common connection on the VC or perhaps the VC
is mounted to something metal that is not common to the amplifier chassis?

Is the RCA input to the amp unplugged while you have been doing these tests?

John :-#)#

[MarkS]

Post by John Robertson

Post by MarkS

Post by MarkS

Post by John Robertson

Post by Michael_A_Terrell

Post by John Robertson

Post by Michael_A_Terrell

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is
different (as stated before), the model I have mostly resembles
the 12SQ7 version. I'm thinking mfg was done of the fly depending
on available components. I want to get this damn thing off my bench
so I can dig into an Ampex F44. I've never had something so simple
give me so much grief (doing this for a friend of course). Been
around tube electronics for >44 years. Restored many radios; built
many amps. I have a decent scope if need be.

Webcor built some models for several years, so ECOs would occur.

Post by John Robertson

Post by Michael_A_Terrell
What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its
heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
82 is the closest standard value. That, or use a Variac set to about
105 volts and short the two heater pins with the tube removed. One
other possibility is a damaged tube socket that is leaking AC to the
grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then
you have something sneaking in via C5, R3 or C4.
Or the wiring for same components.
Can't see the 12SQ7 as the cause as it is after the pot.

He said he had tried multiple tubes. A bad pot can cause problems,
as well. Some need the case soldered to ground, because the thing isn't
built on a metal chassis. The tip about eliminating a tube for a test is
a general suggestion for troubleshooting. If it uses wafer sockets,
leakage is a real possibility.

Post by John Robertson
Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

Is the shield intact? I replaced the lead on more than one turntable
because the repeated flexing had worn through the braid.

Good point, that fits equally well with my thought the ground connection
at the RCA jack is possibly compromised.
So, Mark, what is the word?
John :-#)#
--

Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .0022 cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp like this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.

Is the Volume Control (VC) floating from the chassis? Perhaps the hum is
coming in via either the Common connection on the VC or perhaps the VC
is mounted to something metal that is not common to the amplifier chassis?
Is the RCA input to the amp unplugged while you have been doing these tests?
John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
[phone] (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

Hi, the VC terminals are floating from the metal chassis but the VC body is mounted to the chassis. One thing about the hum I hear, the frequency seems to change as you rotate the VC from min to max. When at minimum, no hum- rotate towards max and ~ mid span there are higher frequency harmonics then back to lower frequency hum at max volume. Phono RCA is disconnected & connected through out these tests, doesn't seem to effect the hum. Going to poke around more tonite, maybe hang a scope on it.

[MarkS]

Post by John Robertson

Post by MarkS

Post by MarkS

Post by John Robertson

Post by Michael_A_Terrell

Post by John Robertson

Post by Michael_A_Terrell

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is
different (as stated before), the model I have mostly resembles
the 12SQ7 version. I'm thinking mfg was done of the fly depending
on available components. I want to get this damn thing off my bench
so I can dig into an Ampex F44. I've never had something so simple
give me so much grief (doing this for a friend of course). Been
around tube electronics for >44 years. Restored many radios; built
many amps. I have a decent scope if need be.

Webcor built some models for several years, so ECOs would occur.

Post by John Robertson

Post by Michael_A_Terrell
What happens when you short the wiper of the volume control to
ground? Jeff may be right about the Heater to Cathode leakage. If you
don't have another tube, you could use a resistor in place of its
heater 84 ohms at 2Watts would give a 12.6 drop in the heater string.
82 is the closest standard value. That, or use a Variac set to about
105 volts and short the two heater pins with the tube removed. One
other possibility is a damaged tube socket that is leaking AC to the
grid.
http://www.r-type.org/pdfs/12sq7.pdf is a datasheet for the tube.

If R2 is the volume control, and the hum goes away when reduced then
you have something sneaking in via C5, R3 or C4.
Or the wiring for same components.
Can't see the 12SQ7 as the cause as it is after the pot.

He said he had tried multiple tubes. A bad pot can cause problems,
as well. Some need the case soldered to ground, because the thing isn't
built on a metal chassis. The tip about eliminating a tube for a test is
a general suggestion for troubleshooting. If it uses wafer sockets,
leakage is a real possibility.

Post by John Robertson
Is the phono input ground shell connection compromised? Any chance it is
actually not tied to ground?

Is the shield intact? I replaced the lead on more than one turntable
because the repeated flexing had worn through the braid.

Good point, that fits equally well with my thought the ground connection
at the RCA jack is possibly compromised.
So, Mark, what is the word?
John :-#)#
--

Hi Guys, been away this weekend, I am going to go back to it after work tomorrow. Thanks for all the suggestions because of which I think I know where the problem is and why I didn't find it for so long but will verify tomorrow 8>).

OK, update, first lets review the schematic. In this version, there is no C5 or R3. Phono plug right to the VC to .01 cap with a 4.7M R to B- leading to G1 of the 12SQ7; there is a terminal board lug where C5 & R3 could be added. Also, no R6; the .0022 cap is connected to the TC wiper to B-. So, I disconnected the VC leaving the G1 of the 12SQ7 terminated with just the 4.7M R- essentially wide open and it was very quite when powered up just some lower level hum which you might expect in an amp like this- touch the wiper arm of the VC to the .01 cap and bam! hum comes back in force.

Is the Volume Control (VC) floating from the chassis? Perhaps the hum is
coming in via either the Common connection on the VC or perhaps the VC
is mounted to something metal that is not common to the amplifier chassis?
Is the RCA input to the amp unplugged while you have been doing these tests?
John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
[phone] (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

Hi, the VC terminals are floating from the metal chassis but the VC body is mounted to the chassis. One thing about the hum I hear, the frequency seems to change as you rotate the VC from min to max. When at minimum, no hum- rotate towards max and ~ mid span there are higher frequency harmonics then back to lower frequency hum at max volume. Phono RCA is disconnected & connected through out these tests, doesn't seem to effect the hum. Going to poke around more tonite, maybe hang a scope on it.

Update- the VC measured over 3 meg when it should be 1 meg. Also, R 10 is connected to pin 3 of the 35Z5 versus pin 2 as shown in the schematic. R5 might be an issue. So, new components ordered, will update when they are received and in place.
Thanks, Mark

[MarkS]

Post by MarkS
Update- the VC measured over 3 meg when it should be 1 meg. Also, R 10 is connected to pin 3 of the 35Z5 versus pin 2 as shown in the schematic. R5 might be an issue. So, new components ordered, will update when they are received and in place.
Thanks, Mark

OK, we have success, thing works great. Replacing the VC with a new 1M and replacing R10 & R5 did the trick. Regarding R10; the Sams has a picture of the chassis showing component location and in the pic, R10 is obviously connected to pin 3, not pin 2 so I left it that way. I didn't do it one component at a time so not sure which made the biggest improvement. Shot gunned so to speak 'cause I just wanted to be done with it.

Want to thank you all for the inputs, really helps when sorting things out. Also great to see some of the old names I remember back here in a single thread.

Very best regards,

Mark

[t***@gmail.com]

Post by MarkS
https://www.dropbox.com/s/vir6vfy8uj8yqq4/b-124.docx?dl=0
OK thanks! see if it works. I should tell you 1) The OPT is different (as stated before), the model I have mostly resembles the 12SQ7 version. I'm thinking mfg was done of the fly depending on available components. I want to get this damn thing off my bench so I can dig into an Ampex F44. I've never had something so simple give me so much grief (doing this for a friend of course). Been around tube electronics for >44 years. Restored many radios; built many amps. I have a decent scope if need be.

post it as an image file maybe

[John Robertson]

Post by MarkS
This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams circuit has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!

Does the hum vary up/down with the volume control?

If yes, then it is pre-amp or cartridge (wiring) related.

If the hum does not change with the volume setting then something is
either wrong with the output tube(s) or the B+ supply.

John :-#)#

[MarkS]

Post by John Robertson

Post by MarkS
This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams circuit has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!

Does the hum vary up/down with the volume control?
If yes, then it is pre-amp or cartridge (wiring) related.
If the hum does not change with the volume setting then something is
either wrong with the output tube(s) or the B+ supply.
John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
[phone] (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

OK,

Peter: Sorry I didn't update the rectifier but I swapped the 35Z5 with several other known good with no change. I shorted the cart lead at the cart and no change so I went to the amp phono plug and shorted there- no change! So I'm sure its in the amp.

John: The hum changes volume with the volume control.

So, is it possible there is something with either the volume or tone pots or associated?

Thanks,

Mark

[John Robertson]

Post by MarkS

Post by John Robertson

Post by MarkS
This thing is killing me. Webcor B-124 phonograph. Has hum I can't get rid of. All caps replaced, leads dressed, you name it. According to the Sam's photofact, the circuit has several variants. The only difference I can see is the OPT. The Sams circuit has an OPT that has what looks like a bucking winding in it for the 50L6GT output tube cathode. This one does not. Any ideas or a mod that can minimize the hum? Thanks!

Does the hum vary up/down with the volume control?
If yes, then it is pre-amp or cartridge (wiring) related.
If the hum does not change with the volume setting then something is
either wrong with the output tube(s) or the B+ supply.
John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
[phone] (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

OK,
Peter: Sorry I didn't update the rectifier but I swapped the 35Z5 with several other known good with no change. I shorted the cart lead at the cart and no change so I went to the amp phono plug and shorted there- no change! So I'm sure its in the amp.
John: The hum changes volume with the volume control.
So, is it possible there is something with either the volume or tone pots or associated?
Thanks,
Mark

Unlikely to be any of the pots, but usually this sort of hum is either
injected (noise from wire placement for example) or B+ hum that is not
well filtered. If the rectifier tube was bad chances are the hum would
be loud enough even with the volum turned down.

Can you unplug the input to the amplifier? Is that what you called the
'amp phone plug'?

Might there be some sort of Mute circuit that has a short to the frame?

Otherwise might be a pre-amp tube that has an internal short - should
show up on a half-decent tube tester.

If you post the schematics one of us will probably take a look and
advise better. Pretty much guessing right now!

John :-#)#

[Jim Mueller]

Post by MarkS
This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
rid of. All caps replaced, leads dressed, you name it. According to the
Sam's photofact, the circuit has several variants. The only difference I
can see is the OPT. The Sams circuit has an OPT that has what looks like
a bucking winding in it for the 50L6GT output tube cathode. This one
does not. Any ideas or a mod that can minimize the hum? Thanks!

Not to be obvious, but did you try replacing the tubes with known good
ones? Or trying these tubes in a known working radio? Sometimes audio
output tubes develop leakage from the heater to the control grid. This
is more common in 50C5 and 35C5 types because of their small size, but
octals can do it too. It's an easy test.

The extra winding on the output transformer is for negative feedback; it
isn't a hum bucking winding.

[MarkS]

Post by Jim Mueller

Post by MarkS
This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
rid of. All caps replaced, leads dressed, you name it. According to the
Sam's photofact, the circuit has several variants. The only difference I
can see is the OPT. The Sams circuit has an OPT that has what looks like
a bucking winding in it for the 50L6GT output tube cathode. This one
does not. Any ideas or a mod that can minimize the hum? Thanks!

Not to be obvious, but did you try replacing the tubes with known good
ones? Or trying these tubes in a known working radio? Sometimes audio
output tubes develop leakage from the heater to the control grid. This
is more common in 50C5 and 35C5 types because of their small size, but
octals can do it too. It's an easy test.
The extra winding on the output transformer is for negative feedback; it
isn't a hum bucking winding.
--
To get my real email address, replace wrongname with eggmen.
Then replace nospam with expressmail. Lastly, replace com with dk.

Yeah, I've replaced the tubes with at least 2 sets of known good ones, no difference. Agree, it does seem the extra winding is cathode feedback which is kind of cool in such a small amp. But this one doesn't have it.

[Jim Mueller]

Post by MarkS
This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
rid of. All caps replaced, leads dressed, you name it. According to the
Sam's photofact, the circuit has several variants. The only difference I
can see is the OPT. The Sams circuit has an OPT that has what looks like
a bucking winding in it for the 50L6GT output tube cathode. This one
does not. Any ideas or a mod that can minimize the hum? Thanks!

Somewhere in this thread, you said that turning the volume down
eliminated the hum but shorting the phono input jack did not. Someone
else said that means that the problem is in C4, C5, or R3. I would add
the phono jack itself to that list. But my guess at this point is C4.
Is it the correct value? Is it good (not the same as being new)? Is it
connected correctly, including the ground connection? Try using your
ohmmeter from B- to the bottom end of C4 and the shield of the phono
jack. They should both show 0 ohms. Use the ohms function of your
meter, not the continuity function.

[Jim Mueller]

Post by Jim Mueller

Post by MarkS
This thing is killing me. Webcor B-124 phonograph. Has hum I can't get
rid of. All caps replaced, leads dressed, you name it. According to the
Sam's photofact, the circuit has several variants. The only difference
I can see is the OPT. The Sams circuit has an OPT that has what looks
like a bucking winding in it for the 50L6GT output tube cathode. This
one does not. Any ideas or a mod that can minimize the hum? Thanks!

Somewhere in this thread, you said that turning the volume down
eliminated the hum but shorting the phono input jack did not. Someone
else said that means that the problem is in C4, C5, or R3. I would add
the phono jack itself to that list. But my guess at this point is C4.
Is it the correct value? Is it good (not the same as being new)? Is it
connected correctly, including the ground connection? Try using your
ohmmeter from B- to the bottom end of C4 and the shield of the phono
jack. They should both show 0 ohms. Use the ohms function of your
meter, not the continuity function.

OOPS. The shield of the phono jack should NOT show continuity to B- but
should show the capacitance of C4 if you have a capacitance tester.