Zenith 12S265 that keeps me guessing.

4/30/2010 10:17:45 AMRadio Joe

I have a beautiful Zenith 12S265 that has a hard to fix hum. I recieve many channels and she is loud as hell but I constantly get a mid to low volume hum all the time. I know this is not normal, have restored this zenith chassis before and it isn’t my house AC lines. I love it because when I get a good song playing it literally rattles the windows with bass and sends the cats running. It is so close to working well that it haunts me in my sleep. It really does. I need some help locating the issue and hope someone can give me some direction on locating the problem.
Not new to repairing Zeniths so I have some knowledge on what is going on. Any help is greatly appreciated.
Here is what has been done so far.
1. I have recapped the whole radio and have double-checked every connection to be correct. All new caps and all old style “linken log” resistors have been replaced with equivalent sizes.
2. Completely swapped out the toxic bar resistor with high quality 5 Watt resistors equal to the original resistances.
3. Checked and swapped out with different tube all the 12 tubes to make sure it wasn’t a bad tube causing the hum.
4. Cleaned ALL sockets and selector switches.
5. New electrolytic caps at schematic listed sizes.
6. Speaker has been re-coned and old voice coil replaced and the original speaker field coil is still intact. Ohms out good.
7. All tube shields in place and variable tuning capacitor has been thoroughly cleaned along with isolating rubber grommets replaced with new. Brass connecting forks cleaned.

I need a plan to track down the problem and remove it. I thought I would start with disconnecting C10 from the volume pot,the signal to the 6J5 the 1st IF tube, and see if the hum stays or disappears. If it stays I know it is in the audio/amp section of the radio. If not it is back in the detection section, correct??? Beyond this I am slightly lost. Like I said, I need a plan or some things to look for common to THIS radio. Way past swapping out obvious parts.
Any of you Guru’s out there can help a brother radio guy out? Unfortunately Google is my professor and I have been learning by doing. This radio has so many parts to it gets real crazy real fast. Any points I can check for continuity or whether ground should be there or not? Thanks for any help.

4/30/2010 11:59:34 AMNorm Leal

Hi

Is hum there with volume turned down? Did you notice negative of filter caps connect to center tap of the high voltage winding rather than chassis?

C21 positive goes to chassis, ground.

Are shields in place on audio tubes, 6J5G? If you replaced these tube with metal types is pin #1 grounded? Zenith didn't always have a pin #1 in their tube sockets.

http://www.nostalgiaair.org/PagesByModel/734/M0024734.pdf

Norm

:I have a beautiful Zenith 12S265 that has a hard to fix hum. I recieve many channels and she is loud as hell but I constantly get a mid to low volume hum all the time. I know this is not normal, have restored this zenith chassis before and it isn’t my house AC lines. I love it because when I get a good song playing it literally rattles the windows with bass and sends the cats running. It is so close to working well that it haunts me in my sleep. It really does. I need some help locating the issue and hope someone can give me some direction on locating the problem.
:Not new to repairing Zeniths so I have some knowledge on what is going on. Any help is greatly appreciated.
:Here is what has been done so far.
:1. I have recapped the whole radio and have double-checked every connection to be correct. All new caps and all old style “linken log” resistors have been replaced with equivalent sizes.
:2. Completely swapped out the toxic bar resistor with high quality 5 Watt resistors equal to the original resistances.
:3. Checked and swapped out with different tube all the 12 tubes to make sure it wasn’t a bad tube causing the hum.
:4. Cleaned ALL sockets and selector switches.
:5. New electrolytic caps at schematic listed sizes.
:6. Speaker has been re-coned and old voice coil replaced and the original speaker field coil is still intact. Ohms out good.
:7. All tube shields in place and variable tuning capacitor has been thoroughly cleaned along with isolating rubber grommets replaced with new. Brass connecting forks cleaned.
:
:I need a plan to track down the problem and remove it. I thought I would start with disconnecting C10 from the volume pot,the signal to the 6J5 the 1st IF tube, and see if the hum stays or disappears. If it stays I know it is in the audio/amp section of the radio. If not it is back in the detection section, correct??? Beyond this I am slightly lost. Like I said, I need a plan or some things to look for common to THIS radio. Way past swapping out obvious parts.
:Any of you Guru’s out there can help a brother radio guy out? Unfortunately Google is my professor and I have been learning by doing. This radio has so many parts to it gets real crazy real fast. Any points I can check for continuity or whether ground should be there or not? Thanks for any help.
:
:

4/30/2010 1:40:24 PMRadio Joe

Hi Norm, I thank you for your help.
The hum IS there when volume is all the way down. Indication of something? I was thinking bad internals to the volume pot creating hum. Have come across this before but it feels smooth to the turn and it controls volume well. Thoughts?

I have the negative of C26 and C27 goin to the center tap and the positives going to the speaker field. Will check again tonight but confident that they are. What was there to begin with was a cob job.

Got to check on the C21 positive is going to chassis ground. I will triple check.

I have glass 6J5s in place. Not all of them have shields just the ones that had them originally. The Osc. 6J5 is on the left and has shield which means there are definitly three without on the right. I DID NOTICE THE #1 pin is not connected to even a receptical on the socket for these tubes. Should I render this for glass tubes to?

4/30/2010 2:48:28 PMNorm Leal

Hi Joe

Since hum is there with volume tuyrned down work on circuitry to right of the volume control in schematic. Wouldn't worry about the volume control or anything to the left.

http://www.nostalgiaair.org/PagesByModel/734/M0024734.pdf

The two audio 6J5G's have a ground symbol going to outside. This would usually mean tubes have a shield?

Hum is in the power supply or audio portion of your radio. Be sure any old electrolytic caps have been electrically removed. Try bypassing electrolytic caps and see if there is a change in hum.

Leakage between cathode and filamement can cause hum but this isn't as common..

Norm

:Hi Norm, I thank you for your help.
:The hum IS there when volume is all the way down. Indication of something? I was thinking bad internals to the volume pot creating hum. Have come across this before but it feels smooth to the turn and it controls volume well. Thoughts?
:
:I have the negative of C26 and C27 goin to the center tap and the positives going to the speaker field. Will check again tonight but confident that they are. What was there to begin with was a cob job.
:
:Got to check on the C21 positive is going to chassis ground. I will triple check.
:
:I have glass 6J5s in place. Not all of them have shields just the ones that had them originally. The Osc. 6J5 is on the left and has shield which means there are definitly three without on the right. I DID NOTICE THE #1 pin is not connected to even a receptical on the socket for these tubes. Should I render this for glass tubes to?
:
:

4/30/2010 2:51:07 PMNorm Leal

One other thing, you can pull 6J5G tubes to help isolate a stage. Only work with audio tubes.

Was any shielded wire replaced? If so it needs to be shielded again.

Norm

:Hi Joe
:
:Since hum is there with volume tuyrned down work on circuitry to right of the volume control in schematic. Wouldn't worry about the volume control or anything to the left.
:
:http://www.nostalgiaair.org/PagesByModel/734/M0024734.pdf
:
:The two audio 6J5G's have a ground symbol going to outside. This would usually mean tubes have a shield?
:
:Hum is in the power supply or audio portion of your radio. Be sure any old electrolytic caps have been electrically removed. Try bypassing electrolytic caps and see if there is a change in hum.
:
:Leakage between cathode and filamement can cause hum but this isn't as common..
:
:Norm
:
:
:
:
:
::Hi Norm, I thank you for your help.
::The hum IS there when volume is all the way down. Indication of something? I was thinking bad internals to the volume pot creating hum. Have come across this before but it feels smooth to the turn and it controls volume well. Thoughts?
::
::I have the negative of C26 and C27 goin to the center tap and the positives going to the speaker field. Will check again tonight but confident that they are. What was there to begin with was a cob job.
::
::Got to check on the C21 positive is going to chassis ground. I will triple check.
::
::I have glass 6J5s in place. Not all of them have shields just the ones that had them originally. The Osc. 6J5 is on the left and has shield which means there are definitly three without on the right. I DID NOTICE THE #1 pin is not connected to even a receptical on the socket for these tubes. Should I render this for glass tubes to?
::
::
:

4/30/2010 3:24:22 PMRadio Joe

Awesome. Forgot about being able to remove tubes. I can't wait to get home tonight and get my beauty working tonight. I WILL let you know what I find tonight. Thank you, I have something to go on now.

5/1/2010 12:00:28 AMEdd

Sir Joe . . . . . .

Considering that all suggestions previously submitted have been conformed with and reinstating the necessity of their definitely being no new electrolytics installed . . . with them merely shunting across old units that might have been cosmetically and mechanically left in place.

With your finding that when the volume control rotor is swung CCW to its lowest to ground position, that then suggests the present hummmmmmmm fault being relevant to :

1 . . . Inadequate filtering by your initial C-L-C elements in the B+ power supply, being acquired off the filament of the 5Y4.

2 . . . Corruption coming in from the negative biasing supplies to the 1st grids of the AF amp stages to then be amplified .

3 . . . Poor or misplaced ground in the 1st audio stage.

Now probably not the 3rd item . . . since this system does not require a great deal of gain, with its small step up of the detected audio level and amplification, and then impedance shifting to optimally match power out to the speaker.

As for item 2 . . . Look at the supplied mark-up working schematic and note the YELLOW BOXED electrolytics.

Surely you had no problem with the C26-C27 initial filtering units polarity, but over to the right you need to confirm set wiring as being in compliance with the C21 and C25 items polarities.

They are associated with the filtering of a negative bias supply voltage that is derived by floating the center tap of the HV winding above ground . . . by a dropping resistor portion . . . of your replaced MUTER / CANDOHM
power resistor unit.

The last two electrolytics, to confirm installed polarization, are the small C19 and C20 units. That's it.

NITTY GRITTY TIME:

If unfruitful in the above check outs . . . and you have, in previous repair . . . replaced C15 and the C16 pair of coupling capacitors such that there is ZERO DC leakage thru them, to be upsetting the 1st grids biasing of following stages and thereby upset overall current comsumption of the stage . That bias upsetting condition shifting towards running closer to Class A could ramp up current demands and ripple level present from the power supply and increase ambient hummmmmmmm level all by itself. All is O.K. ?

Now then, let's evaluate the power supply proper by warming up the set a few minutes and . . . probably, almost immediately detecting that pesky hummmmmm . . OR . . . IS some warm up time required for its onset ?

At any rate, lets shut down the set and then take a pair of ~10-20 ufd 450 electrolytics and use clip leads to connect the - of the electrolytics filters to chassis ground and place each positive lead to a respective 6V6 plate.

Plug in the set and the warm up time should bring up the charge on those capacitors to the plate supply level and the set should be quenching off all hummmm, excepting that being created from the power supply itself

So now . . . is the hummm appreciably lower or non existant unless with ear almost AGAINST the speaker . . which would be an unrealistic situation. As you were previously detecting it WITHIN the programs audio levels.

If this knocks your hummm then we need to power down and pull the electroltytics and discharge them thru a 10-1k bleeder resistor and then finally short them out get zero volts.

We then take the positives of the filters, and they go to ground, with one negative lead of each filter going to the respective 1st grids of the 6V6's.

Power up the set and then listen for hummm level, if no hummmm present there, then the remaining 1st audio stage would be the suspected culprit of introducing in the humm.

Sooooo . . . power down and lift the electroltics, but only a few volts will have been acquired this time, so just short them out.

This test only one electrolytic will be needed, with its positive to ground and the negative WIRE going to the
1st grid of the 6J5 1st audio amp tube, (we don't need the extra baggage of a long test lead wire picking up stray signal).

This time I would expect no hummm, as you were doing basically the same thing with the vol-you-me control set to minimum.

By your previous stage by stage testing, you should now have an idea of the stage where the hum is being introduced into the audio chain.

Will be standing by for your findings . . . .

ASIDE:

On the AF output transformer to speaker interfacing, nothing but a standard 2 wire output of the output transformer secondary going to the 2 terminals of the speaker is depicted on the schematic . . . can you confirm that physically on the set ?

73's de Edd

Zenith Working Schematic mark-up:

Alternate . . . link to schema:

http://img62.imageshack.us/i/zenith12s265schematic.png/

:Awesome. Forgot about being able to remove tubes. I can't wait to get home tonight and get my beauty working tonight. I WILL let you know what I find tonight. Thank you, I have something to go on now.
:

5/1/2010 12:16:13 AMSchema link was COLD !

Sir Radio Joe . . . . . .

Considering that all suggestions previously submitted have been conformed with and reinstating the necessity of their definitely being no new electrolytics installed . . . with them merely shunting across old units that might have been cosmetically and mechanically left in place.

With your finding that when the volume control rotor is swung CCW to its lowest to ground position, that then suggests the present hummmmmmmm fault being relevant to :

1 . . . Inadequate filtering by your initial C-L-C elements in the B+ power supply, being acquired off the filament of the 5Y4.

2 . . . Corruption coming in from the negative biasing supplies to the 1st grids of the AF amp stages to then be amplified .

3 . . . Poor or misplaced ground in the 1st audio stage.

Now probably not the 3rd item . . . since this system does not require a great deal of gain, with its small step up of the detected audio level and amplification, and then impedance shifting to optimally match power out to the speaker.

As for item 2 . . . Look at the supplied mark-up working schematic and note the YELLOW BOXED electrolytics.

Surely you had no problem with the C26-C27 initial filtering units polarity, but over to the right you need to confirm set wiring as being in compliance with the C21 and C25 items polarities.

They are associated with the filtering of a negative bias supply voltage that is derived by floating the center tap of the HV winding above ground . . . by a dropping resistor portion . . . of your replaced MUTER / CANDOHM
power resistor unit.

The last two electrolytics, to confirm installed polarization, are the small C19 and C20 units. That's it.

NITTY GRITTY TIME:
If unfruitful in the above check outs . . . and you have, in previous repair . . . replaced C15 and the C16 pair of coupling capacitors such that there is ZERO DC leakage thru them, to be upsetting the 1st grids bising of following stages and thereby upset overall current comsumption of the stage . That bias upsetting condition shifting towards running closer to Class A could ramp up current demands and ripple level present from the power supply and increase ambient hummmmmmmm level all by itself. All is O.K. ?

Now then, let's evaluate the power supply proper by warming up the set a few minutes and . . . probably, almost immediately detecting that pesky hummmmmm . . OR . . . IS some warm up time required for its onset ?
At any rate, lets shut down the set and then take a pair of ~10-20 ufd 450 electrolytics and use clip leads to connect the - of the electrolytics filters to chassis ground and place each positive lead to a respective 6V6 plate.