Zenith 6-S-152 Humm

10/12/2009 7:30:10 PMDaveF

Hello everyone. My Zenith console was recently recapped and I still get a noticable humm when the radio is on, even at no volume. I cannot read the schematic to be sure if the 3 10@450 electrolytics are too low in voltage and may be the problem. Can someone interpret this for me? Also tnere is a round switch at the back of the chassis and there are some wires underneath the chassis that are cut (2), was this for a phono?

10/12/2009 9:35:09 PMSteve - W9DX

:Hello everyone. My Zenith console was recently recapped and I still get a noticable humm when the radio is on, even at no volume. I cannot read the schematic to be sure if the 3 10@450 electrolytics are too low in voltage and may be the problem. Can someone interpret this for me? Also tnere is a round switch at the back of the chassis and there are some wires underneath the chassis that are cut (2), was this for a phono?
:

Voltage ratings on your electrolytic caps are fine, but I think your 10 mfd values may be too low. The schematic on this site shows C21 at 16 mfd, C25 at 18 mfd. I would try 22 mfd for all 3, and see if that eliminated most of the hum. Also be sure any tube shields are well seated and grounded.
Steve

10/12/2009 9:41:31 PMSteve - W9DX

::Hello everyone. My Zenith console was recently recapped and I still get a noticable humm when the radio is on, even at no volume. I cannot read the schematic to be sure if the 3 10@450 electrolytics are too low in voltage and may be the problem. Can someone interpret this for me? Also tnere is a round switch at the back of the chassis and there are some wires underneath the chassis that are cut (2), was this for a phono?
::
:
:Voltage ratings on your electrolytic caps are fine, but I think your 10 mfd values may be too low. The schematic on this site shows C21 at 16 mfd, C25 at 18 mfd. I would try 22 mfd for all 3, and see if that eliminated most of the hum. Also be sure any tube shields are well seated and grounded.

Correction: I had to get my Beitman's out and use the magnifying glass. The two C25's to indeed appear to be 8 mfd, and your 10 mfd's should be okay. C21 however should be 16 mfd where a 22 mfd at 450v should work fine.
Steve
:Steve
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10/13/2009 2:44:42 AMWarren

If you look at the schematic, there is a section in dotted lines "circuit changes for phonograph " Shows 11 as a switch, and 12 being a phono jack. What are the two cut wires connected to?

10/13/2009 10:11:05 AMDaveF

:If you look at the schematic, there is a section in dotted lines "circuit changes for phonograph " Shows 11 as a switch, and 12 being a phono jack. What are the two cut wires connected to?
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It appears that it is a round switch that can be turned on/off, from what I recall, one goes to one of the tubes, the other to a ground, and then the two ends are cut (unconnected).

10/13/2009 1:30:18 PMWarren

If I understand what you are saying, you do have the " circuit change for phonograph " That wire you say that is connected to the tube and cut lose, could be the cause of picking up hum. The schematic is easy to follow the wire connections and switch for this. Just follow the connections in the dotted area showing the 6H6 optional wiring for phonograph. It's a handy option to have anyway, as to play something else through it, such as an iPod.

10/13/2009 2:40:49 PMEdd

Sir Dave . . .Effff:

Ahhhh sweet mystery, will the hummmmmm be from the power supply---an earler audio stage amplifying hummmm--- from a ground loop---shielding problem ---wiring misrouting--- a poor ground or yet . . . . even something else ?

Might I suggest a differentiating type of initial test; in order to home in on the guilty culprit(s) of the situation ?

That would be done by the taking of a comon low voltage rated electrolytic, in the order of 100 mamma- farads at 25VDC , which should adequately cover the expected highest voltage spread to be encountered by the involved circitry.

Now, in order to cover all hummmmmmm contaminated signal injection possibilities PRIOR to the 6F5 1st audio ampli-fryer, have a cold set condition and take ye olde 100 ufd unit in hand and tack solder connect its negative lead to the 1st grid of the 6F5 and its "positively for certain" lead is going to the cathode of the 6F5 tube.

We have now shunted ALL audio presence above ~15 cycles to ground.

You then have the volume control at MINIMUM and power up the set and after its tube warm up, take an evaluative hummmmmmmm check . . . . . .has the hummmm disappeared or abated appreciably ?

If the previous test was . . . no change . . . power down the set and pull and move ye olde community electrolytic down the audio chain to the finger blistering 6F6, where you will be installing its negative lead to the 1st grid of the 6F6 and its positive lead to the
cathode of the 6 effff 6.

Same old power up the set with the volume control (still at its minimum) and listen for the same old Where's Waldo ? hummmm test.

If the initially experienced hum level has abated now, the circuitry of the 6F5 would be suspect, with it having created the hum or having affected it.

HOWEVER, if the humm level is now a the initial level at the onset of testing, that is pointing to power supply ripple being the source of the hum or a rare case of higher level filament to cathode leakage in the 6F6. Now, if you don't have another 6F6 to sub in, you can use a total pulling out of that 6F6 and the tack in of a 5K 10 W wire wound resistor from plate to ground. Power up and then see if the hummmmm is present.

If still present then the final consideration would be confirming the degree of filtering being provided by the pi network of the C21 input filter cap, the field coil of the dynamic speaker and the C26 output filter capacitor. I don't see the spec of the C21 on this sites schematic but would initially suspect in the order of~ 12 mama farads. 20-22 ufds being in the order for the C26.

Another rare situation would be the deterioration of the enamel on the windings of the field coil and forming a cluster of shorted turns within one close area. That would decrease the filtering efficancy of the unit and yielding a lesser degree of ripple supression.

The only referencing on it is the 1000 ohms DC resistance spec and the involved turns shorting would not be easily reflected within that high of a resistance figure.

Concentrate on the further filtering of the B+, unless a touch test reveals a high temperature of the speaker field coil after proper potential warm up time.

Heyyyyy . . . my letters basket is empty . . . nothing but a couple of j. f xx zz and q's left.

73's de Edd

10/13/2009 7:07:43 PMDaveF(80277:80273)

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:I replaced a few last caps that were the originals then checked the tubes again and discovered in lieu of the 6F6, someone used a 6K6 as a substitute. I replaced this with a 6F6 and cut the connection for the phono switch and the hum is down about 50%. I think I am going to call it a day. Perhaps my audio tube is weak or bad, I will try to swap and see.
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:Sir Dave . . .Effff:
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:
:
:
:Ahhhh sweet mystery, will the hummmmmm be from the power supply---an earler audio stage amplifying hummmm--- from a ground loop---shielding problem ---wiring misrouting--- a poor ground or yet . . . . even something else ?
:
:
:Might I suggest a differentiating type of initial test; in order to home in on the guilty culprit(s) of the situation ?
:
:
:That would be done by the taking of a comon low voltage rated electrolytic, in the order of 100 mamma- farads at 25VDC , which should adequately cover the expected highest voltage spread to be encountered by the involved circitry.
:
:
:
:
:Now, in order to cover all hummmmmmm contaminated signal injection possibilities PRIOR to the 6F5 1st audio ampli-fryer, have a cold set condition and take ye olde 100 ufd unit in hand and tack solder connect its negative lead to the 1st grid of the 6F5 and its "positively for certain" lead is going to the cathode of the 6F5 tube.
:
:
:We have now shunted ALL audio presence above ~15 cycles to ground.
:
:You then have the volume control at MINIMUM and power up the set and after its tube warm up, take an evaluative hummmmmmmm check . . . . . .has the hummmm disappeared or abated appreciably ?
:
:
:
:
:If the previous test was . . . no change . . . power down the set and pull and move ye olde community electrolytic down the audio chain to the finger blistering 6F6, where you will be installing its negative lead to the 1st grid of the 6F6 and its positive lead to the
:cathode of the 6 effff 6.
:
:
:Same old power up the set with the volume control (still at its minimum) and listen for the same old Where's Waldo ? hummmm test.
:
:
:If the initially experienced hum level has abated now, the circuitry of the 6F5 would be suspect, with it having created the hum or having affected it.
:
:
:HOWEVER, if the humm level is now a the initial level at the onset of testing, that is pointing to power supply ripple being the source of the hum or a rare case of higher level filament to cathode leakage in the 6F6. Now, if you don't have another 6F6 to sub in, you can use a total pulling out of that 6F6 and the tack in of a 5K 10 W wire wound resistor from plate to ground. Power up and then see if the hummmmm is present.
:
:
:If still present then the final consideration would be confirming the degree of filtering being provided by the pi network of the C21 input filter cap, the field coil of the dynamic speaker and the C26 output filter capacitor. I don't see the spec of the C21 on this sites schematic but would initially suspect in the order of~ 12 mama farads. 20-22 ufds being in the order for the C26.
:
:
:Another rare situation would be the deterioration of the enamel on the windings of the field coil and forming a cluster of shorted turns within one close area. That would decrease the filtering efficancy of the unit and yielding a lesser degree of ripple supression.
:
:
:The only referencing on it is the 1000 ohms DC resistance spec and the involved turns shorting would not be easily reflected within that high of a resistance figure.
:
:
:Concentrate on the further filtering of the B+, unless a touch test reveals a high temperature of the speaker field coil after proper potential warm up time.
:
:
:
:Heyyyyy . . . my letters basket is empty . . . nothing but a couple of j. f xx zz and q's left.
:
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:
:
:
:
:73's de Edd
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