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 Post subject: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Jul Wed 12, 2017 10:20 pm 
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Joined: Mar Sun 19, 2017 6:15 am
Posts: 23
Location: Bainbridge Georgia 39819
I just got the Zenith Console back together( My first recap job) and now have a static hiss(white noise) in the right speakers. The noise is not volume level related. It happens instantly when the unit is powered up before the tubes have warmed up and the radio signal begins . It occurs no matter what source is selected, am,fm phono or tape. It occurs with the volume in the most minimum position. I have tried switching the speaker wires and the noise moves to the other side when wires are switched. The level of the noise never increases or decreases even when turning up the volume, it is a steady constant. I am not sure if the noise was present before the recap job or not as the unit would barely produce sound. Other than this noise the unit seems to working as it should with great sound and sensitivity. Anyone ever experienced this or have any ideas where to start looking for the cause of this? Thanks Dan


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Jul Thu 13, 2017 3:47 am 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1241
Location: Brooklyn, NY 11217
Is it hiss or hum? Did you install a grounded power cord?


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Jul Thu 13, 2017 4:01 am 
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Joined: Mar Sun 19, 2017 6:15 am
Posts: 23
Location: Bainbridge Georgia 39819
Its definitely not hum, I know what that sounds like. Best I can describe it is like when tuning a radio between stations, the static hiss you hear but soft. It never goes away, it stays constant but you can turn the volume up enough that you don't really hear it unless you really pay attention. No the original power cord is also in great condition(no cracks, almost as a new cord)so I left it alone


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Jul Thu 13, 2017 10:52 am 
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Joined: Aug Sat 02, 2014 3:59 am
Posts: 1241
Location: Brooklyn, NY 11217
Is there a safety cap installed from line to ground?


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Jul Thu 13, 2017 4:36 pm 
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Joined: Mar Sun 19, 2017 6:15 am
Posts: 23
Location: Bainbridge Georgia 39819
I'll have to take a look, It's been a few months since I recapped the tuner and amp and I cant remember. I should probably mention that this is a Hybrid unit. The tuner and pre amp circuit have both tubes and transistors and the separate amp is all transistor.


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Sep Tue 15, 2020 9:10 pm 
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Joined: Apr Sun 16, 2017 7:57 pm
Posts: 291
dcrews1969 wrote:
I just got the Zenith Console back together( My first recap job) and now have a static hiss(white noise) in the right speakers. The noise is not volume level related. It happens instantly when the unit is powered up before the tubes have warmed up and the radio signal begins . It occurs no matter what source is selected, am,fm phono or tape. It occurs with the volume in the most minimum position. I have tried switching the speaker wires and the noise moves to the other side when wires are switched. The level of the noise never increases or decreases even when turning up the volume, it is a steady constant. I am not sure if the noise was present before the recap job or not as the unit would barely produce sound. Other than this noise the unit seems to working as it should with great sound and sensitivity. Anyone ever experienced this or have any ideas where to start looking for the cause of this? Thanks Dan


What was the cause?


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Sep Wed 16, 2020 2:13 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 884
Location: dayton oh usa
since its a old ss amp its likely a leaky/noisy germanium transistor.
a simple audio tracer should pinpoint the noisy stage.


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Sep Wed 16, 2020 2:31 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 6794
Location: Near Brandon, Iowa
This is hybrid-technology set: mostly vacuum tubes in the receiver but there is a solid-state amp in the unit. I recommend looking there for a leaky transistor in the signal path of the noisy channel. Transistors are almost certainly germanium in this set.

An oscilloscope would be helpful in localizing the bum gain stage... you can compare the "bad" channel signals with the "good" one.


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 Post subject: Re: 1963 Zenith Console Model ML2685 Right Channel Hiss
PostPosted: Sep Fri 18, 2020 4:50 am 
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Joined: Dec Sat 24, 2011 9:17 pm
Posts: 6689
Location: Vancouver Island, Canada
I would suggest it could also be a tube? Just that a tube swap would tell you right away and would be the easiest fix.
Focus on the audio section after the volume control. Unfortunately, from the description it sounds like that section is transistors only. Good luck.

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