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 Post subject: 1967 Astro-Sonic hum problem.
PostPosted: Jan Wed 31, 2018 6:48 pm 
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Joined: Jan Fri 05, 2018 7:54 pm
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I have recapped it. The hum is there when the volume is all the way down. The hum increases with volume and the amp input is very sensitive to "finger" proximity when you flip the polarity on the AC MAINS plug.

Either way the AC MAINS plug is in, there is a low level hum that seems above what should be normal.

Should I expect "dead quiet" 60 Hrz (or octaves)?

On the bench the amp seems fairly hum free, or at least the hum is below or at the noise floor of the amp.

In the chassis, and with the input connector disconnected, the hum remains the same as the above description. Indicating to me some kind of ground loop or AC getting into the amp chassis from the tuner module.

There is 1 cap I haven't replaced, because it's capacitance measured fine(didn't test ESR) It is the bi-polar 70-82mfd in the tuner module.

Thoughts?


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 Post subject: Re: 1967 Astro-Sonic hum problem.
PostPosted: Jan Wed 31, 2018 9:32 pm 
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Joined: Dec Sat 24, 2011 9:17 pm
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Location: Vancouver Island, Canada
Transistors or tubes? Just so we know.

How is your lead dress? If there are bare leads in close proximity to wires or components carrying current, you could be picking up hum that way. Check the work you did especially in the front end. Long wires should follow the chassis as much as possible.

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Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
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 Post subject: Re: 1967 Astro-Sonic hum problem.
PostPosted: Feb Thu 01, 2018 7:15 am 
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It's transistors.

The hum is the same level with the input to the amp disconnected.


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 Post subject: Re: 1967 Astro-Sonic hum problem.
PostPosted: Feb Sat 03, 2018 1:46 am 
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Check your solder joints, and make sure no traces on the CB were damaged during the recap.
Check for a misplaced connection, or an incorrect value.
Check for a faulty cap, which can happen, intermittant contact inside.

_________________
Watch the doughnut, not the hole.
Burl Ives, RIP, oldtimer.
[:l>)


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 Post subject: Re: 1967 Astro-Sonic hum problem.
PostPosted: Feb Sat 03, 2018 6:38 pm 
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Joined: Jan Fri 05, 2018 7:54 pm
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Quote:
Check your solder joints, and make sure no traces on the CB were damaged during the recap.
Check for a misplaced connection, or an incorrect value.
Check for a faulty cap, which can happen, intermittant contact inside.


Yes, thank you. All that is good.

What we have here is a very sensitive input cap or ground loop from the tuner module. On the bench, with my overhead lights OFF, the amp chassis is very quiet. The only thing that is different is hooking up the tuner module power plug etc.

Can anyone shed light on why the unit is sensitive to AC mains plug polarity changing? In one position, the input to the amp becomes MUCH more sensitive to induced hum, say from like your finger being in close proximity to the input caps or the plug itself.


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 Post subject: Re: 1967 Astro-Sonic hum problem.
PostPosted: Feb Tue 06, 2018 10:09 pm 
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Joined: Jan Fri 05, 2018 7:54 pm
Posts: 4
So, this turned out to be environmental.

Running underneath the house where the radio sits is something that is emanating a strong hum.

Any ideas on shielding the input section of the amp?

What about ground the chassis of the amp?


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