Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jun Wed 23, 2021 4:00 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 8 posts ] 
Author Message
 Post subject: Aux input HUM
PostPosted: Jun Thu 10, 2021 5:41 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2429
Location: Sterling Heights, Michigan 48314, USA
I've installed an AUX input circuit in a Zenith 7s363 per this diagram:

Image

The circuit works but there is an annoying HUM when using the AUX setting. No hum is present in the RADIO setting. It's not overbearingly loud but it's definitely audible. Can't figure out where it's coming from but I do know it's not from an external source (sounds more like a "ground problem hum" )
The hum is present even with no device plugged into the input jack.

Any ideas ?

_________________
.....Dennis.....
Live Long and Prosper


Top
 Profile  
 
 Post subject: Re: Aux input HUM
PostPosted: Jun Thu 10, 2021 6:04 pm 
Member

Joined: Jan Thu 24, 2013 11:50 pm
Posts: 3485
Location: Bristol, Rhode Island
Is the jack's ground lug grounded to chassis?


Top
 Profile  
 
 Post subject: Re: Aux input HUM
PostPosted: Jun Thu 10, 2021 6:24 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2429
Location: Sterling Heights, Michigan 48314, USA
Diverted wrote:
Is the jack's ground lug grounded to chassis?



Yes it is.

Since my original posting I've been in the workshop fooling with the problem. It turns out that if the input device's volume is not turned up to a reasonable level the hum becomes quite apparent. Not good to use the radio's audio amp to boost a weak input.... doing so induces hum.

Setting the output level of the input device (iPhone, iPad etc) at a comfortable level and doing the same with the radio's volume pot results in a clean sound.

Thanks for your response.

_________________
.....Dennis.....
Live Long and Prosper


Top
 Profile  
 
 Post subject: Re: Aux input HUM
PostPosted: Jun Thu 10, 2021 10:51 pm 
Member

Joined: Jan Tue 16, 2007 7:02 am
Posts: 4496
Location: Lexington, KY USA
The audio stages in this set do not use the chassis for their signal common. There is a 2VDC offset, and possibly a bit of power supply ripple, between the audio common and the chassis.

The CCW end of the volume control is connected to this audio common bus.

http://www.nostalgiaair.org/pagesbymode ... 025354.pdf

On the schematic, you can see this bus running between the volume control and the negative end of filter cap C25 and also the Candohm X (120 ohm) tap. The detector and audio tube cathodes connect to it. All of these points show up as -2 V on the voltage chart.

I suggest trying a large capacitor between this audio common and the chassis. 100uF or more. + to the chassis. 6.3V or 10V would be reasonable.

Be sure both the radio and aux input function OK before deciding to add this capacitor to the set.

It might also be a good idea to include a capacitor in series with the signal input, to keep the DC off the volume control. 0.022uF between the switch contact and the resistor network might be good.

Ted


Top
 Profile  
 
 Post subject: Re: Aux input HUM
PostPosted: Jun Fri 11, 2021 3:43 am 
Member
User avatar

Joined: Dec Sat 24, 2011 9:17 pm
Posts: 7800
Location: Vancouver Island, Canada
Have you tried connecting the return side to B- instead of the chassis?
If that does nothing, follow Ted's guidance for removing hum, good tips.

Using a 1:10 or similar step-up transformer on the auxiliary input boosts the signal, allows your device to be turned down, you get isolation and less distortion. Also you may need to use a shielded input, coaxial cable if there's a long run to and from the switch.

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


Top
 Profile  
 
 Post subject: Re: Aux input HUM
PostPosted: Jun Fri 11, 2021 12:20 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 2429
Location: Sterling Heights, Michigan 48314, USA
No bench work today….back at it tomorrow.
I will try the above suggestions…..and report my results.

Thanks for your input.

_________________
.....Dennis.....
Live Long and Prosper


Top
 Profile  
 
 Post subject: Re: Aux input HUM
PostPosted: Jun Fri 11, 2021 1:37 pm 
Member
User avatar

Joined: Jul Tue 15, 2008 6:13 pm
Posts: 9107
Location: Gretna, Nebraska
Zenith provided a bulletin on modifying their sets for phono input back in the thirties. On their late 30s transformer sets the modification almost always recommended connecting to the points shown on the schematic clip below.

The selector switch wiring that Zenith recommended would switch the audio and also shunt the radio audio to ground when the switch was in the phono position to help reduce audio bleed through when listening to the phonograph.

I always include an audio transformer between the external audio device and the radio when I modify a radio for an auxiliary input. A transformer will almost always improve audio fidelity and level.

Attachment:
Untitled.jpg
Untitled.jpg [ 131.47 KiB | Viewed 156 times ]

_________________
http://www.vintagerestorationservices.com
Paul
...... how hard can it be?


Top
 Profile  
 
 Post subject: Re: Aux input HUM
PostPosted: Jun Fri 11, 2021 3:30 pm 
Member
User avatar

Joined: Jul Tue 15, 2008 6:13 pm
Posts: 9107
Location: Gretna, Nebraska
Here is a 9 tube Zenith model from 1939 that came with a factory phono input. The detector and audio circuitry is similar to the 7-S-363.

You can see how the audio is routed through the selector switch for phono and radio in red. The other switch contact shunts the radio audio to ground when the phono input is in use.

It is fairly common to get some low level radio audio when switched to audio from the auxiliary/phono input.

Attachment:
Untitled1.jpg
Untitled1.jpg [ 211.62 KiB | Viewed 146 times ]

_________________
http://www.vintagerestorationservices.com
Paul
...... how hard can it be?


Top
 Profile  
 
Post New Topic Post Reply  [ 8 posts ]  Moderators: Norm Leal, Marcc

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 45 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB