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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 17th Jun 2019, 4:33 pm   #1
vinrads
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Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
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Default Fitting a external input socket !

How hard can it be ? are you sitting down !. first of all the chassis on this Zenith didn't have a gram position on the wave change switch ,so I needed to arrange something to switch the input ,this problem was solved when I found a 3.5 mm jack socket with a two pole two way switch .The other switch was used to switch a relay disconnecting the heaters on the rf section ,this caused another problem ,because this chassis derives a neg bias voltage from the HT using a series of resistors in the neg of the reservoir capacitor by disconnecting the three valves I reduced the HT current so therefore reducing the neg voltage , upsetting the biasing on the two audio valves.
The problem was solved by using another relay to switch a loading resistor on the HT line when in ext input mode , this turned out to be 25k ohm 7 watt. this brought things back to normal , except , the switches on the 3.5mm socket I guess were designed to switch low level audio ,not 250ma that the two relays required , a nice npn power transistor was fitted ,now the switching current was less than 1ma . yes a diode was fitted across the coils of the relays.
Oh forget to say the output of my Pure dab radio with headphone out only required boosting ,this was done using a mono ic pre amp found on Ebay very cheep , of course this needed it's own power supply ,which I built , the supply for the relays was derived from a spare 6.3v winding on the newly fitted mains tx . Phew . I think that is it . I have just got to fit the chassis back in the newly refurbished cabinet . More on the restoration of the chassis to follow . Mick.
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Old 17th Jun 2019, 5:03 pm   #2
Herald1360
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Default Re: Fitting a external input socket !

Hmmm,

Why not just use one contact on the socket to disconnect the detector audio from the volume control and the other to connect the audio input to it?
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Old 17th Jun 2019, 7:04 pm   #3
vinrads
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Default Re: Fitting a external input socket !

Now that would be to simple Chris , the main thing was I wanted to disconnect the rf section because I am sure this radio will be mostly used to play an I pod etc .Mick.
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