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 Post subject: Crystal Earpiece
PostPosted: Dec Thu 15, 2016 11:59 pm 
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Joined: Mar Tue 11, 2014 2:54 am
Posts: 44
A very good "tool" to have on the bench is a crystal earpiece. It can be used to check where you lose a signal in the audio chain and does not load down the circuit. Just be sure to use a small value capacitor for isolation.


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 Post subject: Re: Crystal Earpiece
PostPosted: Dec Fri 16, 2016 9:19 am 
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A better tool would be an amplifier; a Transistor radio, with leads connected to ground, and through a cap to the VC wiper, can serve as a cheap audio signal tracer.

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 Post subject: Re: Crystal Earpiece
PostPosted: Dec Fri 16, 2016 2:39 pm 
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fifties wrote:
A better tool would be an amplifier; a Transistor radio, with leads connected to ground, and through a cap to the VC wiper, can serve as a cheap audio signal tracer.


I could also use a scope as a signal tracer, which I do have on my bench. The reason I posted this is because not everyone may have a spare tools or test equipment laying around, but the job could still be done with alternatives.


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 Post subject: Re: Crystal Earpiece
PostPosted: Dec Fri 16, 2016 3:58 pm 
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RCA and other manufacturers that published service handbooks for their receivers often recommended a battery and a pair of magnetic headphones. Depends what one is trying to measure as well as training the ear to a "defective" sound...

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 Post subject: Re: Crystal Earpiece
PostPosted: May Wed 17, 2017 6:47 am 
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Joined: May Fri 12, 2017 7:07 am
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Its a good idea i used the same idea for signal testing along the circuit chain ,i used it before i had a scope but hearing it is just as good in some ways .


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 Post subject: Re: Crystal Earpiece
PostPosted: May Thu 18, 2017 9:44 pm 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 2974
Location: Florida
A crystal earpiece makes a good, simple, cheap tracer.

Using the audio part of a transistor radio will also work but the input impedance is low and may load enough to make a good circuit appear weak or defective.

There are circuits on the web that precede an LM386 amp with a FET input stage. I built one years ago and it is a very handy little item.

RRM


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 Post subject: Re: Crystal Earpiece
PostPosted: May Sun 21, 2017 4:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 869
Location: St. Louis, MO, USA
Awhile back, I put one of those old radio shack 4transistor amplifiers in an old aluminum box with a small speaker and used a piece of shielded cable as a probe. Very useful.

Dennis


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 Post subject: Re: Crystal Earpiece
PostPosted: May Sun 21, 2017 6:14 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 9096
Location: Santa Rosa, CA
There are very inexpensive LM386 amplifier boards and kits available on Ebay; some with free shipping.

http://www.ebay.com/itm/1pcs-LM386-Supe ... Sw14xWMfNq

If you need a pre-amp stage, they also have microphone amps that will serve.

http://www.ebay.com/itm/Multi-Stage-Ele ... SwYIxX92zc

My homebrew signal tracer is a 1458 op-amp followed by an LM386 audio amp. A couple of hundred mW is more than enough audio.

Rich


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