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Offline Homefire
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« Reply #30 on: September 30, 2018, 09:14:13 pm »
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   Super Reference Material.  Carl is currently working for  First Texas Products metal  detector makers  100 miles down the road from me in El Paso I believe.   He's been at the game for quite some time. 

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« Reply #31 on: October 04, 2018, 02:22:13 am »
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I need help with electronics...

I think this is not a problem for the detector functionality but... I'm asking because I'm on a costant learning process.

In the picture you can see the simulation of the search oscillator for the russian BFO, you can see an imperfection at the top of the sinewave, this imperfection is present in the circuit too (remember the picture of oscilloscope in the previous posts).

First i was thinking to lower the quiescent point of the transistor lowering R1 (from 43K to 39K) but nothing happend.

It seem that the transistor cannot "drain current fast" as the sinewave start the falling edge... I'm confused beacause is not a clipping of waveform... probably capacitance of capacitors in the emitter make a charge reservoir that the transistor cannot drain as fast creating a "initial slow slope falling edge".

please be patient can you point me in the right direction ?

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« Last Edit: October 04, 2018, 02:24:01 am by giuhalftrack »
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« Reply #32 on: October 04, 2018, 10:39:29 am »
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   Let it be known I am NOT a EE.  Only know what I know via working in the field and self taught.

 Actually I think you have the cause nailed.  First that little bit of weirdness on the wave is not going to effect anything noticeable I'm thinking.

As this is a Simulation it may not even happen in the real circuit, eaa?  

As it seems to be on the Positive Swing, Try adding a REALLY little bit of Capacitance between the Base and Collector.  Series capacitance would lower the total value and cancel some out I think.  1pf may be more then you need.  

Is this really worth the efforts?  

LOL, I wouldn't bother trying.  

  

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« Last Edit: October 04, 2018, 10:44:39 am by Homefire »
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« Reply #33 on: October 04, 2018, 02:53:58 pm »
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Quote:Posted by Homefire
 
Is this really worth the efforts?  

LOL, I wouldn't bother trying.  

  


ohh may be you are right !!  Smiley I use my effort to get the project completed instead of take care about details... I'll try your solution anyway to experiment.
thank you


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« Reply #34 on: October 06, 2018, 07:16:00 pm »
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Not a Solution.  Just a IT MAY HELP .  If your going to make changes at least do it logically ?  LOL   Adding capacitance in series with the cause should reduce the capacitance causing the issue if that is what is going on.  Only way to find out is try.  I would try.  LOL

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« Reply #35 on: October 11, 2018, 02:10:15 am »
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Quote:Posted by Homefire


So can you get it Putt Putting ?  Low Roar maybe ?  Getting the two oscillators close enough to hear the Heterodyning process is not so easy some times.  I've never achieve it without some tweaking the first few goes . 

Ok, waiting !  Would be most interested to hear a Audio File if you could manage something like that ?



I have not fully understood how the audio circuit works ... (tuning) because the russian translation  by google is ... not so good.

this little video shows a 50 cent euro coin at 5-7 inch and a big stuffed tuna can at 10 inch and over that exibit nice "arcade game style" sound  .... so if you ear those sounds that reminds you the old time games you are on a big cache  Cheesy

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« Reply #36 on: October 11, 2018, 12:28:26 pm »
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Emm Sounds almost like it's being gated some how.  Must be something about the way the Mixer is working.

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« Reply #37 on: October 11, 2018, 02:28:28 pm »
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seems like the 2 pnp transistor at the end stage transform a sine wave in a square wave of 1/2 of the frequency...
i think because square waves are more easily to ear..
probably in the original design the speaker or earphones are from the type at high impedance (i think 2K or similar), because using 8 - 16 ohm speaker load the transistor too much and the sound is barely audible.

Now i have added a buffer stage to drive a 8 ohm speaker.... this design have some cool feature that improve the sensitivity producing a sound more easily audible.


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« Reply #38 on: October 13, 2018, 09:30:57 am »
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  I picked up a hand full of these for that sorta stuff.  You can't build them this cheap !

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« Reply #39 on: October 14, 2018, 01:22:34 am »
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sure ! i already have some LM386 in my arsenal Smiley but this solution is very cheap i'll go for that !!

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