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A Receive Pre-Amp you can build!

Discussion in 'CB Radio Modifications' started by Dmans, Jul 8, 2018.

  1. Dmans

    Dmans Active Member

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    All,
    Rambling through my junk box, I ran across a receive pre-amp I picked up many years ago (Don't remember where). It is a very simple circuit that I have tried to draw although I am sure it is not to engineering standards!
    I did install it in a Cobra 29 many moons ago that was in my service truck. As I recall it worked well! Subsequently removed the pre-amp and sold/traded/misplaced the Cobra 29 (and the install instructions for the pre-amp). As I recall it installed between the first inductor coil (closest to the SO-239) and the first tuning can by removal of a small coupling capacitor (20pF or so) between the two. I'm sure it will work in most rigs the same way. (Experts please chime in!)



    I do not know who made this and am not trying to infringe on anyone's copyrights just sharing a clever idea that you can build and experiment with. (If you know the origin of this, please share)

    My NOT to Engineering standards of the circuit.
    (But very good therapy for my partially paralyzed dominant hand!)

    DSCI0037.JPG

    The actual board.
    DSCI0039.JPG

    DSCI0040.JPG

    DSCI0041.JPG

    It can be used with a switch to put it "In" or "Out" of circuit but it might be best to use with a small relay to keep leads short. Using a relay you could also switch the small capacitor, removed to install this circuit board, back into the receive circuit and be "back to stock" receive. I used the RF gain on the Cobra 29 to reduce receive gain as needed.

    73's
    David
     
    Shadetree Mechanic likes this.

  2. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

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    Looks like a neat little project, will be looking in to this for sure. Thanks for sharing.
     
  3. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    KEWL!

    Thought I'd help a bit here...

    Preamp.jpg

    It's (the above) is preliminary - no "Mask" for the traces yet.

    But managed to glean this from your schematic and parts list - thank you.

    Looks like something that can be "inserted" directly - the "13.8V" deal is just that - power supply. Anything over 6 volts the thing worked fine. So this works for any RX/TX switch radio and you simply soldered this ahead of the dropping resistor for the RX LED - so it "toggled on or off" with the RX side of things - took like 50mA at the most.

    As so made, the design was for INSERTION across the leads of a relay - like those used in Amps that did not come with a built-in pre amp. So TSW2 was INPUT (Signal In) FROM your antenna and TSW1 was the OUTPUT (Signal Out) to the radios' coax connector.

    Sidebar SEGUE -
    Note the Orientation of Q1 - Original part outline was for a BCE PINOUT - but the 1675 used has a PINOUT of ECB - so you know.

    The 13.8 thingy was to have been set up to use the POWER side of the Relays unused contacts (Assuming when you're RX-ing the amp was off) when the Amps' Relays contacts were at idle - so it received Power from the amp - but only when the amp was NOT engaged, when the AMP kicked in - the relay broke the power feed and switched it to the amps own internal feeds to the amplifier - turning off this device. (Basically a DPDT style amp - one side for antenna switching the other for power switching.) So you have to locate the amps bypass RX cap for when the amps not on, and this is where TSW1 and TSW2 went.

    The TSW1 and TSW2 - note the diode used ... helped keep the diode conducting into Q1's BASE lead, keeping it and Q1 from self destruct (NPN) so that went across the RELAY terminals that had the antenna set up to BYPASS the amp when it's not engaged.

    These were offered as kits on an older 2290-based amp that people could buy and install in a Grant, Cobra 148 or even their RCI2950's as a helper - Modulator. They were the predecessor to the RFX MOSFET kits you'd see today. These boards were the pre-amps, designed for them and came as a separate kit from the supplier - either RF Limited or CB City had offered these as accessories.

    Fun little device, brings back TONS of memories - and an Old Grant I once had that the XYL never returned.

    SIGH.

    :+> Andy <+:
     
    #3 Handy Andy, Jul 8, 2018
    Last edited: Jul 8, 2018
    357magnum and Dmans like this.
  4. Dmans

    Dmans Active Member

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    Great information Andy! (That's what I meant by "Engineering Standard!!)
    Thanks

    73's
    David
     
  5. nomadradio

    nomadradio Analog Retentive

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    Or would that be the ex-XYL?

    73
     
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  6. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    Yes, Nomad - the x-XYL...

    (*ahemn*)

    Like the Jerry Reed song...

    :+> Andy <+:
     
  7. Handy Andy

    Handy Andy Do Your Research First, Then Decide...

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    I can't take credit for this Dmans - It's all yours!

    To me, I'm thankful that you kept what you did. Because not too many people these days have keepsakes and took notes from those (wonderful) days back then.

    It's like coming across an old pile of Electronic Magazines and seeing - reviewing them for the 'umpteenth time but still, all those articles, ideas, hopes and dreams - and we look around today - WOOF!

    I am finding that a lot of my work I took for granted is now being upgraded to SMD devices, so I have to figure out ways to keep the ideas "fresh" and workable.

    I had the "equipment" up and running from another project so I did this to help me keep me and my own tools - like my head, working right.

    I enjoyed it - and more than happy to help!

    :+> Andy <+:
     
  8. 357magnum

    357magnum Well-Known Member

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    Regardless of where the thanks goes .... Thank You "Andy" for what you bring to this forum . It's a pleasure having you aboard ! IMO (y):):cool:
     
  9. jon666

    jon666 Well-Known Member

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    i have one here someplace mine is much smaller never hooked it up. so dont know how well it works
     
  10. Martian

    Martian New Member

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    These amplifier do "work" as they are designed.

    The biggest problem with them is that they amplify everything.
    More often than not they do more harm than good.

    One way to improve this design is to incorporate a filter.
    Another is to use a device more suited for the application like an MMIC from minicircuits.

    There is someone making them but I cannot remember who.
     
  11. jon666

    jon666 Well-Known Member

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    i suppose this is why i never hooked it up. a friend gave it to me years ago when i was on CB. now im a ham i have a ton of stuff ill never use on ham radio. i may sell it all. it will be a long list. im a compulsive buyer i got 3 plastic tubs of CB stuff.
     
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  12. Dmans

    Dmans Active Member

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    Martian,
    I agree that they do amplify everything. All the more reason to make it switchable, or use it along with your radios RF gain knob. In my limited experience, an RF gain knob and a switchable Tone circuit along with this receive pre-amplifier will pull in those stations you are "just barely able to hear" to a level you can have a conversation.

    73's
    David
     
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  13. 357magnum

    357magnum Well-Known Member

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    I guess it will be the same as the one on my Gray 200 ? Sorry after all these years some things still confuse me .lol If it works the same it's not for me just makes loud crap louder . Do the internal ones have a filter ? I just picked up a Km Italy pre- amp @ a yard sale for 50 cents . It lights up but I haven't put it in line yet to try it . But more noise with my A-99 is not what I'm looking for Good Gravy I get enough !:ROFLMAO:
     
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  14. Dmans

    Dmans Active Member

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    I believe this unit was designed for internal (to the radio) installation. It could be very well the same unit as installed in transistor amps??
    It has no filter on it. Wondering who made it as I have had it since the early to middle 90’s I think. Lost instruction sheet long ago.

    73’s
    David
     
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  15. sp5it

    sp5it Master of puppets

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    Using a preamp on 27MHz is stupid idea. Mike
     
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