R.F. Preamp Gain

- J
- Jamie
  
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posted
11 years ago

Tue, Sep 4, 2012 12:24 AM

I am doing a restore for a friend on an old chicken band transceiver which is actually in meant shape.. I have a service manual however, the receiver seems to be a little weak and the manual does not give you enough info to determine if the RF preamp is at full gain.

I use both my service monitor, HP signal generator and HP RF millivolt meter for several references to make sure I had an agreeable reading.

From what I can gather, I am getting 20 db gain on the R.F. preamp, does this sound feasible? If not, I will need to locate a 1 ghz transistor to replace it. The transistor seems to check fine DC wise and all check points for DC test match the service manual, close enough.

20 db gain sound correct?

Jamie

- D
- Dave Platt
  
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posted
11 years ago

Tue, Sep 4, 2012 1:24 AM

Without knowing anything about the actual circuitry... I'd say it sounds as if it's in the right ballpark... about what I'd expect from a single-stage preamp. CB radios aren't usually well designed for DXing and weak-signal use, and a super-hot front end might actually be a disadvantage in urban and highway use due to strong-signal overload from nearby transmitters on adjacent channels.

What's the circuit like... got a schematic for that section?

--
Dave Platt                                    AE6EO
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- J
- Jamie
  
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posted
11 years ago

Tue, Sep 4, 2012 2:05 AM

it's just a basic RF transformer input with a RF transistor output on the collector.. It uses a bipolar transistor. It's very simple.

The RF gain control comes through the secondary side of the RF input transformer where there is 2 back to back clamping diodes to prevent over drive accidents.

The supply side of the primary output RF transfomer does use a tantalum cap for the bypass, as such in may other locations in that unit. I don't get any stray RF there so I can only amagine it is ok.

20 db gain

+--------------------. ,- 2SC1730L | | )|( RF in + --- ++|( RF out |/ --- +' '- -. ,+-----+--+--------+-| | | )|( | | |> +----+ )|( | | +------+----+ +----+ -' '+ | + | | | | + | V - 1S2473 .-. + | | --- 22n | - ^ | | 47n | | | --- +-----+--+ 330| | --- | + | | | '-' --- - | === .-. | | | ^ .-. GND | | --- 1u | | | | | 10k | | --- === === | | | 100 '-' | GND GND .-. '-' | GND | | | | | | 9.34v _V_ '-'330 +----+-|___|+------------+ | | | | .-. 100k RF level TX switch to inhibit +->| | | |100k '-'RF min | GND

Jamie

- R
- Robert Macy
  
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posted
11 years ago

Tue, Sep 4, 2012 1:58 PM

d

ut

----------------. ,-

=A0 =A0 =A0 =A0| =A0 =A0 )|(

=A0 =A0 =A0 =A0 --- =A0 ++|( =A0 =A0RF out

=A0 =A0 =A0 =A0 =A0 =A0--- =A0 +' '-

| =A0 =A0|

=A0 =A0 =A0 =A0 =A0 +----+

---+ =A0 =A0 =A0 +----+

| =A0 =A0| =A0 =A0 =A0 | =A0 =A0+

=A0| =A0 =A0 =A0 | =A0 --- 22n

=A0 =A0| =A0 =A0 =A0 | =A0 ---

=A0 =A0 =A0 + =A0 =A0|

=A0--- =A0 - =A0 =A0 =A0 | =A0 =3D=3D=3D

=A0| =A0 =A0^ =A0 =A0 =A0.-. =A0GND

| =A0 =A0| =A0 =A0 =A0| |

=A0=3D=3D=3D =A0 | =A0 =A0 =A0| | 100

ND =A0.-. =A0 =A0 '-'

=A0 =A0 =A0| | =A0 =A0 =A0|

=A0 =A0 =A0 =A0| | =A0 =A0 =A0 =A09.34v

=A0 =A0 =A0 =A0 '-'330

=A0TX switch to inhibit

run it through LTspice and see.