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R.F. Preamp Gain

J

Jamie

Jan 1, 1970
0
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
 
J

Jamie

Jan 1, 1970
0
Dave said:
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?
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

Jan 1, 1970
0
Dave said:
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

run it through LTspice and see.
 
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