Ramsey Electronics HR40 Manual de Instruções descarregar pdf (Página 18)

FHT-1 • 18

the side with the markings faces towards the jumpers that you just

installed(JMP5 and JMP6). See the parts placement diagram for correct

orientation.

 100. Install R15, 2.2K (red-red-red). Notice that this part “stands up”. See

the parts layout diagram for correct placement.

 101. Install R16, 470 ohm (yellow-violet-brown). This part also “stands

up”.

 102. Identify and install the last 2N3904 transistor, Q7. Be sure to install

with the flat side oriented correctly. See the parts diagram for correct

placement.

 103. Install R17, 10K (brown-black-orange).

 104. Install R18 and R19,the two large 47 ohm resistors (yellow-violet-

black).These resistors function by limiting the current to the RF “final”

amplifier. This in turn lowers the power output of the transmitter.

Lets “pick apart” the switching network consisting of Q7, Q8 and associated

components. Have a look at the schematic diagram and follow along. When

voltage is applied to the base of Q7 (from the microcontroller U2) transistor

Q7 is turned “on”. This causes current to flow through Q7’s collector and

creates a voltage drop across resistors R15 and R16. This “voltage divider”

is tied to the base of Q8, the power PNP transistor, forward biasing Q8 and

supplying 12VDC to the final transistor, Q6. This is the “high power” mode.

Conversely, when there is no voltage applied to Q7, both Q7 and Q8 are

turned “off”, and the current must flow through resistors R18 and R19. The

voltage drop across these resistors causes less potential to be applied at the

collector of Q6, lowering the output power.

Note to more experienced kit-builders

The “low power” setting can be adjusted by changing the values of R18 and

R19. Please keep both values the same (for equal power dissipation), The

higher the total value - remember that these resistors are in parallel - the

lower the RF output power will become. Please use at least one watt

resistors for this modification.

Component side of

board

Solder side of

board

Solder can to PC

Transistor

can

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Ramsey Electronics HR40 Manual de Instruções Página 18