sceiv

help reduce the " thump" problem. From
here the audio signal is fed to a high gain
preamplifier with Q5, Q6 and Q7 before driving
the LM 386 audio power amplifier. These
stages are not muted during transmit and the
small level sample from the keying sidetone
is fed to Q7. The simple sidetone circuit offers
a very comfortable tone and is simple to
construct. Orig inally, the bag of transistors I
used for Q3 and Q4 were such that I had to
play with the value of the Q3 base resistors
before it would oscillate.
The VFO board is one where you should
considertaking every precaution necessary to
eliminate problems. At 14 MHz, VFO drift is
more apparent than at 3.5 or 7 MHz. Be sure
that you obtain good temperature grade ca-
pacirors on the frequency determining LC
sections . Glass or silver mica capacitors
would be a good choice for the 25 pF values
and LA should be coated with Q·Dope and
mounted rigidly. le 3 is an 8-volt regulator
and is placed as physically close to the MPF­
102 as possible. Q8 and Q9 are 2N2222 transistors
used to raise and buffer the VFO output.
While you may not have the 10 pF tuning
capacitor that I used, anything that you have
that will provide the proper range will work.
VFO shielding is a must and will get rid of
many problems later on.
The switch PCB uses three transistors
to handle
the keyi
ng and
L1 , L3
L2
L5
L6,L7
L4,lB-l11
T/R line, antenna signal switching to the receiver.
and a VFO amplifier which is used to
feed the detector as well as to drive the RF
amp stages. Adiode is used in the collector of
Q16 to block or pass the VFO output to the
RF amp. The original transceiver used a
IN9 14, but you may choose to improve this
to a better switching type.
When the RF is sent to the PA stages, IC4,
Q11and Q12 raise the signal level enough to
drive the MRF-433 final. Thesearc all broadband
stages and information is provided later
on transformer construction. A low-pass filter
will reduce spurious signals on the output.
Table 1. Coil Information
10 turns of #24 wire on Am idon T-37-6 (yellow) torods.
Each coiltapped at center (5T).
38 turns of wire on a single Amidon T·42-6 toroid .
13 turns on Amidon T-42-6 toroid core. Use c-nooe.
12 Turns #20 wire on Amidon T-50-6 cores.
10 turns of #24 wire on T-37-6 toroid.
OUTPUT
COP
+12\1 78L11I8 HPF11112
Figure 6. a. PC board fo il pattern for the VFO board. b.
Pan s placement.
..12 .... I NPUT
j::INTENNj::I
."
r;illlj
Figure 7. a. PC boardfoil pattern fo r the switching board.
b. Pans placement.
30 73 Amateur Radio Today • June, 1992
Figure 8. a. PC board foil pattern for rhe power amplifier board. b. Pans
placement.