Vectronics VEC-1402DK - Manual (Page 21)

VEC-1402DK Instruction Manual High Performance 2 Meter Pre-Amp Kit
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IN CASE OF DIFFICULTY:
Preamp Stops Amplifying:
At worst, a working preamp that fails may indicate a failure of transistor Q1.
First make sure the supply voltage and RF cable connections are okay. If the
preamp failed after being installed in a transceiver, the problem may be
excessive transmitter RF “leaking” back into the preamplifier and damaging Q1.
Cannot Align or Test Preamp:
A newly constructed preamp that fails to work requires a careful recheck of all
work. Low gain, or the inability to properly tune the preamp, may indicate
damaged capacitors or parts in the wrong positions on the PC board. A
intermittently operating preamp may have poor solder connections, a problem
with coax connections, or an intermittent power source. Instability, or self-
oscillation, may be caused by RF cable problems, or by excessive component
lead lengths on the PC board.
Voltmeter Checks:
With 13.8 volts DC applied to the preamp, the collector voltage on Q1 should
read approximately 9 volts, with the base voltage at 0.76 volts. These voltages
are easily measured at the opposite ends of 100-k bias-resistor R1. They may
vary slightly depending on the individual characteristics of Q1, but large
variations indicate that Q1 or other components are damaged or installed
incorrectly.
When all else fails, please refer to the warranty instructions regarding factory
technical assistance or repair discussed earlier in this manual.
THEORY OF OPERATION AND SPECIFICATIONS
Operation:
Coils L1, L2 and L3, L4 form two High-Q tuned LC bandpass filters. The skirt
selectivity of these dual filters combines to offer an excellent degree of out-of-
band signal rejection. This preamp is well-suited for use in strong RF
environments, or for repeater system receiver operation. Q1 is a high-gain low-
noise "microwave"-type transistor with good strong-signal handling
characteristics operated as a common-emitter amplifier. It is biased for
minimum noise figure, and normally exhibits a stage-gain of about 15 dB.
Specifications:
Tuning range: ..................................136 - 154 MHz