Service manual

Page 25
SVR-250 Service Manual
Control Board
Power Supply: DC power comes from the mobile radio via P1 pins 1 and 5. Fuse F1 and MOV VAR1 provide
over current and voltage spike protection. Q3 is the remote enable/disable pass switch, controlled by Q1 and Q2
via P1 pin 3. Q3 output is switched 12VDC and is presented to audio amp U6, and voltage regulators U7 and U8.
Bias voltage for the op-amp circuits is provided by U10 pin 10 and buffer amp U9D.
Transmit audio path: Receiver audio from the mobile is input on pin 6 of P1 and routed to op amp U16B; RV5
sets the sensitivity. The output of U16B goes to the mic amp portion of U10; PC programming of the SVR-250
provides flat response or +6db/octave pre-emphasis. U10 provides all of the audio processing, limiting and
filtering of the transmit audio. Transmit voice audio is output on pin 19 of U10, sub-audible signals on pin 20.
U17B and U17C select the voice and subaudio paths from U10 (simplex) or U11 (duplex operation). The selected
audio is summed by U9A & U9B and passes through the final lowpass filter U9C to remove any clock noise
generated by U10 before being presented to the RF module on P4 pin 6.
Receive audio path: Receiver audio from the receiver module is input on P5 pin 13 and routed to U10 pin 11. U10
provides all of the receive audio processing and filtering, in-band and subaudible tone decoding. The receive audio
is output on U10 pin 22, where it is sent to the local receiver audio amp and mobile transmit audio output amp U1A.
J1 selects either high sensitivity (open) or low sensitivity (shorted) and J2 selects the output impedance (600/2.2K
Ohms). J11 inserts (open) or removes (shorted) a 100KOhm series resistor for radios with low level mic audio
and alternator whine problems.
RF Tx and Rx paths: For simplex operation, U15 switches the single TNC antenna connector P10 between the
Tx and Rx module. In duplex operation, the coax between MCX connectors P8 and P9 is removed and the receive
module is connected directly to TNC connector P11 via J14. If the Auxiliary receiver is installed, it shares the
Rx module antenna connection via the RF attenuator comprised of R201-203. D4 forms the Tx RF detector and
U12 is the Automatic Power Control for the transmitter. RV10 sets the reference voltage for the RF power out
and control TX9V pass transistor Q4.
LED Display: The front panel display contains 2 seven segment displays and 8 LED status indicators. The 3
displays are driven by 3 shift registers in series under control of the main μP and updated every 10mS via P3.
Logic and control: U3 is an Atmel 89C51RB2 microprocessor with flash E²PROM memory. The microprocessor
provides all of the logic and control functions for the repeater including mobile/repeater PTT output, local mobile
PTT sense, mobile transmitter activity sense, audio switching, in-band & CTCSS detect, channel number and
repeater status indications via front panel led arrays.