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Installation
10
025-9368H
no signal), change the COR polarity with JP31. If the LED doesn't change, adjust COR Threshold
Pot R121 until a change takes place.
Summary:
JP21 = COR source, B = external COR
JP31 = COR polarity, A = negative COR, B = positive COR
R121 = COR threshold adjustment
I
NTERNAL
COR: The Model 748 has an internal Dual-Time Constant noise detector circuit for COR
indication. This circuit can be used if a good COR signal cannot be found in the radio. This circuit
requires high frequencies above 6 kHz to work, which means discriminator audio. To use the
internal COR, JP21 should be in the A position, JP31 should be in the A position, and R121 should
be set midpoint. R120 is the squelch control for the internal COR circuit and must be adjusted for
proper COR indication. With no signal applied to the receiver, adjust R120 counter-clockwise until
the CARRIER LED comes on. Then adjust clockwise until the LED just goes off. This adjustment is
just like the squelch control on a mobile radio. Adjusting the RX Audio Pot R59 affects this
adjustment, so R120 will need to be readjusted after doing the complete installation. See Tests and
Adjustments number 6.
Summary:
JP21 = COR source, A = Internal COR
JP31 = COR polarity, A = negative for internal COR
R121 = COR threshold adjustment, set midpoint
R120 = Squelch control, adjust for critical squelch
A
A
B
B
A
A
B
B
JP31 Set to A
JP31 Set to B
Figure 3. Jumper JP31 Settings
8.
POWER SUPPLY: Locate the 12V DC supply for the repeater/transmitter. With a VOM
measure the DC voltage. It must be between 10.5 and 16 volts, and capable of at least 700
milliamperes of current. When using a DC supply the positive (+) connection must be to J6
terminal 1 and the negative (-) to J6 terminal 3. If proper DC voltage is not available, an AC
source of 9 to 12 VAC, at 11 watts, can be used. When using an AC source connections should
be made to J6 terminals 1 and 2. The Model 748 is internally fused with a 2-ampere fast-blow
fuse. Verify that the Power LED lights when power is applied.
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