User's Manual

Removing and Reinstalling the HT750 Transceiver
1. Remove the nuts and washers holding on the Flex retainer bracket (X).
2. Remove the Flex retainer bracket (X).
3. Unplug both the HT750 flex cable (X) from the 20 and 40 pin latch connectors and
the VRS coax cable (X), from the pc board (X).
4. Loosen the 4 screws (X) that hold the HT750 retaining rails (X) to the pc board.
5. Slide the HT750 retaining rails apart so that the HT750 transceiver can be lifted
away from the pc board.
6. To reinstall the HT750 transceiver, reverse the preceding steps.
VRS Transceiver (HT750)
VRS units use a Motorola HT750 synthesized radio. The HT750 transceiver contains
VRS software and can no longer be used as a stand-alone portable. The transceiver does
retain its HT750 model number and HT750 serial number internally. No hardware
modifications have been made to this radio for use in the VRS. Consult the Professional
Radio Basic and Detailed Service Manuals for more information on circuit board layout
and tuning procedures.
HT750 Transceiver Alignment for VRS
The HT750 alignment for the VRS is identical to that shown in the HT750 Instruction
Manual, with the exception of the power output and squelch setting. For all VRS models,
the HT750 RF output power is adjusted so that the VRS transmits 275 mW for low power
and 2 W for high power. The squelch settings for all VRS models are adjusted to closely
match that of older crystallized VRS models.
The software inside the transceiver is unique to the VRS application. The I/O between
the HT750 microprocessor and the VRS 750 controller card is unique. Field repair of the
transceiver is not recommended. Contact your local service representative for a
replacement transceiver.
VRS Tuning and Alignment
The VRS is tuned at the factory and should not need to be realigned. Refer to the proper
manuals for programming and alignment for the associated radio and control unit before
attempting to troubleshoot the VRS unit. If proper tuning of the associated radio fails to
correct a problem, the following procedure may be used to align the VRS.
Setup
ASTRO Spectra Station Cable Setup
Ensure that the ASTRO Spectra B+ cable is disconnected from the power supply and the
control head is off (WARNING: Never turn B+ power off with the control head still on).
Connect the VRS DB-25 cable (HKN6153A) into the accessory connector of the mobile.
Connect the other end of the VRS cable to the VRS. Set the test station power supply to
13.8 Vdc and current limiter to 12 amps. Connect the mobile's B+ cable to the power
supply. See Figure 14.
Figure 14 General Cabling Diagram