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  Notes on Converting a
Motorola VHF PURC Station
for Repeater Use

By Matt Krick K3MK
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The Motorola PURC (Paging Universal Remote Control) is very similar in appearance to a traditional VHF MICOR Station. The key differences are the cards that are present in the chassis. The PURC is always equipped with a Digital Modulator Card. It may also have a Transmitter Site Interface and the optional Voice Actuated Response Module.

The conversion involves modifying the TRN4860A Remote Control Backplane, the TLD5322B PURC Digital Exciter, and the TRN4854B PURC Station Control Module. Each of these topics will be covered individually.

Modifying the TRN4860A Remote Control Backplane:

The TRN4860A backplane is very similar to ones found in normal repeater and base versions of the MICOR such as the TLN5644A. However the silk screen labeling of the jumper locations are somewhat different, the screw terminals have been assigned to various card slots and some connections were not made between the receiver and transmitter connectors.

A receiver RF board will need to be procured; this can come from a MICOR Mobile. The chassis we converted came with an Audio Squelch board, if yours didn't you will need that as well.

Start by removing all cards from the chassis except for the Station Control Module.

Receiver F1 Select: Install a jumper between J2 (Receiver) pin 26 and J2 pin 30.

Transmitter F1 Select: you have two choices.

Option A, Continuous ON: Install a jumper between J5 (Transmitter) pin 19 and J5 pin 20.

Option B, Station Control Module Keying of F1 oscillator ground: The Station Control Module must also be modified to control transmitter oscillator ground. Then install a jumper between J5 (Transmitter) pin 19 and J5 pin 14.

Backplane signal routing requires a few additional wires.

  1. Install a wire (Yellow) from J2 (Receiver) pin 14 to J2 pin 6 and TB-3 terminal 9.
  2. Install a wire (Green) from J2 Pin 10 to TB-3 terminal 11.
  3. Install a wire (Red) from J2 Pin 13 to TB-3 terminal 13.

If you have or want PL Encode, you have two options as well.

Option A, Allow "Code In" from Audio Squelch board to Transmitter: This allows the use of a single tone board such as a Communication Specialists TS-32P or TS-64 in the Receiver compartment to pass the encode tone to the exciter.

  1. Install a wire on the backplane from J5 (Transmitter) pin 29 to J2 (Receiver) pin 16.
  2. Install JU960 on RX Interconnect Board TLN5648A.

Option B, Repurpose "Code In" for TX PL Disable: For separate tone boards a TX PL Disable may be provided to allow the transmitter PL tone to be removed or changed.

  1. Install a wire (White) from J5 (Transmitter) pin 29 to TB-3 terminal 7.
  2. On exciter remove negative side of C408 from board and leave floating.
  3. Connect the PL encoder tone output to C408 negative.
  4. Connect the TX PL tone disable line to PCB hole cleaned from original C408 negative location.

For best results when using a SS-32P, connect PL tone disable signal to an open switch position. It was found that most encoder take a good deal of time to ramp up the tone output to a stable level, however frequency hoping the PL frequency is instant.

Your external repeater controller connections are made at TB-3 and are detailed below.

TerminalSignal Name or Function
1 Ground
7 TX PL Disable (Option)
9 RX Audio Output
11 RX PL Decode
20 TX Audio Input

Modifying the TLD5322B PURC Digital Exciter:

The PURC uses a True FM exciter that has been modified to accommodate Digital modulation and or Flat Audio. This modification is to install the components necessary to enable the speech audio and PL tone paths. The modification consists of the following steps:

  1. Install at R402 a 330 ohm resistor.
  2. Install at R403 an 18k resistor.
  3. Install at R405 a 33k or 47k resistor.
  4. Remove R401 (560 ohm) if present.
  5. Remove connections from the BNC connector on the rear panel to the exciter.

Additional modifications to the TLN6006 Station Audio Control card are necessary if you'll be adding a PL Decoder.

  1. Cut trace at pad on underside of board feeding pin 7.
  2. PL filter send to pad.
  3. PL filter return to Pin 7.
  4. Install JU201.
  5. Install JU202.

Modifying the TRN4854B PURC Station Control Module:

A technician may want to instantly disable the transmitter to check for desense. This modification rewires the "Line Disable" switch to allow disabling of the PTT and enhances the use of an outboard repeater controller. The modification will also eliminate an outside source from inhibiting the PTT. A pilot lamp (LED) can also be added to indicate when the transmitter is active. Also, you may desire to have the transmitter channel element keyed with the exciter. This prevents local oscillator leakage that often can be heard for a good distance from the repeater site.

The modifications to the Keying Switch and adding the LED are described below.

  1. Completely remove Q8. This disables the ability for an outside source from inhibiting the PTT.
  2. Remove JU-2.
  3. Connect a wire from the Line Disable switch S2 contact E (Blue or Grey wire) to the empty hole of JU-2 furthest from the edge connector that feeds the CR1 cathode.
  4. Drill the front panel and install a Red LED.
  5. Connect the LED's anode to A+ at switch S2 contact D (Red wire), and the cathode through a 1k resistor from the collector of Q9.

PTT from the outboard controller is connected to pin 5 (Local PTT) on the card. This is done on the backplane to allow the card to be pulled if needed. The added LED will indicate when the exciter is keyed (On Air Lamp).

If you want to key the exciter channel element along with the rest of the transmitter, do the following steps.

  1. Q6 can be used to control the keying of the exciter channel element for the benefits stated previously.
  2. Remove or cut out CR5, CR21 if present and insure JU-6 is in place.
  3. Use J5 pin 14 (Ant Sw/Audio Mute) on the backplane to supply a ground to enable the Channel Element by running a wire from there to the transmitter interconnect pin 19 (F1 oscillator ground).

The module can be run in the PL Disable mode full time if necessary. Just pull the incandescent lamp out of the socket.

Re-label the Line switch to indicate "PTT Disable." The PTT switch (top switch) will work as originally designed.

Note from Mike, WA6ILQ:

The MICOR station has no "Power On" indicator. While you are digging into the Station Control card, it might be a good idea to mount a green LED into the front panel and add an appropriate series resistor from A+ to ground.

Contact Information:

The author can be contacted at: dcflux [ at ] gmail [ dot ] com.

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This page was HTML'd on 24-Aug-10 by Robert Meister WA1MIK.

Article text copyright 2012 by Matt Krick K3MK.
HTML conversion by Robert Meister WA1MIK.
Motorola, MICOR, PURC, and a bunch more stuff, are registered trademarks of Motorola Inc.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.