Up one level (MSF index)
Up two levels (Moto index)
Back to Home
  Motorola MSF5000
Analog Repeater Interfacing

By Paul Blum K9ARF
  Print this Page


I have done some in-depth reading and experimentation with the Motorola MSF5000 repeaters recently, specifically the "Analog" stations that are EPROM-programmed (these have "CLB" in the model number).

There is difficulty in attaching an external repeater controller or Internet interface to these radios because there is no direct access to either Line Push-To-Talk (PTT) or Repeater PTT unless you happen to be lucky enough to have the optional circuit card (DC or Tone Remote Control) for this function. The problem lies in the fact that these PTT functions exist as individual bits on the internal data bus (known as the MUXBux) that the microprocessor uses to control or monitor the station. I've developed an "External PTT Adapter" to make amateur repeater controller connections easier.

The main reason I went after the line or repeater PTT was to provide a "clean" connection to an external controller. My way allows you to make all of the necessary connections at one place, the expansion connector. You do need to add the COR function by utilizing one of the spare pins though. Some other benefits: using line PTT lets you route audio through the remote card (Tone or DC) if desired, and the transmitted audio will appear at the local station speaker so you can easily monitor what your controller is doing. (This brings up one of my pet peeves about legacy Motorola equipment: they never provided a speaker in their base stations!) Another reason I did this is just to prove I could (I enjoy a good puzzle now and then.

In this article you will find some notes on external connections along with a schematic of an external PTT interface I developed. My interface allows access to the proper bit so that the transmitter can be properly controlled. This same interface may also work on a "Digital-Capable" station and possibly a PURC5000 transmitter as well, but I haven't tested these. The schematic is shown below. Click on it for a larger view.


The MUXBus continuously cycles through all 16 addresses in a binary fashion. This circuit monitors the MUXBus address lines (BA0 through BA3). When address 01 is detected (BA0 high, BA1, BA2, BA3 all low) and the EXT PTT signal is low, a low signal is sent on the data lines to either RPTR PTT (data bit 3) or LINE PTT (data bit 2). The microprocessor then keys the transmitter and allows the proper audio signal to pass until that PTT signal isn't present on a future MUXBus cycle.

Interface Signals:

The following signal information was extracted from the MSF5000 Analog Station Control manual 6881063E61 pages 34, 35, and 36.

SignalWhere To Get It
Local Speaker AudioPin 3 of transformer T801 (near test microphone jack).
Carrier plus PL Detect SignalPin 9 of U829, or Pin 6 of U807. Goes high (+5V) when receiving a signal with the correct PL/DPL.
To disable internal repeat audioCut pin 2 of U831, or remove R895 on bottom of board.
Repeat Audio FeedFeed through a 27K resistor to pin 13 of U834 (the front-most pad where R895 as removed). Isolate with a 1uF capacitor - DC voltage is present from controller.
Line TX Audio FeedIf using the K9ARF external PTT modification, you can input transmit audio at the Expansion Connector (J800) pin 36. There is DC voltage present here, so isolate with a blocking capacitor.
Discriminator AudioTP3 gives raw receiver audio, not squelched, not filtered, not de-emphasized. Block DC voltage with a capacitor.
Processed Audio (not squelched)TP5 gives de-emphasized, PL filtered (high-pass), "always on" audio which is adjusted by the front-panel "RX Level" pot. Block DC voltage with a capacitor. This also appears at the Expansion Connector (J800) pin 38.
Processed Audio (squelched)TP2 gives de-emphasized, PL filtered (high-pass), squelch-muted audio which is adjusted by the front-panel "RX Level" pot. Block DC with a capacitor.
Rptr Carrier Squelch indicatorTP6 gives a squelch control voltage, which is adjusted by the front-panel "Rptr Sq" pot. Goes low (0.5V) when squelched, high (2.9V) when unsquelched.
Local PTT (transmit)TP9 is local Push-to-Talk. This is affected by the local time-out timer in station programming! Ground to transmit. This will also generate transmit PL/DPL if programmed.
Local TX Audio FeedTP8 is local microphone audio, active when Local PTT (TP9) is grounded. Block DC with a capacitor.

The front panel "Control" jack has the following signals. Note that this is with the control tray in its normal closed position on top of the RF tray. The pins in the table are numbered according to the analog SCM documentation: 6-5-4-3-2-1 from left to right when looking into the front of ths station. (The pins were renumbered 1...6 on the digital-capable (CXB) stations. Yes, it is confusing.)

3Push To Talk
4Mic Hot
5Audio Ground
6Logic Ground

Other Noteworthy Items:

When burning the EPROM for this radio, be sure to choose the correct time-out timer and PTT priority for your hookup: whether local PTT, line PTT, or repeater PTT. If using an external controller, disable the internal repeat function. The internal ID timer is set for 15 minutes, which is not legal for amateur use.

Interesting note: the front panel "XMIT" momentary toggle switch will cause the transmitter to key up without coded squelch (PL or DPL) modulation. If this switch is activated when the station is already transmitting, it disables the coded squelch signal.

As you know, this station was "top-of-the-line" in its day, and many are available inexpensively today. These are very fine repeaters; however one design flaw I know of is that of metallic dendrite growth of the tin-plated surfaces of the VCO castings or the injection filter. Motorola later came out with improved plating, easily identified by the dull-matte finish. The affected modules are very shiny, and with age grow tiny hair-like dendrites, which eventually short out the involved circuits. If you are unlucky enough to have the shiny castings, the fix is easy enough - just disassemble and wipe out the assembly every couple of years or so. I have heard some folks have tried to lacquer or paint the assembly to prevent dendrite growth, but I have never tried it. [Editor's note: for more information, search the web for "tin whiskers"; NASA has done extensive research on this problem.]

Acknowledgements and Credits:

Most of the information in this article came from the Motorola MSF5000 Station Control Module manual, p/n 6881063E61.

Motorola, MSF5000, PL, DPL, MUXBus, and a whole lot of other terms are trademarks of Motorola, Inc.

Contact Information:

The author can be contacted at: his-callsign [ at ] sbcglobal [ dot ] net.

Schematic diagram and article text copyright © 2010 by Paul Blum K9ARF.

Artistic layout, HTML coding, and conversion to repeater-builder format by Robert Meister WA1MIK.

Up one level (MSF index)
Up two levels (Moto index)
Back to Home

This article first posted 17-Oct-2010

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.