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  Interfacing the
Link Receiver

By Robert W. Meister WA1MIK
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This receiver was designed for installation into a Motorola PURC5000 station. These were primarily high-powered paging transmitters that used a stripped-down synthesized MSF5000 RF tray as the exciter, followed by one or two MSF5000 power amplifiers. The link receivers allowed several PURC5000 stations to be controlled simultaneously from a central location using signals in the 72-76 MHz, 400-512 MHz, or 928-960 MHz band.

This article describes the various interface signals available inside and outside the chassis. While it is based on the 72-76 MHz receiver I have, the information should apply to the other bands as well.

Manuals and Board Descriptions:

The PURC5000 Link Receiver Options manual, p/n 6881064E10, is No Longer Available. This manual covers receivers for the 72-76 MHz, 400-512 MHz, and 900 MHz bands. When equipped with a 72-76 MHz receiver, this becomes Option C661 (DPL) or Option C850 (carrier squelch). This article deals with Option C661.

The 72-76 MHz receiver board is a TLC6112A or B. It uses a K1003A channel element. The crystal frequency is the receiver carrier frequency plus 5.26 MHz, all divided by six, so a typical crystal would operate around 13 MHz. The channel element output is rich in harmonics, and the 3rd multiple (around 33 MHz) is selected and doubled for use by the mixer. The chassis accepts and uses a standard MICOR receiver board; in fact I've replaced the mid-band board with a UHF board and will be using it on that band.

The flat audio and squelch board is a TRN5363A. This is essentially a MICOR base/repeater audio and squelch board without the power transistors (which are mounted on a separate heat sink). It would appear you could supply your own power transistors and have a 10w speaker output after adding a volume control. A line level output adjustment pot resides on this board. Audio is not passed through the PL/DPL board. As the name implies, the audio frequency response is flat, meaning there is NO de-emphasis of the audio signal. In the MICOR radios, de-emphasis occurred in the 10w audio amplifier circuitry (missing on the flat board) so only the speaker output was de-emphasized.

The DPL board is a TRN5768A. While it may be capable of both encode and decode operation, it is only used for decoding in the link receiver chassis. There is no PL filter on the DPL board, and since audio doesn't pass through it, a filter is not necessary. A standard MICOR base/mobile PL decoder board can plug in here, and will function the same as the DPL board once you remove JU201 on the flat audio board (to route audio through the PL filter).

The front panel or interconnect board is a TRN9273A. It contains the 9.6V regulator, squelch pot, metering jack, and audio muting circuitry. It also interfaces the receiver and audio boards to the outside world.

All of the above boards are fully documented in the manual. Full alignment instructions are provided for all receivers.

Outside The Box:

The chassis is normally mounted on slides in an MSF or PURC cabinet. Plastic latches on the front panel retain the chassis in the station cabinet. A squelch adjustment potentiometer and an MSF5000-compatible metering jack for the receiver are the only visible items on the front panel.

On the back, a BNC female jack accepts the antenna input. A two-row, 10-position AMP MODU-style connector (actually an EMI filter) handles all other input, output, and power signals. A flat cable would connect the link receiver to the PURC5000 station control board.

I mounted a 10-position barrier strip on the back of the chassis and made a short cable that plugs into the stock connector and is wired to the barrier strip. This is just one noninvasive manner to utilize the available filtered I/O. The photo below shows this modification:

I unplugged the internal blue cable (P905 in the wiring diagram below) from the EMI filter and took a photo looking into the business end:

When viewed in this manner, unplugged from the I/O connector and looking into the jack, pin #1 is in the upper left corner. It can only go in one way due to the keyway at the top. When you look into the connector on the rear of the chassis, pin #1 is in the upper left corner as well. A mating connector that would plug into the I/O connector outside the chassis, looking into the back of the chassis, would have pin #1 in the upper right corner. The pin orientation on the schematic, while correct, is unclear because it doesn't indicate the location of pin #1, nor does it take into account the re-sequencing of the pin numbers when you plug something into the back of the chassis. Looking at the back of the chassis with the latches at the left and right and the keyway above pin #5, the pins are oriented in this manner. USE THIS PIN LAYOUT FOR INTERFACING.

135 79
246 810

A mating connector that plugs into the back of the chassis would have its pins numbered differently, flipped left-to-right (mirror image). One connector that fits perfectly is Jameco part number 1578082. If you buy a two-row, 10-position, keyed Insulation Displacement Connector (IDC), this is the order you would have at the end of a cable (keyway above pin #5) and is how the blue cable connector is oriented:

975 31
1086 42

If you make a cable with a standard connector, you must take into account the reversal inherent in the EMI filter on the Motorola chassis. (See the J902/J2522 connections in the wiring diagram below.) The proper wiring, if you want the signals to come out in order of 1 through 10, using such an IDC connector, is 9, 10, 7, 8, 5, 6, 3, 4, 1, 2. Naturally the first cable I made was incorrect, but it was easy to crimp the connector onto a second cable and do it right. Although this may be confusing now, when you make your connector, take a good look at it and make sure the wire going to the upper left pin of the EMI filter, when looking at the back of the chassis, is what you end up wiring to pin 1 of your terminal strip.

The pin configuration for the back of the chassis is described in the table below:

PinSignal NameDC Voltage [1] Notes
1PL Indicator0.0 / 5.3 0V not present, 5.3V present
4Un-squelched Indicator2.5 / 0.0 2.5V un-squelched, 0V squelched
5A+14.00.18 Amps at 14.0 volts
7PL Disable8.5Float: CSQ, Ground: PL/DPL
8Discriminator Output0.0 0.14 volts per kHz deviation [3]
10Muted Audio Output4.8 0-0.5 volts per kHz deviation [4]

[1]: DC voltage measured with no signal applied. See Notes in right column.
[2]: While many receivers have room for four channel elements, this chassis does not bring out the F4 select line.
[3]: 0 to 6 kHz deviation, response within +0, -3dB from 5 to 5500 Hz modulating tone.
[4]: response within +0, -3dB from 50 to 6000 Hz modulating tone. See text below.

The top cover receiver coil locations are well marked, so alignment can be done without opening the chassis or removing any covers. Just plug an MSF5000-style metering panel into the jack on the front and select the appropriate circuit (3=multiplier, 4=discriminator, 5=limiter). There are adjustment holes for F1 and F2 only; you can't get to F3 or F4 without removing the top cover. The only other adjustment you can't get to is the audio output level potentiometer that controls the output signal on pin 10. Note that this cover matches the mid-band receiver installed inside; if you replace the receiver board with one from a different band (as I did after writing this article), the holes will no longer line up.

Inside The Box:

Remove the top cover (four T15 screws) to gain access to the receiver and flat audio board. Each board has its own snap-in shield; the receiver coil locations are marked on the receiver shield.

With the shields removed, you have access to the solder side of the receiver and flat audio boards:

Both boards can be removed by fully loosening the Phillips head screws (four for the audio board, eight for the receiver board) that hold each board into the chassis. The flat audio board plugs into the front panel board and the DPL board beneath it. The receiver board plugs only into the front panel board and has two additional cables (antenna and metering) plugged into it. Here's the component side of these two boards:

And here's the view of the chassis with the DPL board, front panel board, and the two cables:

The DPL decoder board is mounted vertically under the flat audio board, but it can't be extracted from the top; however you CAN access the DPL code plug from the top. You need to remove the black front panel (four T15 screws) and the bottom chassis (six T15 screws); then the two Phillips head mounting screws can be loosened from the bottom and the board can be extracted. It can be unplugged from the flat audio board if that is still in the chassis. Here's the view from underneath:

Here's a photo of the DPL board. The code plug has tape on it with the code "351". I don't think this is an official Motorola code plug. There's an article in the Motorola section of this web site that explains how to make your own DPL code plug. You can replace the DPL board with a PL decoder board from a mobile or base station, but due to the orientation, you will need to remove the bottom chassis to access the plug-in PL reed. The PL boards have a high-pass filter in them that removes the sub-audible tones from the audio path; you'll need to remove JU201 from the audio/squelch board to allow receive audio to pass through this filter.

After removing the bottom chassis, you can access the front panel board. The blue flat cable is permanently attached to the circuit board and can only be disconnected at the rear chassis connector. The metering extension cable can be unplugged from the front panel. Disconnect the blue cable and remove three T10 screws to extract the front panel board. On the unit I purchased, the mounting nut for the 9.6V regulator was missing; I subsequently added one (#4-40). Also, the volume control was very loose and would fail intermittently. This was due to excessive pressure applied to the knob which pushed the back of the pot inward. I had to completely remove it and tighten the back cover crimp lugs around the phenolic. While I had the pot off the board, I squirted some contact cleaner into it.

The low-band (25-50 MHz), mid-band (72 MHz), and UHF receivers use a standard RF-type RCA plug for its antenna input. (800 and 900 MHz receivers have an SMA female jack on them and would need a different antenna cable.) The manual has part numbers for the various antenna cables, but they're probably No Longer Available and too expensive to buy from Motorola, and you can make your own rather easily. The seven-pin metering jack accepts an extension cable that brings the metering signals out to the front panel's RJ45 jack. The chassis should accept any MICOR Sensitron receiver board you have. I subsequently replaced the mid-band receiver with a UHF receiver and it worked just fine. The antenna coax could have been a bit longer, however, and I ended up replacing it with a slightly longer assembly that came from a UHF base station.

Receiver Performance:

I connected +14V to pin 5 and ground to pin 9. I did not have to manually select F1; apparently a jumper on the front panel board did this for me. It would need to be removed if I ever wanted to use the F1/F2/F3 inputs on the rear panel to select more than one channel. The entire chassis drew about 180 milliAmps of current.

As I got it, the chassis was set up to receive a signal on 72.2800 MHz with a DPL code of 351. The front panel squelch pot closed the squelch around the ten o'clock position. At this threshold setting, the receiver opened squelch (i.e. the Unsquelched Indicator pin went to ground and audio came out of the Muted Audio Output pin) with a signal level of 0.15 uV. Full squelch kept the receiver quieted until it had an input signal of about 0.7 uV.

Raw Discriminator Output (pin 8) was essentially flat from 5 Hz to over 5000 Hz. The amplitude of the recovered audio was linear up to 6000 Hz of deviation; after that the crystal filters did a good job of limiting the acceptance bandwidth of the receiver.

The Muted Audio Output (pin 10) had some low-frequency roll-off that started becoming noticeable at 50 Hz. It was flat to over 5000 Hz. This is quite acceptable for voice reception. My unit was set for approximately 0.3 volts per kHz deviation, but it is adjustable using a pot on the flat audio board up to about 0.5 volts per kHz. Above that, it starts distorting with a 5 kHz deviated signal.

Since the audio outputs are not de-emphasized, it was not possible to make a 20dB quieting sensitivity measurement, but all the crackles in the signal were gone by approximately 0.3uV. In fact the manual says you can only do 20dB quieting measurements using a de-emphasized audio output on the PURC station.

The audio board schematic shows three jumpers, two of which are installed. The link receiver manual only mentions whether they're installed or not, without giving their purpose. The table below documents their function:

JumperIn/OutFunction and Notes
JU201IN PL/DPL filter bypass; remove if decoder has filter
JU202OUT Low-pass audio filtering; almost de-emphasis
JU203OUT In for 10w audio level
JU204IN PL/DPL decoder present; remove for CSQ only

JU201 is installed to pass audio around the decoder board. The DPL decoder board present in the link receiver does not provide audio filtering, unlike the PL decoder board, which has a high-pass filter on it. If you replace the DPL decoder with a PL decoder, you would remove this jumper to utilize the PL filter. The graph below shows the Muted Audio output with this jumper installed (default) and with the jumper out and a working PL decoder board installed.

JU202 adds some low-pass filtering to the muted audio coming out of pin 10. It's almost as effective as a de-emphasis circuit but not quite. The audio response doesn't follow the standard 6dB per octave de-emphasis curve. Received audio sounds better, but the 20dB quieting sensitivity is still poor, as would be expected without a proper de-emphasis circuit present. The graph below shows the Muted Audio output level with this jumper missing (default) and installed:

JU203 is installed on boards that are driving loudspeakers, usually mobile radios.

JU204 is installed if the receiver has a PL or DPL decoder board installed, to keep the radio squelched until the correct PL or DPL code is present on the incoming signal.

I plugged an MSF5000 metering panel into the front metering jack. Here are the readings I got with no input signal after performing a full alignment. Your mileage may vary.

M3 (Multiplier): +24uA; M4 (Discriminator): +0.5uA; M5 (Limiter): +4uA

There was already a very healthy M5 metering value with an input signal level of 0.3uV and M5 saturated with only a few microvolts of input signal.

Note that the low-band, mid-band, and high-band VHF receivers generate positive metering signals. The UHF, 800 MHz, and 900 MHz receivers generate negative metering signals. There's nothing like a bit of consistency to make a product outstanding.

Here is the M4 (discriminator) value as a function of frequency shift. The meter was symmetrical on both sides of zero.



Here's the block diagram of the 72 MHz link receiver chassis. Click on it for a bigger image.

Here's the physical wiring/interconnection diagram of the link receiver chassis. Click on it for a bigger image.

If you need or want more information, you should purchase the Link Receiver manual from Motorola.

Acknowledgements and Credits:

PURC, PURC5000, MSF5000, MICOR, Sensitron, PL, DPL, and a whole bunch of other terms are trademarked and/or copyrighted by Motorola, Inc.

Schematics and wiring diagrams came from the PURC5000 Link Receiver Options manual, p/n 6881064E10, used with permission from Motorola.

Additional receiver information came from the SpectraTAC Receiver Instruction Manual, p/n 6881039E45.

Contact Information:

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

Article text, photographs, and hand-coded HTML © Copyright 2008 By Robert W. Meister WA1MIK.

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This article first posted 30-Oct-2008

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.