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Motorola GM300
Narrow-Band
Conversion Kits

25 kHz to 12.5 kHz Channel Spacing Conversion Kits for GM300, M10, M120, and M130 Series Mobile Radios
Kit Numbers HLN9575 and HLN9576

Transcribed by Robert W. Meister WA1MIK
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PLEASE NOTE: This instruction sheet is provided as a guide for technically competent service personnel who are familiar with electronic communications circuitry and test equipment. If you are not confident in your ability to understand and complete the instructions, or you lack the proper test equipment: STOP -- Do not continue.

NOTE: These conversion kits can also be used on limited versions of the Radius M100, M200, and MaxTrac mobile radios. The VHF RF boards must be HLD4321C (or later) or HLD4322C (or later). The UHF RF boards must be HLE9310B (or later). Refer to the specific radio service manual to find the circuit details and PC board overlays for each model. It might even be possible to apply similar parts changes to an 800 MHz MaxTrac RF board (HLF9122) for narrow-band use on the 900 MHz amateur band.

ALSO NOTE: The GM300 radio, after conversion, will only be capable of receiving narrow-band signals properly. The transmitter deviation can be set for 2.5 kHz or 5.0 kHz. Regardless, the GM300 radios do not support the ability to select narrow-band or wide-band operation on a channel-by-channel basis. You'd need a CDM-series radio to have that ability.

Description:

The HLN9575 Conversion Kit for the VHF models and the HLN9576 Conversion Kit for the UHF models are used to convert the GM300, M10, M120, and M130 series of Radius mobile radios from 25 kHz to 12.5 kHz channel spacing. These kits WERE available from MyRadioMall for about $65 each in October 2011. The Motorola On-Line retail price is about $85, but when I tried to order one, the system said they were Not Available yet they anticipated shipment in 2-3 days. I checked the availability again in June 2012 and Motorola has changed their status to "obsolete, no replacement available." I even called Parts ID and they have nothing to offer as a substitute, so it seems they don't have any of these kits left. They want you to buy a newer radio (from them, of course) and there probably are plenty to choose from.

Equipment Needed:

Conversion Kit HLN9575 for VHF Models (136-174 MHz):

Table 1 lists the reference (from the schematic diagram and parts list in the service manual), the part number (value) to remove from the 25 kHz board, and the part number (value) to replace it with, to convert the board to 12.5 kHz.

Table 1. HLN9575 for VHF Models
Component
Reference
Part Number (Value) on
25 kHz Board TO BE REMOVED
Part Number (Value) for
12.5 kHz Board TO BE REPLACED WITH
FL519180097D06 (455 kHz 6D) 9180097D04 (455 kHz 6F)
FL529180098D06 (455 kHz 4D) 9180098D04 (455 kHz 4F)
R610611077B19 (68k) 0611077B07 (22k)
R620611077B09 (27k) 0611077A26 (10k)
R1630611077A50 (100) 0611077B11 (33k)
R1650611077B03 (15k) 0611077A98 (10k)
R2230611077A66 (470) 0611077A92 (3.6k)
R2250611077B01 (12k) 0611077A92 (5.6k)
Y51 (a&b)9180022M02 (45.1 MHz, 25 kHz) 9180022M03 (45.1 MHz, 12.5 kHz)

Conversion Kit HLN9576 for UHF Models (403-512 MHz):

Table 2 lists the reference (from the schematic diagram and parts list in the service manual), the part number (value) to remove from the 25 kHz board, and the part number (value) to replace it with, to convert the board to 12.5 kHz.

Table 2. HLN9575 for UHF Models
Component
Reference
Part Number (Value) on
25 kHz Board TO BE REMOVED
Part Number (Value) for
12.5 kHz Board TO BE REPLACED WITH
FL519180097D06 (455 kHz 6D) 9180097D04 (455 kHz 6F)
FL529180098D06 (455 kHz 4D) 9180098D04 (455 kHz 4F)
R610611077B19 (68k) 0611077B07 (22k)
R620611077B09 (27k) 0611077A26 (10k)
R1630611077A50 (100k) 0611077B11 (33k)
R1650611077B03 (15k) 0611077A98 (10k)
R3010611077A82 (2.2k) 0611077B03 (15k)
Y51 (a&b)9180022M02 (45.1 MHz, 25 kHz) 9180022M03 (45.1 MHz, 12.5 kHz)

Converting Your 25 kHz Radio to a 12.5 kHz Radio:

Replacing the Board Components:

Carefully remove and replace receiver components FL51, FL52, R61, R62, and Y51 (a&b) listed in Table 1 or Table 2, depending on the radio's model.

Receiver Alignment:

Use a Communications System Analyzer, operating in the "Generate" mode, connected to the radio's antenna jack. The photo below of a UHF RF board identifies the pots and coils that need to be adjusted; the VHF RF board is similar. The "I-F" test point is pin 24 (lower left pin, closest to FL52) of the receiver IF chip (the 24-pin IC with the tuning data sticker on it); this doesn't appear to be marked on the circuit board.

gm300-info/gm-uhf-rf-align.jpg

Receiver 45.1 MHz IF Alignment:

  1. Apply an on-channel signal modulated with a 400 Hz tone at 3 kHz deviation (120% of the maximum rated system deviation of 2.5 kHz).
  2. Connect an AC voltmeter between chassis ground and the "I-F" test point on the RF board (near ceramic filter FL52). The AC voltmeter must be capable of operating at 455 kHz.
  3. Starting at approximately -70 dBm, increase the RF signal level as necessary until an indication of 10 - 30 mV rms is measured on the AC voltmeter.
  4. Adjust coils L55, L56, L57, and L59, in that order, for a maximum indication on the AC voltmeter. Reduce the RF signal level as necessary to keep the meter indication within the range of 10 - 30 mV rms. After adjusting the radio once, readjust L56, L56, L57, and L59, in that order, for a maximum indication on the AC voltmeter.
  5. Disconnect the AC voltmeter from the "I-F" test point.

Squelch Sensitivity Adjustment:

  1. Connect the SINAD input of the Communications Service Analyzer to the chassis (for ground) and to the (+) speaker terminal.
  2. Apply an on-channel signal modulated with a 1 kHz tone at 1.5 kHz deviation (60% of the maximum rated system deviation of 2.5 kHz).
  3. Turn the "SQ" (Squelch) control potentiometer on the RF board fully counterclockwise. The "SQ" control is located between the 14-pin connector and U51 on the "components" side of the PC board.
  4. Adjust the RF signal level until 10 dB SINAD is obtained.
  5. Turn the "SQ" control clockwise until the flashing "TX/BSY" LED just goes out.
  6. Reduce the RF signal level to minimum. Slowly raise the RF level until the "TX/BSY" LED indicator just begins to flash. Re-measure the SINAD. If a minimum reading of 9 dB SINAD is not obtained, repeat steps 7, 8, 9, and 10 above.

Transmitter Alignment:

Replacement of the transmitter circuitry chip resistors shown in Table 1 (R163, R165, R223, and R225) for VHF models, or Table 2 (R163, R165, and R301) for UHF models, may not be necessary. Use the following procedure to determine if further modifications are required. For the transmitter measurements, use the Communications System Analyzer operating in the "Monitor" mode.

  1. Using the RSS, program Channel 1 and Channel 2 for the same frequency at or near the mid-frequency of the range (e.g., use 454 MHz for a 438-470 MHz M120).
  2. Program Channel 1 with "Tx Squelch Code" = "CSQ".
  3. Program Channel 2 with "Tx Squelch Code" = "XZ" (TPL 67.0).
  4. Connect the Communications Service Analyzer to the radio antenna connector. Set the frequency to the same frequency used for Channels 1 and 2.
  5. Connect the 1 kHz tone oscillator output of the Communications System Analyzer to the microphone jack of the radio.

VCO Modulation Adjustment:

  1. Place the radio on Channel 1.
  2. Press the PTT. Adjust the oscillator level (while the radio is keyed) to 800 mV rms.
  3. Measure and record the deviation of the 1 kHz tone; this should be 4 to 5 kHz. Adjust R302, the "VCO MOD" potentiometer, for exactly one half (50%) of the recorded deviation. (R302 is located near the VCO / Synthesizer shielded compartment at the pin-1 end of connector J6).
  4. If the minimum setting of R302 produces the 50% reading, skip to step 13. If it does not produce the 50% reading, continue with the steps below:
  5. On VHF radios, replace chip resistors R223 (470) with 5.6k, and R225 (12k) with 5.6k.
  6. On UHF radios, replace chip resistor R301 (2.2k) with 15k.
  7. Re-measure the 1 kHz tone deviation on the CSQ channel. Readjust R302, "VCO MOD", for exactly one half (50%) of the deviation measured in step 8.
  8. Release the PTT.

Reference Oscillator Deviation:

  1. Place the radio on Channel 2.
  2. Turn OFF the 1 kHz tone. Press the PTT. Measure and record the PL deviation; this should be 0.5 to 1 kHz. Adjust R164, the "REF MOD" potentiometer, for exactly one half (50%) of the recorded deviation. (R164 is located near L51 at the pin-14 end of connector J6.)
  3. If the minimum setting of R164 produces the 50% reading, skip to Step 20. If the minimum setting of R164 does not produce the 50% reading, continue with the steps below:
  4. On VHF radios, replace chip resistors R163 (100) with 33k and R165 (15k) with 10k.
  5. On UHF radios, replace chip resistor R163 (56k) with 100k and R165 (15k) with 10k.
  6. Re-measure the PL deviation on the second channel. Readjust R164, "REF MOD", for exactly one half (50%) of the deviation recorded in step 15.
  7. Release the PTT.

Acknowledgements and Credits:

The information contained in this article came from the official kit instruction document: 6880904Z18-O, dated June 1995, as well as perusal of the GM300 Service Manual 6880902Z32, both of which are available as PDF files on this web site. Additional comments have been added and the format has been changed. I have also corrected several errors that were present in the original document.

Disclaimer:

The person who transcribed this information has NOT performed the conversion himself. He's just the messenger. Don't blame him for any errors or discrepancies.

Contact Information:

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


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This article first posted 29-Nov-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.