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  Improving the Performance of
the Low-band Syntor X9000

By John Haserick W1GPO
With Help From Robert Meister WA1MIK
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One of Motorola's best ever low-band radios can still be enhanced for amateur use. As with every commercial low-band radio, the harmonic filter starts to attenuate above 50 MHz. In this radio, there is more than 1 dB loss above 53.5 MHz, which translates to at least 20 watts lost to heat. There is also reduced receiver sensitivity, as the harmonic filter is between the antenna and the receiver. With the advent of good solder vacuum desolderers, the removal of the PA board is now much easier. We used a ZD 985 90-130 watt model available from Memotronics for about $140. The 1.3 mm tip needed to be enlarged slightly to fit loosely over the feed-through capacitor leads, and worked well removing the coil shields, but the bottom harmonic shield removal required a 100 Watt iron, solder wick, and patience.

Getting to the Filter:

After removing the screws and two nuts that retain the PA circuit board to the heat sink, remove the solder from the nine feed-through capacitor leads holding down the board. Remove the RCA jack connector from the exciter, and push on the associated RCA plug to help free the board. There are ferrite cores beneath the board on each of the feed-through leads, so take care not to lose them. The photo below shows the board after being removed from the radio. Click on any of the photos or images for a larger view.


Modifying the Harmonic Filter:

A large shield must be removed from the bottom of the circuit board as it restricts access to the harmonic filter shields. Remove the individual shields over the five harmonic filter coils to be spread. For each coil, evenly spread the turns with a thin non-metal blade, starting with the coil end turns. Only the tops of the coils will spread, because the bottoms are fixed. Set each to the dimensions shown below, measured across the top of the coils from end to end:

L890 6.5T14.5mm22.5mm
L891 5.5T12.0mm19.0mm
L892 4.5T5.5mm14.0mm
L893 6.5T14.5mm19.5mm
L894 7.5T17.0mm23.0mm

The image below shows the harmonic filter schematic with the coils that you need to spread out. The relevant information has been previously tabulated.


The photo below shows what the harmonic filter coils look like after having their turns spread according to the table above. Note the shields at the top of the photo.


Do not be tempted to touch the 7-turn unshielded PA coil L810, or you will lose 10 meters. Just changing the harmonic filter has no effect on frequencies below 6 meters, as it is strictly a low-pass filter without ripple below cut-off. Needless to say, wipe off and apply new heat sink compound the three power transistors. There is clear greasy compound on the square insulator for the control transistor, so keep it clean.


With power at 52.49 MHz set to 105 watts, current limit to 22A at 14V, the results we got after filter change is tabulated below:


All scope pictures are with marker at the center at 54 MHz, 25 MHz sweep width, so each vertical line represents 2.5 MHz. The top line represents 0 dB of loss. This image shows the original harmonic filter response that starts rolling off just below 54 MHz.


This image shows the modified harmonic filter response that starts rolling off at just over 57 MHz.


Additional Improvements:

Another modification should be made to the mike to improve audio from tinny to good communications quality. The radio itself has a very straight 6dB pre-emphasis from 300-2800 Hz; all speech tailoring is in the mike. Change the 0.047uF C1305 to 0.33uF. C1305 is closest to where the mike cord exits the circuit board. 0.47uf gives flat natural audio. See the images below.



Receiver sensitivity can be improved to 0.25uV for 20 dB quieting by adding a 0.001uF chip cap across the pre-amp Q100 emitter resistors. We have not noticed any inter- modulation problems as a result of this change. See the images below.



Acknowledgements and Credits:

Schematics and printed circuit board X-ray views came from the Motorola Syntor X9000 Low-Band Radio System Service Manual, p/n 6880101W95-D.

Photographs were taken by the author unless otherwise indicated.

I thank Bob WA1MIK for his invaluable advice and assistance with creating this article.

Contact Information:

The author can be contacted at: jhaserick84 [ at ] comcast [ dot ] net.

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This article originally written 12-May-15.

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.