KB8ZQZ's Syntor X retuning page

Herein I try to describe the hack job I do on Syntor X radios to retune them for the ham bands. If you want to do this job correctly, obtain a large quantity of expensive test equipment and scarce special items, then follow the procedure described in the Motorola manuals.

I have done the retuning successfully with a digital multimeter. It's tedious and time consuming, but it's possible. If you have or can borrow a cheap sweep generator and spectrum analyzer, it's easier. I obtained my sweep gen from eBay, the spectrum analyzer frame from the half-price sales at Dayton, and borrowed the spec-an plugin. Someone you know probably has this gear.

See all the fine material on the Extended Frequency Modifications page on Mike Blenderman's site (link below) for the real explanations which taught me how to do this.

Equipment used
  • Digital multimeter
    Used to check v-bit crossover voltages.
  • Spectrum analyzer
    Needs to cover the desired transmit and receive ranges, as well as the VCO operating range (53.9 or 75.7 MHz above or below the operating frequency) used on receive. As long as the unit produces repeatable results, it doesn't need to be all that calibrated.
  • Sweep generator
    Needs to cover the desired transmit and receive ranges, as well as the VCO operating range (53.9 or 75.7 MHz above or below the operating frequency) used on receive. As long as the unit produces repeatable results, it doesn't need to be all that calibrated.
  • Frequency counter
    If the test gear to which you have access is of questionable calibration, or if the settings are analog and it's hard to tell exactly what frequency you're covering, a counter may be helpful, but it isn't essential.
  • Soldering iron and solder
    The VCO retuning procedure entails reconnecting one or two wires.
  • Basic hand tools
    You'll need a torx driver to get into many of these radios (some are crosspoint), a small straight-blade screwdriver to adjust the filters, and whatever small pliers or prodding devices you find useful for manipulating small items.
  • Ability to generate code plugs
    This means access to appropriate software, and an EEPROM programmer. See my syntorxgen tool for one way to do former; find a buddy if you lack the latter. To the best of my understanding, it's not possible to make the suitcase programmer do the necessary programming unless you're willing to hack on its ROMs.


The Syntor X VCO is essentially equipped with four operating ranges. One overlapping pair is used for transmit; the other overlapping pair is used for receive. Two bits set in the programming of the radio determine which range is active. Programming software must know at which frequencies to switch these bits. The factory used fixed values which allowed enough leeway for slight variations in components and for temperature-induced variations. When the radio is re-tuned, new values for these crossover frequencies must be determined in order for the programming software to correctly generate working code plugs.

Steering lines

The VCO is controlled by means of steering line voltages generated by the PLL circuitry. For the radio to properly lock on frequency, the PLL must be able to generate a steering line voltage which is in-range. The factory settings kept the voltages between about 3 and 7.5 volts; if the voltages drop to about 1.5 or rise to about 9.6 volts, the PLL becomes unable to shift them further, and the radio will no longer be able to lock. The difference between the factory-used ranges and the unlock points provide for some margin for varying operating conditions such as extreme temperatures. As part of retuning the radio, you will determine whether you have made large enough changes by monitoring these voltages; you will also determine the new band limits of your retuned radio by finding the new frequencies at which the lowest and highest desired voltages occur.


There are two major filters which need to be retuned. The first is the front end filter which allows only the desired range of operating frequencies to enter the first mixer. Its adjustable parts consist of several variable inductors. If improperly set, this filter will prevent adequate received signal from reaching the rest of the receiver, making the radio somewhat to extremely deaf. The second is the injection filter which allows only the needed frequencies to pass from the VCO to the first mixer. Its adjustable parts consist of three variable inductors. If this one is badly adjusted, the VCO signal reaching the first mixer will be too weak, and the signal exiting the first mixer will also be weak. The band pass graph of each pole of these two filters resembles a single sine wave peak with a flattened top. The tuning of the several poles overlaps, resulting in coverage across the entire desired range.


Images showing locations of various boards and components of the radio are available to help describe the conversion. Due to their large size, I've placed them on a separate page. The total size of the page with images is about about 1.5 MB.

Phase I: Prepare the code plugs

These test plugs should cover the existing and desired operating ranges fairly evenly. For example, on a high-band rig, you probably want your test plug to cover roughly 136 to 174 MHz in 32 channels of one to one and a half MHz. You'll need two:

  • One with it's v-bit changeover set to the lowest possible operating frequency for the radio you're converting, probably 136 MHz for a high-band radio.
  • One with it's v-bit changeover set to the highest possible operating frequency for the radio you're converting, probably 174 MHz for a high-band radio.

I've provided some sample code plugs here. These may or may not cover the frequency ranges in which you're interested, so look first before using.

Band Low freq High freq V-bit x-over syntorxgen source s-record hex file
Low band 28 54 28 Download Download
Low band 28 54 54 Download Download
High band 136 174 136 Download Download
High band 136 174 174 Download Download
UHF Range 2 440 460 440 Download Download
UHF Range 2 440 460 460 Download Download
Phase II: Measure the unmodified radio

Determine what range of steering line voltages you're willing to run. Factory spec was about 3 volts to about 7.5 volts. At room temperature, most radios seem to be willing to lock below 2 volts and above 9 volts. I've set a number of mine to operate within the range of about 2.2 volts to about 8.8 volts, but haven't made exhaustive tests to determine what temperature ranges will cause them to fail. Given the fact that you're about to analyze your specific radio, you may be able to get by with a wider range than the factory could. If you're industrious, do the tests below under extreme temperature conditions to make sure you're inside the margins.

Take the bottom cover (the one which is held on with screws) off the radio. Position the radio on the bench exposed side up, handle towards you. Remove any existing EEPROM module and install the module containing your low-crossover test plug. Remove the cover from the VCO bay. Turn the radio onto its left edge; remove the top cover (the one which is released by the push button latch). Select mode 1 and apply power. Locate the PLL unlock LED on the RF board. Step through the modes, and determine which ones lock (LED off) in both receive and transmit. Take notes. Remove power.

Set your DMM for DC voltage. Turn the radio on one of its long edges. Connect the ground lead of the DMM to the ground lead near the upper right hand corner of the VCO board. Connect the positive lead of the DMM to the steering line near the top center top center of the VCO board. Remove the other cover of the radio. Select the lowest numbered mode which locked in the previous test and apply power to the radio. Work your way up through the modes until you reach the first one which has steering voltages inside the range you selected in the first paragraph above. Remember to do both receive and transmit. Record the modes and steering line voltages. Now, select the highest numbered mode which locked in the second paragraph above. Work down until the steering line voltages comes into your selected range. Record the modes and voltages. Remove power.

Swap in the EEPROM comtaining the other test plug. Repeat the steps in the second and third paragraphs above.

Now, translate the mode numbers you recorded above to frequencies by referring to your programming sheet. You should be able to see at about which frequencies the v-bits should cross over for this unmodified radio by splitting the difference between the high end of the lower range and the low end of the higher range.

Phase III: Convert the VCO

If you were lucky, you discovered that you don't need to modify the VCO to get the radio to work in your desired operating range. Otherwise, you now need to change the tuning capacitor strip on the VCO board.

Put the top (button latch) cover back on the radio. Lay the radio flat, open side up, handle toward you.

Modify the receive strip first. The simplest thing to try is to simply bridge the cut the factory made in the wire on the receive strip. In my experience, the factory usually made just one cut. If that isn't true, you can either put the leftmost cut back together and test again, or you can put them all back. To put them back, bend the tips of the wire in the factory cut back together as close as you can, and stick a small blob of solder to them. Use an pencil type iron, not a big iron or gun. Be careful not to heat the ceramic substrate up too much or you will damage it. Check the resistance of the strip using an ohmmeter; it should be quite low.

I've found that some radios need receive work but do fine untouched on transmit. If your notes from the previous measurements indicate that you have to make changes, repeat the process above on the transmit strip.

Now, repeat the measurements from Phase II. Record your results. Look up the frequencies to see if the VCO is now usable over the desired operating range. If so, continue to Phase IV. If not, cut the strip wires to remove pads from the strips if you need to raise the frequency range, or add pads back if you need to lower the frequency. Remember that the strip starts at the left end. Repeat until you get it where you want it.

Phase IV: Retune the front end filter

There are several ways to do this phase. One is the full factory specified procedure, which involves a fair amount of test equipment and use of the Motorola-supplied test EEPROM. I don't discuss this one here; see the manuals. I describe two others. I'm assuming the radio is still positioned as above, open side up, handle towards you. You'll want to remove the top (button release) cover, then set the radio on two blocks of wood, one at the fins, and one under the handle. Be careful not to damage the exposed RF board at the handle end while working on the radio in this condition.

Regardless of which method you choose, the principle is the same: adjust the variable inductors in the front end filter to shift their pass band from the factory settings to your desired operating range. There are several general things you'll need to know. First, there are two sets of inductors. One is a group of three on the left side of the silver chassis; these form the injection filter which you will adjust later. The other is a group which varies in size depending on band. Second, the screws are covered with thread-lok at the factory, and you will have to chip this off before you can adjust the inductors. Sometimes it pops right off. Other times, it resists vigorously. Third, use of a metal screwdrive to adjust the inductors may affect the tuning of the inductor while the metal is near the inductor; make the adjustment and then move the tool away before measuring the result, or use a plastic tuning tool. Note that the screws can be rather stiff to move, so the plastic tool may not be practical.

My gut feeling is that the left hand inductor usually has more effect at the bottom of the tuning range, and the right hand one at the top of the range. Turning the screws into the radio (clockwise) seems to lower the operating range.

Using sweep generator and spectrum analyzer

This is probably the easiest method. If you have a spectrum analyzer with tracking generator, it's truly simple. If you have only separate sweep gen and spec an, it's a little tougher.

I set the sweep generator to cover the desired operating range over the course of 10 seconds or so. Inject the signal from the sweep generator into the antenna port.

The spectrum analyzer is set to sweep the whole range many times per second, and to use its max-hold function. Max hold will let you build up a picture of the current state of the filter over the 10 second period. Since you're changing the filter, you'll have to periodically clear the curve and start fresh. Turn the radio over for a moment to clip the probe from the spec an onto the middle pin of the rear side of T150 on the first mixer board.

I suggest scanning across the combined factory and desired operating ranges. Sketch the curve of the filter, and note the dB values shown on the spectrum analyzer. These don't need to be calibrated accurate, but will give you a an idea of how you're doing relative to the way you started. If your unit will do it, do the scan of the factory tuning with the "a" trace, then leaving that displayed, switch to the "b" trace to do the retuning.

Now, begin adjusting. The objective is to produce a curve on the spectrum analyzer which is the same shape as the factory tuning, but shifted in frequency to the desired operating range. Be systematic; e.g. try turning all of the inductor screws half a turn in the same direction, then clear the held curve and wait for it to redraw. Once you've gotten the curve shifted about where you want it, work to flatten out the top; it won't get perfectly flat, but shouldn't vary by more than a few dB across the range. It should also be at about the same level (dB) as the original curve.

NOTE: avoid moving the probes around as you take measurements; this can significantly affect the results.

Using a voltmeter and signal generator

This is extremely tedious and time consuming. It may result in dead spots in the tuning range. The maximum size of the dead areas will be less than the spacing between the frequencies of the test code plug you're using.

The principle is essentially to tune the radio to a known frequency, inject a weak carrier into the radio at that frequency, and measure the audio voltage of received noise. This process is repeated at multiple frequencies across the desired operating range. The adjustable inductors of the front end filter are tweaked to minimize the noise voltage. Since you can only measure a single point at any given time, this requires persistent iteration, but after a while you will start to understand how different inductors are affecting portions of result and will be able to smooth out the response curve.

Phase IV: Retune the injection filter

Using sweep generator and spectrum analyzer

Inject signal from the generator by clipping a probe onto the coax leaving the VCO buffer board toward the injection filter. Tap the signal out for measurement by clipping a probe onto the coax entering the first mixer board from the injection filter.

The actual tuning procedure for this one is essentially identical to the front end filter, except that the range scanned by the signal generator and the spectrum analyzer is the frequency generated by the VCO, rather than the actual operating frequency. For high band and up radios, this is 53.9 MHz away from the operating frequency. For UHF Range 2 and higher, it is below the operating frequency; for the others it is above. The low band radio is special; its VCO operates 75.7 MHz above the operating frequency.

NOTE: avoid moving the probes around as you take measurements; this can significantly affect the results.

Using a voltmeter and signal generator

All the warnings and misery of doing the front end filter by this method apply here too.

Since insufficient injected VCO signal strength will result in weak output from the mixer, the procedure works for this step too, more or less. Use the instructions above, except tune the three poles on the left side of the chassis. Good luck.

Phase V: Test

You can check your work in a couple of ways. Unless you have access to calibrated test equipment, both involve comparison to other radios which you know perform well.

If you have access to an RF signal generator on which you can control the strength of the delivered signal, check the sensitivity of a receiver known to perform well. Set the signal generator and the radio to the same operating frequency. Inject the generator's output into the antenna jack of the comparison radio. Starting at the lowest level available on the signal generator and working upwards, find the level at which the receiver just begins to mute the static. Repeat the test with your converted Syntor X. The results should be similar. If you're off by a lot, revisit your tuning of the filters.

WARNING: The largest signal level you should be injecting into receivers is on the order of a few microvolts. Injecting large amounts of RF into a receiver will destroy it.

If you lack access to a signal generator, you can get a crude approximation by tuning your comparison radio and your freshly converted radio to the same weak signal (distant repeater?) and comparing the relative noisiness of the received signal. Of course you'll need to use the same antenna and feedline for the test to avoid differences which could easily be more significant than the tuning you're trying to measure.

You're done!
That's it. If you got similar performance over your desired operating range, you've successfully done the retuning project. Now go generate the code plug you want in your radio and install it.
This page created by Dennis Boone, KB8ZQZ, jm-sxg at yagi dot h-net dot msu dot edu.