Modification of the Motorola Syntor VHF radio
for use in the Amateur 2 meter band
By Mark A. Tomany N9WYS

Before I take all the credit for this, I must pay homage to another ham, Terry - WA9AWO, whose technical advice kept me consistently pointed me in the right direction. Had it not been for Terry's help, I probably would have tossed the radio off of my roof in frustration, and employed some other, "lower-quality" radio for my station. But because I live in what was once referred to as "the boonies" southwest of the Chicago metropolitan area, I really wanted the 100w output from the Syntor for coverage --- so I tore into it. This effort has taken me about 5 years to get it right - I did this a couple of times, and a couple of ways, before but the receivers died and I ended up shelving the project for a while. I resurrected it just about a month or so ago because packet radio and APRS is becoming increasingly popular with the Chicago Office of the National Weather Service and local amateurs for severe weather reporting.

This article will address my conversion of the Motorola Syntor VHF radio (Model #T83SRA3800AK) for use in the 2-Meter amateur band. As a further clarification - the specific version addressed here is the ORIGINAL Syntor - not the Syntor X, Syntor 9000, or any other iteration of the Syntor.

This particular model of the radio is designed to operate in the frequency range of 146 - 174 MHz, with a "Range III" VCO, which operates from 150.8 - 162 MHz. The VCO would not lock at the amateur frequency range, so modification was necessary. For my application, I wanted to use the radio as a packet station, which meant I needed the VCO to go down to 144 MHz.

My research on the web revealed one modification that employed the connection of one pin off the freq PROM to another point within the radio chassis, programming the PROM to 10.7 MHz off freq from where you want it to operate, etc... The frequency PROM is a "one-time, burn-and-use" item and mine was already programmed to the actual frequencies I wanted to be using, so this wasn't a viable option for me, unless I wanted to spend more money on another freq PROM. Getting another PROM was a pain (and EXPENSIVE anymore!) so I started to dig into the radio.

Now for the up-front words of caution: You will see this repeated OFTEN within this document: Performing this modification requires a soft touch and a cool iron, since you will be working on a circuit board that has a ceramic substrate. The ceramic substrate is prone to cracking if heated improperly or if too much physical force is placed on the board. If you end up cracking this board, it becomes trash and so does your radio!

USE A SOFT TOUCH AND COOL IRON!! I recommend an iron no more than 35w for this operation. (That's what I used, and even though it took a while to get components to heat up enough to work with, the wait is necessary.) Also, allow the components to heat slowly and work them at that point - don't start prying up stuff! You may become a bit impatient while trying to heat the solder points, but don't! Because of the ceramic substrate, this board dissipates a lot of heat quickly.

Photo 1

The circuit board you will be working on is the Synthesizer VCO Controller board. It is located within a rectangular compartment (shown in Photo 1, above) which has a heavy cover secured by four screws. Work with the radio connections toward you, and POWER THE RADIO DOWN while performing these tasks. It is not necessary to remove the assembly from the chassis to perform the mod. The Synthesizer VCO Controller is mounted toward the top of the assembly now visible. You will see two parallel jumpers - these connect to the VCO "tuning caps". It is these jumpers that you will be working with. Note - Photo 1 was taken during the modification procedure. Sorry, I didn't remember to get my camera ready until after I started working.

I found that before the modification on my radio, two jumpers were already cut. These were one lower jumper to the right end of the run, and one upper jumper to the far left of the run. Experimentation determined that I needed to add the upper jumper, and remove others. I removed the upper jumper directly above the far right lower jumper, and a second jumper to the left of that, and reinstalled the missing jumper from the upper far left (See photos and diagram). Once I completed these tasks, the VCO would lock and the radio received on the frequencies desired.

Photo 2

Photo 2 shows my completed modification. It's certainly not pretty to look at, mainly because I wanted to be as careful as possible with the heat, but it works! It is easy to note in this photo which jumpers are removed, and the jumper I added.

The diagram below, scanned from the Motorola Syntor Instruction Manual - Motorola Part # 68P8104E40-A, dated 3/5/81 - indicates which jumpers should be added (GREEN) and which ones should be out (RED). Black jumpers on the diagram remain unchanged. I added the color highlights to assist you in identifying which jumpers required modification on my radio.

Jumper diagram

Again, I reiterate - USE A SOFT TOUCH AND A COOL IRONwhile working on the synthesizer VCO board!!! The desire to use a higher heat iron WILL crack the substrate and ruin the board. As far as this modification goes, patience is a necessity to get it right. Motorola states that even a scratch in the substrate could have adverse effects on the operation of the VCO. My dad always told me "Patience is a virtue." (Although many times I didn't listen...)

Once you reset the VCO for the lower frequencies, you may need to adjust the deviation. Check your transmit deviation once you reset the VCO to operate at your frequencies. As Motorola states in their manual:

"Because of the nature of the VCO, its modulation sensitivity tends to increase with RF carrier frequency. When setting deviation it is important that deviation is checked on all transmit channels especially if wide channel separations (greater than 5 MHz) are required. This will ensure that the radio will never over-deviate on the higher frequency transmit channels.

Motorola suggests a deviation of 4.75 kHz at a frequency near the center of the transmit frequency range for a non-PL/DPL radio. (If you are using PL/DPL, set deviation at 4.0 kHz -- make your adjustments with the PL/DPL board DISABLED.)

Even though I lowered the range of my radio, TX deviation remained approximately the same, so I made no adjustment. However, if you should need to adjust deviation, you make the adjustment at variable resistor R518, which can be found to the lower right of the frequency PROM. Photo 3 (below) shows the location of R518. R518 is the BLUE pot in the photo.

Photo 3

Since the receiver was originally tuned for the 155 MHz range, some adjustment of the receiver preselector needed to take place to regain optimum receiver sensitivity. Of course, the best way to do this is to perform a receiver alignment as described in the Syntor manual. I understand that most amateurs do not have access to a service monitor, but one is necessary at this point of the modification, if you want the receiver to be anything other than deaf. Again, I called on Terry WA9AWO and his service monitor for help...

Motorola has charts in the Syntor manual which specify how high the tuning screws must be from the preselector case at the start of the tune-up procedure. A little extrapolation was necessary, since the bandwidth of my radio only extended down to 146 MHz. I determined that all screws for coils L1 through L5 had to be at 4mm above the case for a starting point. The tune-up procedure is shown below, along with the location of the tuning points you will be involved with.

After setting coils L1 through L5, I tried to follow the tune-up procedure. Unfortunately, in order to be able to read the voltages called for in the procedure, I also needed a Motorola test set - something else I didn't have. So Terry and I resorted to injecting a signal into the receiver that was 10.7 MHz below the intended lowest operating frequency (in this case, the lowest freq to be used was 144.910, so we injected 134.210) and set coils L107 and L109 for the "dip and peak." (This is Step 4 of the procedure.) After that, not being able to get the voltage readings we wanted, we resorted to doing what Terry called a "SINAD tune-up" - we set the service monitor to 144.910, and tuned the radio for 12dB quieting. After that we checked the sensitivity and it appears we required a 0.30 uv signal for 12dB SINAD, and squelch opened at -117dB, as low as the service monitor would go. Signal sensitivity overall appeared to be about 0.15 uv, which is better than the specifications calls for.

The station is currently on the air, happy as a clam, and so am I!! I now have an 85w station operating on packet (currently on 145.010, but plans to move it to 144.390 as soon as I can get a new PROM) for all of northeast Illinois to hear. (The Syntor I used is rated at 100w, so we set the PA limiter at 100w max, and then adjusted for an overall output of about 85w. I didn't want to work the finals TOO hard...) So far, I've been able to reach out about 100 miles with this station, successfully.

Now, the disclaimer... This modification assumes that the person performing the work has a moderately advanced knowledge of electronics and electronic theory. (I most certainly am NOT an Electronic Engineer... I TAKE NO RESPONSIBILITY WHATSOEVER IF YOU DECIDE TO PERFORM THESE MODIFICATIONS ON YOUR RADIO. They worked for me - they may or may not work for you. (You should have noted at the top of this document that I have been working for some time to find a satisfactory mod that would work permanently for my application.)
Therefore - the descriptions disclosed within this document are for informational purposes only - your mileage may vary. This mod probably also voids the FCC Type Acceptance for the radio - but you already knew that, didn't you? Oh, and before I forget: USE A SOFT TOUCH AND A COOL IRON!

Before trying to perform ANY modification on this type of radio, I strongly recommend you go to the Repeater-Builder's Motorola Index page and other Syntor web pages, etc., and thoroughly acquaint yourself with the Syntor radio and its idiosyncrasies. This alone will help keep you from turning your radio into so many dollars worth of useless electronic parts.

Allow me to wax editorially for a moment... I have always had an affinity for Motorola equipment, both for amateur purposes and otherwise. I've been a police officer for 28 years, was also a volunteer fire fighter for 18, continue to be an emergency management volunteer, severe weather spotter, etc. Inasmuch, I have had the opportunity to use Motorola, General Electric - Ericsson - M/A-Com, Standard, E F Johnson, Icom, etc. It should be understood that my comments do not necessarily constitute an endorsement for, or against, any equipment manufacturer or device, but are merely an expression of my own personal preference. Long before I invested in any 2m/70cm amateur gear, I used a VHF MT1000 and an "enhanced" MVA mounted in my car for my mobile. I acquired another MT1000 on UHF to "expand my horizons". I still own and use these radios, satisfactorily and happily. I currently assist the Will County (IL) EMA, in conjunction with the Plainfield (IL) Radio League, in operating a Motorola MSR2000 repeater on 444.550/+, CTCSS 114.8, in the Joliet, IL, area. I've always found Motorola equipment to be extremely serviceable and very durable, with good audio, and fairly easily modifiable for use in the various amateur bands. This is why I'm personally disappointed that the "Batwing boys" of Motorola decided several years ago to "abandon 2-way radio" in favor of cellular. OK, maybe they didn't totally abandon 2-way, but after the MICOR and Syntor product lines, utility and, more importantly, quality seemed to decline considerably. Of course, there were some exceptions, but I speak here in generalities. Maybe at some point in the future Motorola will resume the production of quality 2-way products. But I don't think I'll hold my breath. Oh well, so much for the editorial commentary...

The following are Registered Trademarks of the Motorola® Corporation: Motorola®, Syntor®, SyntorX®, Syntor 9000®, MT1000®, MVA®, MSR2000®, MICOR

The following are Registered Trademarks of their respective corporations: General Electric®, Ericsson®, M/A-Com®, E F Johnson®, Standard® and Icom®

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