Home Engineering Hardware Hacking the Motorola Radius R100 Repeater
Engineering - Rally - December 26, 2015

Hardware Hacking the Motorola Radius R100 Repeater

Here’s an album of pics of me (KC3ASF) and /u/bondiblueos9 (N3WCC/AE) working on the repeater. (http://imgur.com/a/feKLz).


We used information from repeater builder and this index had good guides, as well as PDFs of the service manuals. Finding the actual software was a little more difficult. I got a bit brave and downloaded it from the sketchiest website on the planet. Thankfully it didn’t have any viruses, and was the actual software.


When I picked it up as a $50 repeater, I had no idea what I was getting myself into. Here’s a brief list of some of the issues we ran into:


  • The guy who sold me the radio forgot to give me the key to open it
  • Had to build our own programming cable
  • The software needs a 486-era computer to run
  • The software only allows for modifications to tx and rx frequencies within the commercial band
  • The radio, despite the seller assuring me that “it had been up in their club running no problems” was set to commercial bands
  • The transmitter just blasted RF off any normal frequencies and was stuck continuously on when the unit was powered up
  • The power cable that came with the unit was broken
  • Had to tune for new frequencies for anything to work at all, which involved disassembling the entire unit and both radios to access the adjustment points


The Key


The key wasn’t too bad. According to repeater-builder it takes a standard motorola 2135 key, which I purchased on eBay for like $3. Once the key came, I was able to open the radio up.


Determining initial frequencies


We connected a 2m/70cm antenna placed about 50ft away to the TX output of the repeater, and connected another antenna to the RX input. We fired up the repeater and listened to it with a Baofeng UV-5R, thankfully there were notes penciled in on the inside for the frequencies it had been set at as well as the DPL codes. We pushed the PTT button on the inside of the repeater and used a NooElec R820T SDR to see the frequencies. Fortunately we had previously corrected the frequency shift on the NooElec so that we could see the accurate frequency by firing up the Baofeng and comparing frequencies with a Signal Analyzer. Here’s process for someone with a signal analyzer.


  • 1. Put out a known frequency (used Baofeng), and checked the shift with a known accurate Signal Analyzer to make sure that the Baofeng was transmitting on the frequency it claimed it was.
  • 2. Transmit known frequency and adjust frequency shift on NooElec so that it lines up with the known frequency.


It was indeed set on the commercial frequencies. (tx 463.425 rx 468.425 DPL 743). At this point, the repeater was stuck transmitting so we powered it off. According to the table at the top of this overview, a stuck transmitter could have been caused by one of the capacitors (C894, 100uF, 25V), so we replaced it (100uF electrolytic, 35V).


Programming Cable


The next day, we made the programming cable in order to set it to amateur frequencies. We stopped by a closing radioshack and raided them for all the parts we thought we would need. We built the programming cable based on the schematic found on this page. Note: later in the process we had to reduce the 27kOhm resistor to 15kOhm to correct the signal levels.


Programming Software


Then we downloaded the programming software, and had to hexedit it to allow the software to allow programming on the amateur bands (since this is locked out by default) we did this using hexedit and the process is described on the same page as the schematic. Since the software was designed to run on a 486 and I realized I did not have mine with me, we decided to run DosBox with a cycles per second of 3000, a cpu type of 486_slow, and the following setting for the serial port: serial1=directserial realport:COM3 rxdelay:200


We determined the serial settings by checking device manager under serial devices to determine which COM port it was on. We ran the serial cable to a serial to USB adaptor purchased on eBay for about $1 including shipping. We also determined the rxdelay and cycles through trial and error. We ran the other end of the cable goes to the 6C6P port inside the repeater on the control board. There is one for the Receiver and another for the Transmitter. It is NOT programmed through the external JAux DB25 port.


Programming The Receiver


We flipped the Repeater Disable switch on and powered up the device and verified that it was not transmitting. We then attempted to read data (called a code plug) the receiver EEPROM. The software gave a lot of errors and then eventually after tweaking the cpu and rxdelay settings were able to get it to read data successfully. The software was really difficult to use and I would not have been able to figure it out without referencing the programming manual. For example, you need to use a number pad while numlock is on to navigate the menus (i.e. using the numbers instead of the arrows, so 4 and 6 are prev next, etc).


After reading the data, we changed the settings to our desired frequencies (we looked for a shared, non coordinated pair through our local repeater coordination group), then we wrote it back to the receiver radio successfully.


Programming The Transmitter


The transmitter did NOT go anywhere near as smoothly as the receiver went. Whatever we did, the software said that “The Device Is Not Ready” which is indicated in the manual that it is either not powered on or connected. However, it WAS powered on and it WAS connected, as verified by our multimeter.


To diagnose this problem, we hooked up an oscilloscope to our programming circuit on the data in/out lines on both the radio side and the computer side of the circuit. We attempted to read data, and we verified that signals in the form of a square wave were going both from the computer to the radio, and then in response, from the radio to the computer.


Since we could see there was communication flowing both ways, we downloaded COM port sniffer software 14 day trial from eltima this allowed us to monitor the COM port while dosbox was using it. The sniffers showed that the computer was in fact sending and receiving data from the receiver on the COM port, however on the transmitter the computer sent the initial message to the transmitter, and did not receive any data back.


Looking more closely at the signal levels on the oscilloscope we observed that the data coming into the computer from the receiver was at 200mV, while the data coming into the computer from the transmitter was only at 150mV. This prompted us to replace the 27kOhm resistor in the original schematic with a 15kOhm resistor, which increased the voltage to around 300mV, and the computer was then able to talk to the transmitter.


We verified that with this modification it was still able to talk to the receiver. We have not figured out why there is a voltage difference between the transmitter and the receiver in our radio, and if anyone has ideas, I’d love to hear them.


After sorting out the signal issues, we were able to program the desired tx frequency into the transmitter in the same way we programmed the receiver.



Tuning The Radios


After programming the transmitter with the desired frequency, the PTT button no longer had any observable effect. According to the manual, this is because we had not yet tuned the transmitter.


The instructions for tuning the radios in the repeater are in the service manual in section 6.5. We OCR’d this document to make it easier to search. There are other various adjustments that can be made, however we skipped right to the RF adjustments to make sure the repeater actually worked.


To properly make all the RF adjustments, you need the following equipment:


  • 1. DC Voltmeter
  • 2. Accurate RF power meter that provides a 50 ohm load
  • 3. Accurate frequency counter
  • 4. Suitable attenuator
  • 5. Modulator analyzer or test receiver
  • 6. Audio oscillator
  • 7. Oscilloscope
  • 8. RF Wattmeter with minimum 25W load
  • 9. 4 ohm resistive load (speaker)
  • 10. RF signal generator
  • 11. AC Voltmeter (>1 MHz bandwidth)
  • 12. Small flat and phillips screwdrivers
  • 13. At least 4 hands
  • 14. Patience


In lieu of this equipment, you can improvise as described below. We are hams, right?


Most of the adjustments are straightforward and described in the manual, except for locating test points and adjustment locations on the boards after you remove the RF shielding and take the radios out of the repeater while leaving all the wires connected.


Tuning the Transmitter


To access the RF adjustments for the transmitter, we removed it from the repeater, removed the RF shielding, and flipped it over.


VCO adjustment —


These steps involved the use of a DC voltmeter measuring a voltage of 6V at the SL test point on the transmitter RF board while adjusting a small flathead screw for the VCO frequency control C221. Once this adjustment was made, PTT started having an effect on our NooElec spectrum and the repeater’s heat sink started to warm up, indicating the repeater was actually capable of tranmitting.


Output Power Adjustment —


These steps involved the use of a DC voltmeter measuring various voltages on the Power Supply Regulator board, the transmitter command board, and the PA current limit board, as well as the use of an RF power meter that provides a 50 ohm load, while adjusting three phillips screws–High Power Set R453, Low Power Set R455, and Voltage Limit R463–on the transmit command board. For the 50 ohm load on our RF power meter, we simply used our TX antenna. Our RF power meter was a simple SWR/Power meter for the appropriate band with needles for the reading, so we had to eyeball the exact power setting.


Reference Oscillator Adjustment —


These steps involved the use of an accurate frequency counter through a suitable attenuator while adjusting the Frequency Control R163 on the transmit RF board for a reading of the exact transmit frequency without 100Hz. Since we did not have a frequency counter or an attenuator, we used our NooElec to observe the frequency (the AFC function on HDSDR helped to zero in on this) and the air between the repeater TX antenna and our NooElec’s antenna as a suitable attenuator.


Deviation Adjustment —


These steps involved the use of a modulation analyzer (FM non de-emphasized and CCITT Filter OFF) or test receiver (non-deemphasized) through a suitable attenuator to determine deviation levels of 4.6Khz, an audio oscillator (capable of 1000Hz and 200Hz tones) connected to the Service Handset connector, an AC voltmeter to read voltages of around 800mV RMS, and an oscilloscope while making various adjustments.


We did not have a modulation analyzer, but we used our NooElec instead, along with air as a suitable attenuator. For the audio oscillator, we downloaded a tone generator on the computer, and hooked the computer headphone jack up to the service handset connector (another 6C6P jack) using the connector 6C6P connector from our programming cable. Our AC voltmeter did not give us good readings, so we used the oscilloscope to measure the RMS of any signals.


We were unable to measure any non-DC voltage as P12-3 and no change in voltage when turning on our tone generator. Since the manual implies that we should have been getting an oscillating signal that we needed to adjust to read 800mV RMS, we instead connected the oscilloscope to our computer audio output and adjusted the volume there until the oscilloscope read around 800mV RMS. This may not have been correct, however we wanted to try to do something similar to the instructions.


To measure the deviation level of the signal on our NooElec, we observed the spectrum around our signal and adjusted the VCO MOD control R302 until the spectrum did not have too much power outside of 4.6kHz above or below our center frequency and the audio output from both the NooElec and our Baofeng was not crackling or distorted but also not too quiet.


For the Reference MOD R305 adjustment, we were unable to view a good waveform on the oscilloscope neither when it was hooked up to the audio output of the Baofeng nor when it was hooked up to the audio output of the NooElec. Instead, we listened to the signal on the Baofeng and made the adjustment until the audio was clear, which was approximately in the middle. Note that in this step there is supposed to be a square wave on the oscilloscope (which we did not observe), and this square wave is supposed to be caused by the transmitter radio clipping the low 200Hz audio.


2-10 Watt RF Power Output Field Adjustment Procedure —



These steps involved the use of an RF wattmeter with a 25W load (minimum) connected to the TX antenna connector while making adjustments to the High Power Set Control R453 on the transmit command board. This section may be mislabeled since we connected the RF watt meter and our TX antenna and adjusted the High Power Set R453 to get a reading of 25W.


At this point we high-fived because we got through section 6.5.2 and could get a signal on the correct frequency with audio through the handset connector out of the repeater.



Tuning the Receiver


Fortunately, the RF adjustments for the receiver only involve removing the RF shielding since the adjustments are on the side facing up.


VCO Adjustment —


These steps involved the use of a DC voltmeter measuring a voltage of 6V at the SL test point on the receiver RF board while adjusting a small flathead screw for the VCO frequency control C201. Note that the receiver also has a control at C221 like the transmitter, however this is not used.


RF and I-F Alignment —


These steps involved the use of a 4 ohm resistive load (speaker) across the Local Speaker connector on the main board, a DC voltmeter, an RF signal generator to produce an unmodulated on-carrier signal (for your receive frequency) just strong enough to quiet the receiver (we assumed this meant to increase the power until no noise was heard on the connected 4 ohm speaker) and later to produce a signal modulated with a 1KHz tone at 30kHz deviation, and an AC voltmeter having at least a 1 MHz bandwidth for measurements around 10-50mV RMS. The adjustments are made to the “slugs or screws of the front end coils or helicals” which we determined were the 8 metal rectangles with screws on top nearest the side of the repeater with the connectors, as well as a few other small screw adjustments. At no point did we PL disable the receiver while making any adjustments, since we have a DPL model. If you do want to use the PL disable, you will need to install a jumper switch, and be careful if you try to short the connection points while the repeater is powered as you may get sparks and pops if you make contact with a wrong conductor on the board nearby.


To connect a speaker to the local speaker connector, we solder a 1/8″ headphone jack onto the local speaker connector points on the main board. Unfortunately, after attempting to use a 4 ohm speaker, an 8 ohm speaker, and the oscilloscope, we were unable to get anything out of the local speaker connector, except some faint tones at one point.


For the adjustment of the three cells of injection filter helical FL3 (right-most when repeater connectors are facing away from you), we were able to get a voltage reading of around 2.7V after adjusting the screws. For these adjustments, the peak voltage was obtained somewhere in the middle of the adjustment extremes, i.e. if you keep turning the screws too far the voltage will begin to drop.


We did not have an RF signal generator, however, we used our Baofeng keyed up with the antenna near the RX antenna and observed a voltage of 19mV on the oscilloscope (in place of an AC voltmeter). The instructions call for an unmodulated signal, and the Baofeng would be producing a modulated FM signal of silence, but we worked with what we had. Adjusting the helicals FL1 and FL2 had no effect on the RMS voltage, however adjusting L51 did increase the voltage. Then adjusting FL3 again had minimal effect. We did not secure the adjusting screws with paint as the instructions suggest because we hope to properly tune the receiver at a later time when we obtain access to proper equipment. The existing paint was easy enough to break through.


For the Volume Control R818 and Quad Coil L54 adjustments, we connected our computer running tone generator software with 1KHz tone from its headphone jack into the microphone input on the Baofeng, and ignored the specification in the instructions to use a 30kHz deviation, since we had no way to adjust that. At this point we did hear a faint tone on the connected speaker, but it was too faint to notice any difference during the adjustments. We connected the oscilloscope across the speaker leads to measure the voltage, but we were not able to get the specified 1V RMS reading or anywhere close. Volume Control R818 had no effect, but we adjusted Quad Coil L54 for max voltage, even if less than 1V RMS.


Squelch Adjustment —


These steps involved the use of a 4 ohm resistive load (speaker) across the Local Speaker connector on the main board, an RF Signal Generator to the RX antenna connector to produce an on-carrier RF signal at 1mV RMS modulated with a 1 kHz tone at 3 kHz deviation, and an AC voltmeter (or oscilloscope) while making various adjustments, measuring various voltages, listening to output from the speaker, and measuring SINAD around 17dB.


Again, we could not hear much from the speaker, and could not obtain a 1.4V RMS reading across the speaker connection. We also used the tone generator software with 1kHz tone connector to the mic input of the Baofeng transmitting next to the RX antenna and ignored the specification of a 3kHz deviation. Additionally, we ignored measuring for the 17dB SINAD level.


Really, for the squelch adjustments, one of us walked away from the repeater and transmitted with the Baofeng while talking and the other made various adjustments while listening to the repeater output on an Icom HT until the audio output sounded good and did not cut out. At this point we also tweaked previous adjustment points to verify they could not be improved.


At this point we attempted another high-five because we got through section 6.5.4 and could successfully repeat a signal using the repeater, but we were tired after the whole ordeal and missed.




If we can get access to the specified equipment and not improvise solutions, we would like to go back and make the adjustments again, now that we are familiar with the process, and see if we can make any improvements. The output of the repeater seems like it is a little quiet, and according to the NooElec there are several harmonics. We hope that the harmonics are caused by the proximity of the NooElec to the TX antenna (only 50ft away), but if they are not, we expect to eliminate them by building a duplexer with appropriate filters.







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