MOTOROLA RADIUS P200 - OUT OF BAND PROGRAMMING |
Because i needed a good quality radio for 27MHz (Citizens' Band), i tried to modify an old Motorola Radius P200 original designed for 30-36MHz (LB) range. After range extension by software, some hardware modifications were required. Final results were quite impressive, nice receiver (double balanced mixer, PLL FM demodulator) and enough Tx power.
Same algoritm can be applied to other radio models (ex. MT1000 6 channels), bellow 30MHz (CB) or above 50MHz (6 meters amateur band).
Software modification
Motorola P200 low band programming software vers. D03.00.01, 01-Jun-90 was modified for out of band use (27...54MHz). All instructions can be found on "repeater-builder website". More easy is to download my software version, already extended for 27-54MHz range.
The RSS (radio service sofware) was tested using an old computer at 80MHz and MS-DOS enviroment (DOS 7.1).
Before range extension attempt, read and save the original code plug (archive option). Programming procedure is simple, the new frequencies can be directly introduced in the radio memory.
Hardware modification
1. VCO
VCO module includes Tx/Rx voltage controlled oscillators, PLL synthetiser and 2nd L.O. generator.
2nd L.O. is mixed with 19,95MHz FI signal in order to obtain the 2nd FI 450KHz. Low side injection 19,5MHz or High side injection 20,4MHz can be selected by RSS programming.
RX VCO frequency is placed above RX input frequency (27,225MHz + 19,95MHz = 47,175MHz). Without any modification, the original RX VCO is able to generate 47,175MHz. In this situation the tunning voltage has around of 0,25V. To be sure, the range of RX VCO was shifted between 44-52MHz controlled by 0-5V.
Is easy to recognise the RX VCO by LC resonant circuit built with a toroidal coil.
Coupling capacitors are located between LC circuits and transistors base, were increased from 10pF to 18pF for RX and from 17pF to 40pF (22+18pF) for TX.
Schematic diagram of VCO module. Negative Voltage "-Vee" has -1,88V when the radio was programmed under 30MHz.
Here is more easy to see where are the capacitors located on >VCO PCB<.
2. RX Bandpass Filter
RX 30-36MHz band pass filters need to be readjusted for best sensitivity.
During measurements, the VNA was connected between antenna and base of Q1 transistor.
Response of original input bandpass filter:
VNA connected between antenna and input of diode ring mixer (C10/C11 capacitors). VNA output level was decreased in order to compensate the preamplier gain.
Global response of original RX channel:
RX bandpass filters were modified only by capacitors adjustements. I tried with ferrite slugs but is neccesary to insert only few thread steps, difficult to achive enough stability...
Schematic of RX channel modified for 27,225MHz (CB 22 - common channel in Romania):
VNA connected between antenna and base of Q1 transistor.
Response of 27MHz input band pass filter, M1/M2 markers at -3dB:
VNA connected between antenna and input of diode ring mixer (C10/C11 capacitors). VNA output level was decreased in order to compensate the preamplier gain.
Response of 27MHz RX channel, M1/M2 markers at -3dB:
3. TX Power Amplifier
For TX alignament is mandatory to use a 10V power supply with over current protection at 2 Amps. In this way the output transistor will be protected during mismatch, overload or parasitic oscillation.
Same over current protection was used inside of battery eliminator.
Output power is around 6.5-7W, efficiency ~40%.
After capacitor/resistors modifications, the alignament is done by C116 and C159. Adjust the Tx chain for best stability and steady 6-7W output power. Q103 need enough time for cooling down, don't keep pressed the PTT too much time. The shield of PCB must to be installed for a true alignment.
P200LB Software 27...54MHz
Motorola P200 LB RF Section
Motorola P200 LB Service Manual
Motorola P200 Operating Instructions
Motorola P200 Alignment Procedures
P200LB PCB modified
Update 27.01.2019
MT1000 99CH RSS modified for 27MHz, 29MHz and outband. Thanks to YO2MLR!
LINKS
PC-radio Serial Interface
Repeater-builder P200 modification
MT1000 (30-36MHz) test on 27MHz (yo2lmr). After HW alignment the RX sensitivity was more better and TX output ~5-6W
