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Conversion Circuit Board

Fully Integrated M/P Driven CTCSS 

   This board facilitates the conversion of GE series commercial radios for 'frequency agile' use. As such, it contains the Microprocessor (which emulates CTCSS encode / decode functionality), and the buffer transistors to interface with the 'target' radio. The circuitry is contained on a high quality, silk screened, solder masked, two sided board measuring 3 by 2.75 inches. It contains its own 12VDC to 5VDC voltage regulator (LM7805).

   Sequential construction steps are shown below along with descriptions of each of the components. Components are installed in 'groups', as shown::

Before Starting

    Before you start to 'populate' the P/C board, you might want to print out a copy of the board layout and a sketch of the jack pin numbering as you'll need to know the numbering convention to properly connect the P/C board up in your control head.  'Stuff' the board as you see fit.
Component Placement 
  • Component / Value


    Comments / Action

     28 Pin IC Socket MC908JL16CPE Ensure That ALL pins route through the board.
     JP-13 PWR Main - Pin 2 +5VDC Output Powers Logic Circuits
     C5 - 10 mfDecoupling capacitor-
     IC-2 +5 VDC RegulatorTO-220 device is required.
     X1 - 9.8304 Mhz OscM/P ClockOscillator in a 'can' - Note that the 'dot' is positioned in the upper left corner.
     D3Reverse Polarity ProtectionPlease don't omit.
     JP-1 - (KEYS)Keypad Interface8 pin terminal strip
     RN-3 - (5) 22K NetworkM/P Pull UpsStrip Resistor - Check 'dot' common side - if in doubt, verify with meter.
     RN-2 - (8) 22K NetworkM/P Pull UpsStrip Resistor - Check 'dot' common side, as above.
     RN-1 - (5) 22K NetworkM/P Pull UpsStrip Resistor Network - Check 'dot' common side, as above.
     R1 - 1KCurrent limiter-
     R2 - 1KCurrent limiterNeeded for MVS conversion only - strap out for others.
     R3 - 1KCurrent limiter-
     R4- 1KCurrent limiter-
    R6- 2.7KA/D ConverterFor CTCSS
    R7- 2.7KA/D ConverterFor CTCSS
    C1 - .1 mfA/D ConverterFor CTCSS
    C2 - .1 mfA/D ConverterFor CTCSS
     Q1 - 2N3904 (NPN)SPI Enable-
     Q2 - 2N3904 (NPN)SPI ClockNeeded for MVS Conversion only - strap base to collector if otherwise.
     Q3 - 2N3904 (NPN)SPI Data-
     Q4 - 2N3904 (NPN)Transmit-
     D4 - 1N4001Reverse Surge ProtectionProtects Q4 from an EMF 'spike' when PTT is released - DON'T OMIT
    RLY 1DPDT RelayFor PTT and CTCSS Timer Transmit / Receive Isolation
     JP-4 - (PLL)Terminal StripConnects SPI and Transmit to Radio.
     JP-2 - (LCD)Terminal StripLCD Connections
     D2 - 5.1 Volt ZenerM/P ProtectionLimits COR / CAS voltage to M/P - protects M/P - DON'T OMIT
     R10 - 1KCurrent limiterLimits current on COR (CAS) lead prior to D2 clamping action.
     JP-12 - MAINPrimary Radio InterfaceInterface connections to radio.
     Q5 - 2N3904 (NPN)PLL Unlock InhibitorBlocks Transmit Override function if radio's PLL is unlocked.
     Q6 - 2N3904 (NPN)Transmit OverrideInsures that the target radio will transmit when requested to do so  (if PLL locked)
    Q7 - 2N3904 (NPN)Receive CTCSS BufferBuffers CTCSS signal (square wave) from target radio
     C5 - .1 mfCouplingCTCSS Out
     JP-13Terminal StripTo be strapped later if the radio doesn't transmit reliably.
     JP-10Terminal StripConnects PLL Lock and Transmit Override leads from radio
     R5 - 10K PotentiometerLCD Contrast Control-
     R8 - 5K PotentiometerCTCSS Level Adj.-
     R13 - 10KCurrent limiterQ7 'pull down'
     R12 - 10KCurrent limiterLimits current flow to the base of  Q7.

    Testing the Circuit Board 

        a) Before Installing any Integrated Circuits.

        Apply power (12 VDC) to the PWR - BU (JP3 - Pin 2) and ground to the VSS lead (JP3 - Pin 1). 

        b) Connecting the LCD Display - Careful - - - the LCD is Delicate 

        Connect the 2x16 LCD display. (Any LCD with the HD44780 controller will work). Don't remove any protective plastic strip from the front until the LCD is mounted and you are ready to place the bezel / plexiglass over it.

        If not already equipped with wire wrap pinssolder wire wrap pins (short pieces of stiff wire) to each of the LCD's connection points. Don't use too much solder as this could 'contaminate' the display. These will be used to connect the LCD's inputs to the P/C board. Check for 'shorts'.

        Determine the length of the wire required to connect the LCD in your control head to the M/P board which will probably be mounted on the rear of the enclosure.  Leave enough wire for subsequent servicing.

        Connect the LCD to the P/C board using 30 gauge (wire wrap) wiring. Before applying power, check for crosses between any of the adjacent pins on the LCD display, and that the +5VDC (VCC) lead is connected to LCD pin 2, and that ground (VSS) is connected to pin 1. 

      Also, determine the value of the LCD backlight resistor. I've used resistors around 10 ohms.  Check here for additional information. Encapsulate the backlight resistor in heat shrink tubing.  

    c) Connecting the Key Pad - An Integrated Unit is a Good Choice 

        Make the 8 wire wrapped connections between the P/C board and the keypad.  Here are some suggestions on determining the correct pin-outs for those surplus / unmarked keypads..

    Making the Initial Power On Tests

        Before installing the M/P, apply power to the P/C board and verify that the LED inside the LCD lights. Adjust the contrast resistor until you see dim rectangles across the top of the first row of the LCD.

        With the power off, and following CMOS handling precautions, remove the M/P from it's protective conductive foam. Ensure that all pins are straight, and carefully insert the M/P into its socket, 'rocking' it side to side.

    Note: Ensure that the pins on the M/P are perpendicular to the IC so that they will seat properly.  One way to make this alignment is  to hold the IC pins against your workbench while aligning the IC perpendicular to it.  This aligns all the pins on one side at the same time.

    Operational Tests - For All Frequency Applications

        Turn the power back on. The LCD will briefly display the current software version (T0.02). Adjust the LCD contrast control (R5), as required. 

    Note: Turn the power off for about 30 seconds and power the board up again.  Take note of the Channel Group shown.  If it is set to 1, then nothing else need be done.  However, if the LCD shows something other than 1 (perhaps an odd character), you need to reprogram the Channel Group to 1.  You can do this by entering B and then 1, following the prompts that will be displayed.

         If the LCD displays KEY!, you've simultaneously depressed two keys. Or the wiring between the keypad and the M/P board may be incorrect. Valid keystrokes are closures between ONLY TWO LEADS, one in the row, and the other in the column group. Recheck your wiringIf you have a Key Error, use the D Key to clear it.

        Enter another frequency between 140 and 184 Mhz. For the VHF and UHF, it is not necessary to enter the leading digit (i.e, the 1 or the 4). Since only 5 khz. steps are allowed in the VHF mode setting, the M/P won't accept anything beside 0 or 5 as the last digit. If another digit is entered, the M/P will overwrite this entry to a 0, and set the frequency. Similar logic applies to 12.5 khz (UHF).

    Note: 'Stepping' the frequency up and down is done by either the * or the # keypad keys.

        Ground the Mike PTT lead and note that BAND shows on the LCD. This means that the transmission is not allowed, unless a frequency between 144 - 149 Mhz has been entered. The program allows transmissions only on authorized USA amateur frequencies.

    DISCLAIMER - - If you follow the steps outlined herein, you do so at your own risk. I cannot, nor will not, be responsible for any possible damage to radio equipment, personal property, to yourself or to others caused by modifications that you may make to the radio as a result of your reading this.

    The M/P controls TRANSMITTING as well as receiving on many frequencies, suitable for a wide range of HF, VHF and UHF needs. In the USA, TRANSMIT operation requires a license issued by the FCC  for the class of operation intended.  Amateur radio licensees must maintain strict control over their equipment, preventing unlicensed operation within the amateur bands, or outside of them.

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