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Pro-120 Mk1 History

This page refers to the very early Mark 1 Pro-120 which is now too old for factory repair.

Warning: this listing was originally generated for 4QD internal use only, so is not as fully detailed as it should be if it were intended for general circulation. Also 4QD’s policy is one of continual improvement, so changes tend to be small and frequent. Also changes usually first appear as a board modification on an the earlier board issue. This makes change history difficult to document fully, but it does improve the product. The history is however included here as it may be useful! No liability is accepted for the accuracy of the information in it!

The ‘Bare Board is 4QD’s part number 30-3B. This is etched in the copper of the circuit board, with the issue number, in the format 30-3B-08, where the last -08 is the Issue number.


The Pro-120 Mk 1 controller became Pro-120 Mk 2 in 2003. Mark 1 boards are now  too old to qualify for factory service.

  • Issue 1
    Board Part number 30-3B-01 28 November 1995. This was a small prototype run only.
  • Issue 2
    Board Part number 30-3B-02 5 April 1996. First production run.
  • Issue 3
    Board Part number 30-3B-03 6 August 1996

    • Battery discharge circuit fitted (not used)
    • High Pedal LockOut (HPLO) fitted
  • Issue 4
    Board Part number 30-3B-04 10 June 1997

    • Battery discharge fitted to 24v versions, not on 12v versions.
    • HPLO fitted properly: issue 3 needed a modification!
    • Hiside c/l override – 330K added. Without this, sometimes the circuit could stay latched in when the reversing relays had de-powered.
    • Hiside pullup PNP transistor removed: space for it was allowed initially but never used.
  • Issue 5
    Board Part number 30-3B-05 27 March 1998

    • Reversing threshold altered. Reversing input resistors changed from 100K and 10K to 10K and 1K5. Some early boards had shown problems with humidity. Threshold was altered and the impedance lowered.
    • Auto power down option fitted. This is a manufacturing option.
  • Issue 6
    Board Part number 30-3B-06 19 March 1999

    • Power resistors added in relay coils, to simplify 36v and 48 v versions.
    • 12v zener mod added. See Pro, Scoota and NCC: Internal power supply and protection circuitry
  • Issue 7
    Board Part number 30-3B-07 19 May 1999

    • Pinstrips for Undervoltage and Symmetrical rev. These are now engaged or disengaged by fitting or removing a header on the pinstrip.
  • Issue 8
    Board Part number 30-3B-08 15 October 1999

    • Diode added for pot fault input protection. Two customers had blown pot fault detector transistors on more than one controller by wiring faults at the input!
    • 10n ceramic on motor output. To help protect against motor commutator noise.
    • Transistor leg spacing reduced: to help assembly.
  • Issue 9
    Board Part number 30-3B-09 18 October 2000

    • Diode option added on power relay. See Pro, Scoota and NCC: Internal power supply and protection circuitry
    • Undervoltage cutback reduced to around 16v: can be increased with external resistor in ignition circuitry.
    • Reverse acceptance threshold increased, was 200mv, now 500mV.
    • Expansion connector changed to enable it to be used for Tachogenerator feedback board.
    • Parking Brake fault detection being added.
      This is intended to make the controller switch itself off if the parking brake gets shorted by faulty wiring. There’s little room on the board to put in as much protection as we would like but by using a little lateral thinking we have squeezed in a clever fault detection circuit. Unfortunately it doesn’t work quite as expected, so will not used until a small modification is made to the HPLO circuit.
  • Issue 10
    Board Part number 30-3B-10 16 May 2001

    • Noise suppression cap added to HPLO. Testing the brake driver current limit showed the HPLO to be oversensitive to noise. This has not shown up in the field, but switching off an inductive brake coil (to bring in the resistors to check the over current protection) causes the HPLO to switch off the relay. So we couldn’t test the feature! As it is only very occasionally there’s a fault in a customer’s parking brake wiring, the over current protection is really for our own benefit, to save an occasional repair, so wasn’t fitted on issue 9.
    • Power supply current source increased: 22R changed to 18R. The only reason for keeping the lower value was so the controller could be used on 24v or 36v, but we sell so few 36v this is not useful. The 48v version already has a TO220 transistor fitted. See Pro, Scoota and NCC series controllers. Internal power supply and protection circuitry for more.
  • Issue 11
    Board Part number 30-3B-11 Dec 2001

    • Tr6 (the transistor used as a power dropper in the current source) is now a TO220 type (power transistor): this is required for the 36v version, which is becoming more popular.
    • Also, about this time, the mounting holes in the heatsink are increased from M3 to M4, in response to popular opinion.
  • Issue 12
    Board Part number 30-3B-12 Feb 2002

      Minor changes only, to do with track clearances.

There is no issue 13: in early 2003, that became the Mk 2 Pro-120