This page describes a modification which may be fitted to base-boards, issue 8 through 10. Issue 11 boards include this modification as standard. The circuit shows the new components (33K, 3K3 and 12v zener diode) circled in red.
Currently power is applied to the parking brake (to release it) when demand speed (from the input throttle device) or motor speed are not zero. The power is removed when demand speed is zero and motor speed is also zero (motor has stopped).
The brake driver circuit (below) is a two transistor stage with the first transistor (Tr1) normally biased on from the internal 12v supply via the resistor chain 100K-100K with 22K base resistor, so the second transistor (a complimentary power darlington consisting of Tr2 and Tr3) is held off.
In the stopped condition, (motor) Stop'd line is high and D(emand) Zero line is high - so the input diodes are reversed.
Either line going to zero will turn on the appropriate diode, removing the bias from Tr1 which will turn off, powering the brake.
The modification consists of adding the 33K resistor (22K or 27K is fine) and the 12v zener. A 3K3 resistor also is required.
The 33K prevents the Stop'd line going low (and powering the brake) as it is held high via the (unpowered) brake coil, so that power to the brake is only removed by the D Zero line.
When the brake is powered, no voltage is fed back via the 33K resistor, so any voltage on the Stop'd line will keep the brake powered: Demand speed has to fall to zero and motor has to stop to remove brake power.
The 3K3 defines the brake release threshold: without this, if the ignition is turned off when the brake is powered, current gets fed through the 33K, through the control board into the 12v line where it is enough to maintain some holding current in the brake!
The second drawing shows the 33K (at top) and zener diode (or right). The 3K3 is the bottom addition between 10K and 220K resistors. All three can be soldered onto existing component leads without removing the aluminium base. The 33K resistor is added between the brake driver line (emitter of power transistor - a topside track) and the 10K indicated. The zener is soldered between the anode of the diode indicated and the earth copper track where shown.
The zener is required because the Stopped line feeds some CMOS logic chips on the top board and it is imperative to clamp the line to avoid over-voltaging and destroying these chips.
There is a problem: in the top board, the demand zero line is sensed from the demand speed line, and the Stopped line is sensed from the motor voltage. Both of these are fed to error amplifiers and the source impedance of the demand speed line is such that if enough voltage is present on the motor speed to saturate the error amp, the demand speed line is pulled up enough (via the feedback network) to operate the 'demand zero' detector. This did not matter with the old brake circuit, but allowing the 'motor stopped' line to drive the 'demand zero' down removes the point of the latch!
Fortunately this is easily overcome by halving the resisors sourcing the demand speed line so the fed-back voltage is always below the sense threshold.
The diagram shows the two resistors to change: the arrowed 10K is to be changed to 4K7 and the 2K2 to 1K0.
You will need
For control board
For base board