Rev. A.2, 10/03 Page-ii
APPENDIX B: TROUBLESHOOTING
Note first that the DK-26 replaces older Securitron keypads: the DK-20, DK-20+ and DK-25.
The DK-26 keypad and CPU board are not compatible with any of the older units so make sure
you have both a DK-26 keypad and DK-26 CPU board. If the keypad label is missing, you can
identify a DK-26 keypad by the fact that it has three LED’s. The DK-26 circuit board is marked
with its name. Also, as you are going through these troubleshooting points, note that the goal is
to get the unit working, but if this is not possible, to identify whether the failure is in the keypad
or in the CPU board.
PROBLEM-- Unit appears dead.
First check that power has been correctly connected and use a voltmeter on the CPU board to make sure that the
correct voltage level is present. If the voltage reads very low, the problem may be that a fail safe lock being
controlled by the DK-26 is drawing too much current for the power supply. Remove the lock from the circuit. If this
restores proper voltage and operation of the DK-26, you'll have to determine if the power supply is undersized or if
there is a short circuit in the lock wiring which is pulling down the power supply.
If the DK-26 is receiving specified voltage, briefly short terminal SRC to terminal REX. You should hear the relay
click. This confirms that the CPU board is working but for some reason, it’s not reading the keys. Make sure that
the keypad cable is connected exactly as shown in Figure 2. It is fairly easy to skip a terminal when connecting the
keypad cable and also a strand of wire may jump between two terminals. Pay particular attention to the keypad
white wire going into terminal WHT. If this wire is not connected, the keypad will appear to be dead.
If the relay doesn’t operate when SRC and REX are connected, the CPU board has
either tripped one of its automatic fuses or has some major problem requiring
replacement. The DK-26 employs three special type fuses called PolySwitches.
PolySwitches look like capacitors. To identify them on the board note the drawing to the
right. When a PolySwitch senses a current overload problem, it automatically adds a
high resistance to the circuit which limits current flow to about 100 mA, thereby
protecting the circuit. Each PolySwitch protects against a particular problem and you
need to know how to determine if the PolySwitch has tripped and how to correct the
problem and reset the PolySwitch.
PolySwitch #1 comes into play only if you are powering the unit from an AC source
connected to the AC input terminals. It protects against an internal short on the board in
the components (four large diodes) that convert AC to DC. If you are powering the board with DC voltage into the
DC input terminals, you can ignore PolySwitch #1. PolySwitch #2 protects against an internal DC short circuit on
the CPU board. PolySwitch #3 protects against a short circuit in the keypad which can be caused by skinned
keypad wires or mis-wiring the keypad cable into the CPU board terminals. PolySwitch #3 will also trip if there are
short circuit problems with the SRC, REX, UCD and HCD terminals.
To check the PolySwitches, apply the probes of a voltmeter to both PolySwitch leads. Note that you have to
do this with power on the board. If you are checking PolySwitch #1, set your voltmeter to AC. Set it to DC for
PolySwitches #2 and #3. In the normal condition, the PolySwitch will be conducting current so you will read less
than one volt. A tripped PolySwitch acts as a high impedance resistor so you will read several volts across the
PolySwitch leads. If none of the three PolySwitches have tripped but the properly powered board will not operate
its relay when SRC and REX are briefly shorted together, the CPU board must be returned for replacement to the
factory.
Any time you find a tripped PolySwitch, you have to understand how to reset it. Overload current through the
PolySwitch trips it so that it clamps the current down to roughly 100 mA. The PolySwitch will continue to clamp
until all power is removed for about 5 seconds. It is not enough to correct the overload condition; you have to
depower the board for 5 seconds and the PolySwitch will reset itself.
AC IN
PS1
PS2
PS3