Schematics
Schematics of the timer are available here:
CPU board
Display board
Fault finding
If your completed kit doesn't operate properly, here are some tips for tracking down the source of the problem:
- The timer PCB has a large copper fill area between the tracks that is connected to 0V (GND). Care must be taken not to inadvertently bridge the connection to this area when soldering. The copper fill area is coated with solder resist, but this can be chipped and expose the copper underneath. Your joints should look like the one in the picture on the left, with all the solder within the pad area. The picture on the right shows a joint with solder potentially forming a connection with the copper fill area. This joint needs to be desoldered and remade. To reduce the chance of this kind of joint, use a soldering iron with an appropriately sized tip and use as little solder as possible. Don't use the same soldering equipment you use for your plumbing.
 |  |
| Good joint | Bad joint |
- Disconnect completely from the battery. Separate the boards. Remove the CPU from its socket. Using the the pin on the board interconnector nearest the corner of the board as your GND reference (it's square and the others are round), measure the resistance between this and the other connections on the CPU board interconnector. They should all be >1MΩ relative to GND. The same should be true of the display board interconnector (again, use the square corner pin as GND reference). If any of these points show a low resistance to GND, then check the joints along the appropriate track for shorts to the copper fill area.
- With the battery and display board disconnected and the CPU removed from its socket, check the resistance between the pins of the CPU socket and GND. They should all be >1MΩ apart from pins 8 and 19. If any of these points show a low resistance to GND, then check the joints along the appropriate track for shorts to the copper fill area.
- To see if the CPU is operating, insert the CPU into its socket. Connect the CPU board to the battery. With the display board disconnected, switch the unit on (the toggle switch towards you, away from the battery connector). If you press keys on the keyboard you should hear a beep from the buzzer. If not then you have a fault on the CPU board, or a flat battery.
Once you have determined which board has the problem, you can check your joints and the orientation of the components to try and find the problem.
Digit faults
If some but not all of the digits light up then first check the orientation of the components and the solder joints. If there is still a problem then try the following; With the clock switched on, momentarily connect a piece of wire between pin 8 (+6V) of the display board interconnector to the following points (select the appropriate point corresponding to the faulty digit from the table below). Only connect the wire for a second or so as prolonged connection could damage other components on the board.
- Interconnector pin
- Base resistor top end
- Base resistor bottom end
- Transistor base (B)
- Transistor emitter (E)
- Transistor collector (C)
- CPU (U1) pin connection on PCB
- CPU (U1) pin direct to CPU pin
If the faulty digit
lights during a test, look up the possible fault from the list below. The fault will correspond to the
first test that causes the digit to light:
- Bad joint on interconnector/bad joint on CPU pin
- Bad joint on interconnector/bad joint on CPU pin
- Bad joint on interconnector/bad joint on CPU pin/bad joint on base resistor
- Bad joint on transistor base
- Bad joint on transistor emitter
- Damaged transistor, replace with any small signal PNP transistor
- CPU not seated in socket correctly/Bad connection on CPU pin
- CPU damaged, replace CPU
Email support
If you continue to have problems then send an email
with the results of the above tests to
. If you get no response from that address within 48 hours then please try
.
Don't be afraid to include photos.
