Electronics 2000

hi,
just been presented with a mountain piece of Imaginex train set that should make a noise when a train passes over it - not working so took it apart and inside seems to be simple.
2 x ag44 batteries producing 3v to mini circuit board with reed switch to 1 inch speaker. Checked voltage to board in is 3v ok - checked reed switch normally open and closes when magnet applied; used continuity test for this.
What confuses me is when I check voltage each side of switch when open (no magnet) there is 3v and I don't understand why. only thing I haven't checked is speaker but till I understand this voltage at switch when I expected none I don't want to take anything else apart.
Any help appreciated by confused granddad and grandson
cheers
ian

Very difficult to do fault-finding at a distance. Am I right in thinking that the loco carries a magnet which closes the reed switch when the train goes over the switch? Suggest that you look carefully for any obviously faulty components, wires broken, etc. You could always post a photo of both sides of the board which may help. Often these little sound generators have a small chip in them, and they are normally not repairable. It may come down to mending it with a new one.

thanks David,
guess I will get grandson to make the noise himself. can you help me understand why there are 3v showing at the switch ends when the switch id open though?
regards
ian

Really would have to take a look at the circuit diagram, or trace the connections through the circuit board ...
btw, grandson may well prefer to make the noise!

As the circuit has a relatively low impedance compared to the 20,000 or more Ohms per Volt of the multimeter across the switch then the switch shows the full supply voltage across it when it's open.

HTH