Coin Acceptor CH-925 Pulse
From this Amazon review:


  There are 'live' (12V), 'ground' and 'signal' wires coming out of the
  back. Live and ground are pretty self-explanatory, and need to be
  connected to a 12V supply - I bought a simple 1A DC one. The final
  wire says 'signal', and needs to be connected to your circuit as input
  into a GPIO pin in the case of a raspberry pi. It produces a pulsing
  5V signal (Why not tell us it will be 5V somewhere in the
  documentation? Luckily I have a multimeter) as a way of encoding which
  type of coin it has encountered. However, raspberry pi's only want
  3.3V signals as inputs, so you'll be wanting to make a voltage divider (With a couple of resistors - I used 33k and 18k) to safely bring that
  5V pulse down to 3.3V. (http://en.wikipedia.org/wiki/Voltage_divider)
  You'll also want to connect the ground from the device to a ground on
  your Raspberry Pi, or the signal won't work. I did this by splicing
  the ground cable from the power supply so that it has 2 ends - 1 for
  the power supply to the unit, and one for the unit to the Raspberry Pi
  Circuit.


It seems that a 5v pulse is the expected output. How are you measuring the output? You'll need a 'scope rather than a multimeter measuring voltage...

Anyway, I would not try to 'amplify' the signal, I would look to check the connections are correct and if that's not the issue, I would return the unit as defective.