A radiation source should always work. It only can get weak, when it is older. There can be a defective detector or a defective preamplifier, if there are no output pulses.

Best way to check the PAS-2:
Connect tail pulser or a rectangular pulse generator to the input with a 1 pF capacitor in series. Frequency: about 1 kHz, amplitude: about 1V. For cleanest signal terminate the pulser signal at the preamp input:
coax cable from the pulser -- 50 Ohm to ground -- 1pF (series connected) -- input connector of the PAS-2.

You should observe bipolar output signals. If a rectangular pulser is used, there are also pulses with opposite polarity present, which can be ignored. If there are no output pulses, the preamp input is probably damaged.

An actualized manual containing circuit diagrams is in our website.