Gymna Pulson 330, Service Manual, Page: 26 / 49

Each time the pulse is sent, the voltage over this led is measured,
and fed to Q12.
When the led lights up, the voltage at C28 is about 2.4 V.
To conduct, the voltage at the zener must be 2.7 V, so Q12 will stay
closed.
If the cable is broken, the led will never light up, and the voltage
at C28 becomes 5V.
This is enough for D7, so Q12 will conduct..
As a result, Q13 opens and the voltage on Rpot1* becomes 5 V.
(* When the active US head is connected on Con 2. If the active head
is connected to Con 3, Rpot2 value is significant for the cable
fraction detection).
Via U15 this data is read into the microprocessor.
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