As a part of the internal switched
power supply IC-PS740 there is a ventilator that is mounted on
the right side of the box, from the inside. It is a bit difficult
to get PS740 todays. I already had PS35 which is almost identical
to PS740. Therefore I decided to solve the fan independently on
the internal PSU.
I made an aluminium sub-board to cover the big round hole in the chassis and enable the standard 80mm fan to be mounted on it.
The fan is fed through a small PC board. This board takes 13.8V from the REG unit and also T8 signal (+8V when the TCVR is in TX mode) from it. The fan is supplied from 13.8V and switched by a small NPN transistor controlled by T8 signal. In both leads to the fan there are small resistors and blocking capacitors. The fan rotates only when TCVR is in TX mode. The resistors decrease the voltage to some 7V which makes 12V fan rotate rather slowly and smoothly. The resistors and condensers filter any possible voltage ripples from getting to supply rail and cause audible hum in the audio output.
However even that the fan is
actually powered from 13.8V and I made every effort to filter the
leads to the fan I experienced some hum in the headphones (only
during TX, however). To my surprise the hum was cause by the
magnetic field radiated from the fan!
I tried three different 80mm fans to finally pick the quiet one:
Schematics diagram of IC-PS740 PSU unit
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