A lot of amps have this problem. How about lowering the fan speed with a
resistor then shorting it out when you go to transmit. A 555 timer IC
could
be used to keep the fan running for a while after going back to RX if
desired.
Easy enough since the PTT key line is already there on the amp. Making the
project easier, an OEM time-delay relay could be used where the delay
circuitry is built into the base of the relay and would allow the higher
speed to remain active for a while to: (1) allow extended cool-down; and (2)
eliminate repetitive mechanical relay noise in VOX and CW-QSK modes.
With a couple lines of code added, this entire timing and electronic
switching function for the blower/fan and T/R system is easily accomplished
on my S-QSK Board. In addition to RF sensing at the 100 mW level, it has
four digital input channels and eight digital output channels that are
managed with either a PIC or Arduino microcontroller. Blower control can
also be managed with a temperature sensor - or even a combination of a temp
sensor, the built-in RF sensor and/or PTT line state with a few more lines
of code. The board is described on my QRZ.com page as well as in the 2014
ARRL Handbook, adjacent to W7RY's excellent material.
Paul, W9AC
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