Icom IC-275A (E)

Modification of the cooling

In most TCVRs the colling fan is triggered together with PTT. This makes sence as the most of the heat is produced during the transmission. Also the noise produced by the fan is less disturbing during TX.
The IC-275 doesn't work like this, however. The fan doesn't run during TX normally. The heat is absorbed by the massive aluminium heatsink hidden under the top cover. When the heatskink reaches 50C the fan starts colling. Then the fan runs all the time, at lower speed during RX, at higher speed during TX, until the temperature falls below 50C again.
This way you can have a feeling that the TCVR is extremely quiet when you use it for occasional short QSOs over the local repeater, or something like that. If you use it for 24-hours lasting contest this behaviour is not much convenient.
I use IC-275 in very adverse conditions. Besides that I need to CQ for long tens of minutes or even hours during contests, I use it often in an ambient temperature above 30 C. I needed to make the cooling as efficient as possible.
The following modification changes the behaviour in the following manner. During RX the fan doesn't run. During TX the fan runs at high speed. After the TX -> RX transition the fan runs at a lower speed for about 7 seconds, then it switches off. To protect the transceiver if the temperature of the heatsink reaches 50 C the fan runs at high speed regardless RX/TX status.


Tiny subboard with the above schematics:
Placement of the subboard in the PA compartment of IC-275A:



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