[ICOM] Fan on 756PRO

[Adam Farson]

Hi John,

There is a thermistor circuit which will keep the fan running even after the
transmitter is unkeyed, when the PA transistors heat up. 

>From the Service Manual:

3-2-10 TEMPERATURE PROTECTION CIRCUIT
(PA UNIT)
The cooling fan (MF1) is activated while transmitting or
when the temperature of the power amplifier exceeds the
preset value. The temperature protection circuit consists of
Q10–Q13 and R50.
While transmitting, Q10 and Q12 are turned ON, and provide
a voltage to the cooling fan to rotate at medium speed.
The thermistor detects the temperature of  Q5* and activates
Q11 and Q13 to accelerate the cooling fan when the detected
temperature exceeds 70°C (158°F). The cooling fan
rotates at high speed at 80°C (176°F) or more.
The thermistor keeps the cooling fan rotating even while 
receiving until the Q5 temperature drops to 60°C (140°F) or
below. 

* Q5 [2SC5125 PA output transistor]

See also http://www.qsl.net/icom/ic756pro2/hot.html

The IC-756Pro-series radios do not heat up dangerously in receive, as long
as there is adequate air-flow around the radio case. Installation in an
enclosure without adequate air circulation is not recommended.

Cheers for now, 73,
Adam VA7OJ/AB4OJ


-----Original Message-----
From: icom-bounces at mailman.qth.net [mailto:icom-bounces at mailman.qth.net] On
Behalf Of John Geiger (NE0P)
Sent: 10 March 2006 11:20
To: icom at mailman.qth.net; icom_756pro at yahoogroups.com;
ic756pro at yahoogroups.com
Subject: [ICOM] Fan on 756PRO

I have noticed that the fan on my new acquired (love this rig) Icom 756PRO
seems to only run during transmit.  This means that if the rig is sitting in
receive only mode for awhile, it gets pretty hot.  Is there a mod to make
the fan run continuously, even during receive?  Or does the radio have a
thermal switch that will turn the fan on during receive if it reaches a
certain temperature?

73s John NE0P 



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