By Matt Erickson KK5DR

 

The topic has been raised that the IC-756ProII "runs hot". I feel this is mostly "perception", not reality.

 

Here I provide "real" data I gathered.

 

These are the test parameters:

The radio and power supply have been operating in receive mode for at least 3 hours in a room with an average temperature of 83 degrees F. / 28 C

The test instrument used; Extech IR201 precision Infra-Red temperature scanner.

Measurements were taken for 10 seconds at a distance of 1inch. The maximum reading was then recorded.

 

(See the list below for readings)

 

 

IC-756ProII readings;

(1)   Display face  = 91 F/33 C.

(2)   Top of display, directly into the vents on the top cover = 111 F/ 44 C.

(3)   Below display, bottom cover = 93 F/ 34 C.

(4)   Left side panel = 96 F/ 36 C.

(5)   Right side panel = 96 F/ 36 C.

(6)   Rear apron directly into PA section vents = 110 F/ 43.5 C.

 

PS-125 power supply readings;

(1)   Top cover = 90 F/ 32.5 C.

(2)   Front face panel = 87 F/ 30.5 C.

(3)   Rear exhaust vents = 86 F/ 30 C.

(4)   Right side panel = 90 F/ 32.5 C.

(5)   Left side panel = 90 F/ 32.5 C.

 

These readings will vary with room temperature, and the amount of transmit time placed on the units.

 

The Pro II service manual states that the maximum temperature that the PA section is allowed to reach at any time is; 176 F/ 80 C.

This figure is measured by a thermistor on the body of the final PA transistors. This does NOT mean that the air temperature will reach this level, since air has roughly a 50% heat transfer efficiency rating. Nor does this indicate the transistor "junction temperature" either.

 

The Pro II cooling fan begins operating at low speed when the thermistor temp reaches 140 F/ 60 C.

When the thermistor reaches 158 F/ 70 C, the fan accelerates to high speed.  

If the thermistor reaches 176 F/ 80 C, the PA is shut down until the temperature is reduced to 158 F/ 70 C.

 

Unless you operate a great deal of full power (100 watts output) RTTY, in a very hot room, don't expect to see that kind temperature. High output SWR levels can not cause the PA to overheat, due to the action of the SWR protective systems reduction of output RF when a high SWR is present.

 

Most recently, I measured the temps on my Pro III, they are virtually identical to those of the Pro II.

 

Design:

Thermal-budget and design; The design of the chassis of the 756 series radios is not new, it was used in Germany during WWII, it is a sound engineering design. The way in which it works is a bit of a mystery to some hams now days.

It is far more efficient to couple heat generating solid-state devices to the chassis/heat-sink than it is to use air flow to remove the heat produced by them alone.

In the 756 series, and some other ICOM radios, the heat created by the devices is conducted to the exterior of  the chassis where it is dissipated into the air.  The interior of the chassis reaches what can be called "Thermal-equilibrium" where the heat dissipated equals the heat generated. This also creates a thermally stable environment inside the radio which is good for the devices therein, and helps to stabilize all the systems and devices in it.

The fan is used to remove excess heat from this system during transmit or high ambient temperatures. 

I have found that the fans used in these radios, are some of the quietest I have ever experienced.  Those who complain about "excessive noise" produced by these fans appear to not have any other noise producing devices in their radio rooms, or possess "super-human" hearing.

 

I hope this article has clarified the meaning of "normal" operating temperatures.

 

 

 

Copyright © 2006 M.A. Erickson, KK5DR. All rights reserved.