This series of tests was conducted to determine the effects of AC line voltage on regulator and power supply heating in the IC-781.   A secondary objective was to observe the cooling effects of the Sherwood Engineering cooling fan in both receive and transmit modes.


For the receive mode tests, the IC-781 was turned on and not touched further.  For the transmit mode tests, the IC-781 was turned on, adjusted for 150 watts CW output and a series of  (16) Vs was sent in CW mode at approximately 20 WPM at 150 watts output on 40 meters.  The antenna was a Bird 8251 50 ohm dummy load and the SWR was negligible. 

The temperature measuring device was a Sper Scientific model 800006 digital thermometer with two Type K thermocouples.  One thermocouple was placed inside the regulator compartment by inserting the probe about ½" into the hole left by removing the second-from-the-left top screw in the top of the regulator shroud.  The other was placed just under the front of the scope board, to sense heat generated by the power supply board.  The temperature readouts were recorded manually every minute for the duration of the tests.  The 781 was powered down between test and left to sit several hours until the probes showed the unit had cooled down. 

Line voltage was supplied by a Staco Variac model 1010N, and the voltage was measured with a BK Precision model 5380 DVM.

Test Descriptions
  1. Receive mode, 100V line voltage, Sherwood fan installed
  2. Receive mode, 124.5V line voltage, Sherwood fan installed
  3. Transmit mode, 100V line voltage, Sherwood fan installed
  4. Transmit mode, 124.5V line voltage, Sherwood fan installed
  5. Same as #1, but with stock regulator shroud installed
  6. Same as #2, but with stock regulator shroud installed
  7. Same as #3, but with stock regulator shroud installed
  8. Same as #4, but with stock regulator shroud installed
The test results were paired (1&2, 3&4, etc.) in the following graphs to better illustrate the effects of the two line voltages.   One additional chart is presented at the end to better illustrate the striking improvement offered by the Sherwood fan at the higher line voltage.
The test with the Fan was terminated after 42 minutes when the temperatures stabilized.   The temperature difference at the end of the test period would have been in the vicinity of 25 degrees F.  

Tests #1 and #2 

The black line is the Regulator compartment temperature at 100V and the red line is the corresponding Power Supply probe temperature.  The yellow and green lines are the Regulator and Power Supply temperatures for 124.5V.  Note that the starting temperatures are about 5 degrees higher for the 124.5V test,  resulting from a sudden weather change that caused a rise in the ambient temperature.  I think it's safe to conclude that when using the Sherwood fan in receive mode, the line voltage makes little or no difference in the heat build-up.

Tests #7 and #8

Except for a difference in ambient temperature when the test began, there was little difference in observed temperatures between the two line voltages.  The 124.5V test was stopped at 42 minutes as the temperatures had stabilized.  The 100V regulator temperature continued to drift very slowly upwards after the test period was over.  I can't explain the lower trend of the 124.5V Power Supply temperature vs. the 100V Power Supply temperature.  The 781's PA fan did not come on during this test.
Tests #3 and #4
Tests #5 and #6
The sudden drop in temperatures seen in this chart is the point where the PA cooling fan turned on.  Under almost identical ambient temperatures, the temperature rose to the PA fan turn-on point a full 10 minutes sooner with 124.5V power.  The Regulator and Power Supply temperatures for the 124.5 volt case both continued to drift upwards very slowly after the 60 minute test period.   A later chart will compare the temperature difference between using a cooling fan or not with 124.5V line voltage.  I was surprised to find that the PA fan ran in receive mode at all.  The speed of the fan was clearly much lower than the level it runs at in transmit mode.  I was also surprised at the sudden effect the fan had on the regulator compartment temperature.  Careful investigation later showed that there's a small opening for air to flow from the main cooling fan into the regulator compartment.  The air flow is obviously inadequate, since most of the heat damage to my regulator board was at the far right end of the regulator board, away from the air inlet. 
While the 124.5V line voltage produced higher temperatures without a cooling fan than did the 100V case, the maximum difference is seen here to be only approximately 5 degreesNote that the PA fan runs continuously when in transmit mode, so the sudden drop seen in the receive cases when the PA fan comes on is not seen here.
The following chart shows the improvement made by the Sherwood fan at a line voltage of 124.5V.

The placement of the temperature probes for these tests was such that air temperature was being measured.  It would  have been more interesting if I had secured one of the temperature probes directly to a regulator circuit board hot spot.  However, the method used does accurately reflect the relative impact of the line voltage and the Sherwood cooling fan.  If I had it to do over again, I'd also relocate the probe that measured the power supply to a point near the rear of the PI board instead of at the front.  In the front position, I think the probe was cooled a bit by fresh air coming into the 781, and so some temperature rise in the PI board area was probably not measured as accurately as it could have been. 

Clearly, using the Sherwood cooling fan on the regulator compartment results in substantially reduced regulator compartment temperatures and should be the first thing you do to get the heat problem under control.  While running the 781 at a line voltage of 100V had much less impact upon the temperatures seen, it does make a difference and if your line voltage is as high as mine is, you should consider using a Variac to lower it.  The 781 runs quite well at 100V and I've not noticed any differences in operation at the lower voltage.

73, Floyd - K8AC
Angier, North Carolina

100V vs. 124.5V Power - Receive Mode - No Fan