Today, I ran a stress test on my IC-7700 to check PA integrity under high duty-cycle usage (heat). No failure occurred, and I saw what I assume to be an ICOM safety feature turn on.
I ran this test for 15 minutes using a 75% duty cycle (CW dashes) signal into a 50 + j0Ω dummy load. Ambient temperature was 70°F (21°C). Power output was 170W, not 200W (my choice) as measured on an LP-100A Digital Vector RF Wattmeter which has a calibration pedigree. I had the IC-7700's multimeter display on, so I could watch all parameters simultaneously. Table 1 gives the logged data points.
| Time (min) | Po (W) | ID (A) | VD (V) | Temp Ind (segments) | Fan Speed |
|---|---|---|---|---|---|
| 0 | 170 | 11 | 48 | all blue | low (1) |
| 3 | 170 | 11 | 48 | 1 gray | med (2) |
| 5 | 170 | 11 | 48 | 2nd pink* | high (3) |
| 8 | 170 | 11 | 48 | 3rd pink | high (3) |
| 12 | 170 | 11 | 48 | 4th pink | high (3) |
| 14 | 85 | 8 | 48 | 4th pink | high (3) |
| 15 | 85 | 8 | 48 | 4th pink | high (3) |
| * Pink starts after 7 gray segments. | |||||
After the test, I stopped transmitting until the temperature dropped back into the gray segments then sent dashes again. Power was back to 170W, and ID back to 11A. All other parameters remained normal.
At the highest temperature, 2 segments remained before the red zone started in the line below the segments. It is understood that the temperature indicator segments are arbitrary indicators.
It was nice seeing the radio cut back to 50% power level at the maximum temperature reached.
I used dashes (75% duty cycle) rather than RTTY (100% carrier) because I wanted to be sure the heat from the finals was getting conducted into the heat sink fast enough to give the cooling system a chance. I didn't want to overrun the heat transfer rate.
The fan appears to have a total of three speeds: low, medium and high.
Clearly the IC-7700 can not support 100% duty cycle for more than about 10-15 minutes without automatically cutting back its power output. In my test using 75% duty cycle the radio cut back its power output by 50% at some predefined temperature. I terminated the test 1-2 minutes after power cutback as I saw no reason to continue at this low output level. I'm not sure that was a good reason.
So there you have it. My radio (S/N 0201526) can tolerate the heat created by the test described above. What is not clear is why some of the reported PA failures are occurring at 50-100 watt levels.
73, Bob - W6OPO
Copyright © 2009 R. Lanning W6OPO. All rights reserved.
Editing and page creation: A. Farson VA7OJ/AB4OJ
Last revised: 09/25/19.
