This evening I did some testing to satisfy my own curiousity about the question of which way the fan for the TR7 should be oriented, to blow in or out of the radio. The results were interesting.
The subject was my TR7 that has been fitted with the standard Drake FA7 fan oriented to pull air out of the radio as Drake intended. Two inexpensive digital multimeters with K-type thermocouple bead style temperature probes were used to take temperature measurements. One probe was threaded into the space near the PTO under the DR7 through a gap in the chassis bottom plate. The other was placed through the vent slots in the top cover into the PA heatsink, between the fins and in contact with the heatsink in the vicinity of the final transistors. Both top and bottom covers were screwed tightly in place with all the screws. The radio was allowed a half hour warmup period without the FA7 running. Ambient temperature was measured at 27C. After a half hour warmup, the PTO area measured 41C and the PA heatsink was at 30C. THe FA7 was then powered and the rig was allowed another idle period to stabilize. Heatsink temperature declined to ambient, and the PTO area remained at 41C. The rig was then placed in CW transmit at a power output level of 150 watts at 7 Mhz, into a Bird 600 watt dummy load, for a period of 5 minutes. After 5 minutes heatsink temperature had reached 62C, while the PTO area remained at 41C. The rig was allowed a 15 minute cooldown period, after which the heatsink reached ambient and the PTO area was still at 41C. After cooldown, the FA7 direction was reversed and another idle period was observed to allow the radio to stabilize. The heatsink remained at ambient and PTO area temperature was measured at 36C. The rig was again placed in CW transmit at 150 watts for 5 minutes. Heatsink temperature reached 62C after 3 minutes, and after 5 minutes was at 70C. PTO area temperature measured 38C after 5 minutes of key-down, and peaked at 39C some 3 minutes after key-up. The PTO area temperature began to slowly decline after the heatsink had cooled to below 36 degrees, roughly another three minutes later. PTO area temperature at the end of the cooldown period was 37C. Conclusions: While blowing air into the radio does seem to keep the front area under the DR7 and near the PTO somewhat cooler, the temperature at the PA heatsink rises much more sharply and reaches higher temperatures during transmit than with the fan oriented to pull air out. There is also more variation in temperature in the PTO area as the PA heats up and cools down; although PTO area temps were slightly higher with the fan oriented to pull, they remained at a steady 41C throughout the RX/TX/RX cycle. In my opinion, the better option seems to be to orient the fan to pull air out of the radio, as I had speculated in an earlier post. This was a quick and dirty experiment, admittedly crude, and the measuring equipment used was not what I would call high-precision. I would welcome other hams to repeat this experiment, to confirm or refute my results. Let 'er rip, guys! 73 -Jim NU0C _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
