Taming a Hot Brick
Paul Bock, K4MSG
Hamilton, Virginia

Wednesday, July 20, 2005 10:44 PM
Paul Has Provided Additional Note On This project at:  Follow-Up Note To "Taming a Hot Brick"   and  2nd Follow-Up Note To "Taming a Hot Brick"

Click here for other articles by Paul posted to the LARG Internet Site

        Virtually everyone who uses the VHF/UHF ham bands has used or will use one of the ubiquitous “brick” power amplifiers to generate a little more “oomph” to their signals. As a general rule these amplifiers require no additional cooling during normal operation, although there are a few cases - such as the Mirage B-310-G 100-watt 2-meter mobile amplifier - where a cooling fan is built into the assembly because the heat sink is small and the amplifier may be used in a confined space. As I recently discovered, however, there are occasions when extra cooling is desirable and in some cases absolutely necessary even with a fixed-station “brick”. One such occasion is when operating meteor scatter using a digital mode like FSK441A. Transmit and receive periods are 30 seconds each and long QSOs (up to an hour) are not uncommon unless the “rocks” (meteors) are frequent and the resulting “pings” (ionized trails) allow for faster QSO completion. Under these conditions, heat buildup in the amplifier can become too much for convection cooling alone, thereby causing the amplifier’s high-temperature sensor to trip. If this happens the amplifier truly lives up to its nickname, i.e., it temporarily becomes a “brick.”

        It should be noted that most such amplifiers have a maximum rated duty cycle of 50%; that is, 50% on and 50% off. Digital meteor scatter modes are 50% duty cycle since the transmitter is “on” continuously for the full 30 seconds of transmit time, but the 30-second “off” period is simply not long enough for the amplifier heat sink to cool sufficiently before the next “on” cycle. The result is a heat build-up of the heat sink & chassis. An added downside to this heat buildup is that even if the amplifier temperature overload doesn’t trip the heat will raise the noise figure of the built-in receive preamp (if used) and render the receiving system less sensitive to weak pings.

        My first encounter with this overheating problem occurred while attempting to complete a 2-meter digital meteor scatter QSO with a distant station under conditions where the “rocks” were not that frequent. About 50 minutes into the QSO and still waiting for a signal report confirmation, I heard the tell-tale “click” of the over-temperature relay in the amplifier and saw the red light glaring from the front panel. The heat sink was almost too hot for me to touch - and I had been running the 200-watt amplifier (a TE Systems 1412G) at less than maximum rated output! Obviously, some corrective measures were required.
Radio Shack Fan with mounting hardware. Photogrpah by Paul Bock - K4MSG of Hamilton, Va.
        The owners’ manual suggested forced-air cooling of the heat sink. I had a small Radio Shack 12VDC, 33 cfm box fan (Radio Shack part no. 273-2438) and this was pressed into service. As shown in the photos, four 3-inch long, 8-32 machine screws were mounted on the fan and then covered with heat-shrink tubing. The spacing of the screw holes just happens to be the right distance to allow the four screws to fit snugly between the fins of the heat sink, albeit with the fan at a slight horizontal angle to the fore & aft line of the amplifier. The fan leads were connected to the amplifier’s accessory socket and when the amplifier’s (separate) power supply is turned on the fan runs continuously, blowing cooling air directly down on the heat sink. Power consumption of the fan is less than 2 watts, or about 160 ma of current draw.
        The difference in amplifier cooling is dramatic. With the drive level set so that the amplifier was running 170 watts output and with the fan operating continuously, 45 minutes of FSK441a operation using the meteor scatter sequence time of 30 seconds each “on” and “off” resulted in the amplifier heat sink & chassis being only slightly warm to the touch.

Radio Shack Fan on Brick - Front View. Photogrpah by Paul Bock - K4MSG of Hamilton, Va.

Radio Shack Fan on Brick - Side View. Photogrpah by Paul Bock - K4MSG of Hamilton, Va.

        The fan is not particularly noisy although it can be heard running. If desired, the installation can be modified to have the fan only come on when the amplifier is keyed, but TE Systems recommends that a time delay be used to keep the fan running for a short time after the amplifier is unkeyed. Given the short cycle time of typical digital meteor scatter operation it is hardly worthwhile to go to this extra trouble since the fan will be running almost all the time, anyway.

If you have questions please feel free to email me at [email protected].

Paul Bock, K4MSG FM19ee
QRV Meteor Scatter 50/144

Long live Randoms - only a radio and the rocks required!