From cbfsb!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!cs.utexas.edu!ut-emx!shrike!kriss Fri Jul 24 17:43:42 EDT 1992 Article: 3517 of rec.radio.amateur.packet Path: cbfsb!att!linac!pacific.mps.ohio-state.edu!zaphod.mps.ohio-state.edu!cs.utexas.edu!ut-emx!shrike!kriss From: kriss@austin.lockheed.com (R M Kriss) Newsgroups: rec.radio.amateur.packet Subject: "Cooling" Mod for the PK232 Message-ID: <1109@shrike.com> Date: 24 Jul 92 17:59:51 GMT Sender: news@austin.lockheed.com Organization: "Lockheed Austin Division, 6800 Burleson Rd, Austin, TX 78744" Lines: 66 Nntp-Posting-Host: kestrel "Cooling" Mod for the PK232 MBX by Tom, AB5FH AB5FH @ N5LJF.#AUS.TX.USA.NA 23 July 1992 (REVISED) For all PK232 MBX owners, place your hand on the top, middle, right side of the PK232 case. If it appears warm-to-hot after one hour of operation, read on. The power voltage regulator/heatsink is mounted to the circuit board below this point. This regulator is an LM317T voltage regulator capable of dissipating approximately 15 watts. In normal packet mode, this regulator sources approximately 700 milliamps with a voltage drop (across the LM317T) of approximately 6.7 volts. This results in a power dissipation for the LM317T of approximately 4.7 watts. While this is 1/3 of the capability of the LM317T, this heat is driven into the circuit board. While replacing a defective radio selector switch, I noticed that the circuit board under the heat sink was charred. This particular PK 232 MBX has been in operation approximately 3 years. A review of the schematic revealed that there was no series dropping resistor in series with the LM317T input. This resistor is used to reduce the input voltage to the LM317T which in turn reduces the voltage drop across regulator. With this voltage reduction, the power dissipation of the device is reduced. I paralleled two 10 ohm, 2 watt resistors and placed them in series with the cathode of D11. This reduced the dissipation power of the LM317T by approximately 30%. Consequenty, the entire unit runs exceptionally cooler with no degradation in performance. The heat will be dissipated by the resistors (instead of the LM317T) and then be conducted into the case. The two paralleled resistors were placed adjacent to the heat sink, and along the edge of the circuit board. These resistor should be installed so that they are at least 1/4" above the circuit board. You can make this mod from the top of the board since the circuit board trace to this resistor is on the top side and the hole feedthru is plated thru the board. Locate and lift up diode D11(large bodied diode closest to the side edge of the board near the LM317T regulator.) from the cathode side (banded end). Use a low wattage iron and wick soldlead. The resistor lead that is to be placed in that hole must be trimmed so that no more than 1/16" of lead extends from the underside of the board. If you are uncomfortable with this step, you may remove the circuit board to expose the underside of the board. Place one lead of the paralleled resistor assembly in that hole and solder in place. Be sure that the resistor pair sits above the circuit board. It helps to let the resistors lay over on their while they are soldered in place. This will force the pair to be sit above the board. Position the other lead end of the resistor pair such that it connects to the cathode (band end) of diode D11. Solder the resistor lead to the DII diode lead. Position the resistor pair so that their leads are not in contact with the LM317T heatsink or the 1000 ufd filter capacitor (C26). The resistor pair was obtained from Radio Shack (part no. 271-080 - $.99) and that was done to my PK232 MBX is optional due to its aesthetic result. A pattern of 8 holes (0.1" in dia) were drilled in the cover above the heat sink and resistor pair. With the holes drilled in the case top, there is no noticeable heat rise detected on the cover and no degradation in performance in all modes. I have operated the modified unit for over 2 weeks since the modification without disconnecting power with no degradation in performance It is also important to note that the AEA transformer supplied to power the PK 232 outputs DC voltage in excess of 17 volts unloaded. This can be checked by placing a meter on the connector end of the transformer unit while it is plugged in. When connected to the PK232, the load of the PK232 MBX drops this voltage to approximately 13 volts (nominal). If you have a newer PK232MBX (> s/n 50000 and with multicolored LED's), you will have a 7805 regulator in place of the LM317T. This fix still applies to the 7805 regulator circuit. Tom Rohrer AB5FH Packet Address: AB5FH @ N5LJF.#AUS.TX.USA.NA 2008 Key West Cove Austin, Tx. 78746 512-327-9294