From: kriss@austin.lockheed.com (R M Kriss)
Newsgroups: rec.radio.amateur.packet
Subject: "Cooling" Mod for the PK232
Date: 24 Jul 92 17:59:51 GMT
Organization: "Lockheed Austin Division, 6800 Burleson Rd, Austin, TX 78744"


                    "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