DX-398/ATS-909 FM 76 MHz Mod
This modification extends the lower end of FM reception to 76 MHz.
PROS: Useful if you are planning a trip to Japan or if you want to
pick up the audio of TV channels 5 and 6. Or if you just want to see
what's down there.
CONS: There are many image signals below 88 MHz (this could be why the
extended FM is not the factory default). Most of the image signals
are from broadcasts 21.4 MHz higher. For example a broadcast at 100
MHz will also be picked up at at 78.6 MHz.
TOOLS: You will need a small phillips screwdriver, a soldering iron
and solder wick (or a desoldering tool).
PARTS: One 1N4148 signal diode (a 1N914 should work as well).
Open up the case
Remove the five phillips head screws from the back of the unit
(including the one hidden under the antenna) and pry the two halves of
the case apart. See http://members.aol.com/rickw999/ for details and
warnings. You don't need to remove the batteries if you can remove
the front part of the case from the rest of the unit. Be careful of
the speaker leads and the leads to the buzzer. You may want to mark
these and remove them temporarily.
Remove the smaller Control printed circuit board
Remove the two small phillips head screws holding the board. Release
the five plastic hooks at the top and bottom edges of the board. Lift
the board out. I found it to be easiest to disconnect the the two
coax wires via the connector at the main PCB. This allows the control
PCB to "hinge" on the remaining wires and lie flat in front of the
rest of the unit. With the back of the control PCB facing upward.
Remove the metal shield
Most of the back of the control PCB is covered by a metal shield. If
the shield has a bunch of holes in it, you are on the wrong side. It
is soldered to the PCB at about 10 locations spaced around its edges.
Remove the tape that holds some wires to the shield. Unsolder all the
connections on the edge of the shield. Make sure you don't melt any
plastic or wires during this process, especially when working on the
edge that is closest to the body of the unit. After the connections
are unsoldered the shield lifts right out. There is no bending of
Install the diode
The diode to install is called D310 but there are no markings on this
side of the PCB. For orientation, the multi-wire connectors are at
the bottom of the board, and the large uP chip is up near the top of
the board. Look for three diodes lined up side by side halfway down
the board, and then look for four empty solder pads just to the left
of the diodes and down slightly:
| | | |
| | | |
| | ------
four pads -> o o I I I <- three diodes
-> o o
^ add new diode here
Add the new diode as indicated. It should be mounted in the same
direction and orientation as the three nearby diodes. It is a
surface mount connection, but I had no trouble using a regular diode
by making its leads short and bending them to fit the pads.
Before closing up the unit you can hold the control PCB board
vertically and reattach the connector that has the two coax wires
going to the main PCB. Power the unit up and see if your FM range now
goes down to 76 MHz.
Solder the shield back in place. Retape the coax wires to the shield.
Insert the control PCB back into the unit. Be sure that the wires
between the two PCBs don't prevent the board from seating properly.
Snap the five plastic hooks in place. Remember to put back the two
small screws that hold the control PCB. Reattach the buzzer and
speaker if you have disconnected them. Carefully put the two halves
of the case together. Make sure all the slide switches are working
properly. Put the five screws that hold the halves together back in.
That's it. Again, check out http://members.aol.com/rickw999/ for more
details regarding opening up and closing up the case.
-- Jim Bowlin April 10, 1998