This radio was purchased December 2002 on Ebay. Physically the radio was in pretty good shape but the lettering on the keys was almost worn away and the speaker sounded terrible. I used it like this for a few years but ordered a new speaker from Kiwa Electronics and replaced the old speaker; apparently this is a common problem with these units. I took the speaker apart and found corrosion and flaking of the magnet which rubbed against the coil of the speaker causing the distortion. The radio operated very well but I didn't like the muting action when tuning the bands and the tuning detent was a little annoying. After researching the radio on the web I found several modifications including the new white LED mod so I decided to apply them to my unit. I've posted pictures of my mods below. The owner's manual from the Radio Shack website does not include any of the diagrams so I've created a PDF scan of my manual for downloading (approx. 9 MB). After installing the mods I added new lettering to some of the buttons using "dry transfer lettering" which works fairly well.
Disassembling note: 5 screws hold the back in place but after the screws are removed the radio will not come apart very easily. This is due to some very strong plastic clips built into the front and back covers. Some of these may break when you try to lift the back off. Once the back was off I just broke all the clips off because they are such a pain when trying to take the radio apart. The screws hold everything in place very well and the clips are not needed. The antenna connection just unplugs from the analog board but the speaker and piezo element will need to be unsoldered. The connectors between the two boards need a fair amount of force to remove. Before reassembling the radio carefully align the side switches with the plastic slide covers . These switches are fragil and will break if not aligned (don't ask me how I know); That's my next mod, uh, repair!
The mods I installed are 1) removing the mute function 2) removing the tuning detent and 3) replacing the green LEDs with white LEDs.
1) I've read a couple of different ways to do the mute mod but I feel the correct way is to just remove the mute wire connection. How that's done is a matter of choice. I decided to just unsolder the pin and pull it out completely from connector CN2. Some suggest unsolder it and leaving it in place but the pin will still be close enough to make contact so I just removed it. You could also cut the wire leading to this pin but use heat-shrink tubing or hot glue over the open end of the wire to prevent shorts.
2) Check the pictures below to see the tuning detent details. After unsoldering it from the board, bend the four tabs straight out to open it up. There is a small piece of brass that surrounds the tuning shaft with a small detent on it. All you have to do is flatten it out with a pair of needle nose flat pliers or flatten just slightly if you want a slight detent feel. When I had mine apart I could see some small pits of plastic floating around inside. I had notice that the radio was tuning strangely before I took it apart - going forward a few KHz then going back a few KHz even though I was always tuning forward. After a good cleaning and applying a new layer of silicon grease to the contacts I've had no issues with this problem.
3) I had some white LEDs laying around from a 3-LED light that I was experimenting with; so I decided to replace the green LEDs. Some users have experienced low or no brightness with white LEDs but the ones I have worked with no other modification; in fact they were a little too bright so I added a 220 Ohm, 1/4 watt dropping resistor and it's perfect - drawing about 8 milliamps. The LEDs are wired in series with the voltage coming in on the left as you face the LCD into diode D323 (observe proper polarity with the LEDs or they will not work). I added the 1/4 watt resistor on this side in series with the LED. The LEDs I had were on the large size so I trimmed them down with my Dremel tool so they would snug up against the clear plastic bezel.
After giving it a test drive I just love the way these mods make the DX-398 so much more user-friendly. Tuning is feather light. Also, now in the "seek mode" you can hear the audio while it seeks. I noticed now that the radio was passing up weak but readable signals. If you hear a signal get passed in seek mode just press the UP or DOWN arrow to stop the seek and fine-tune the freq. There is noise when switching modes but the tuning is fairly quiet except for a couple of places around 27.021 and 27.471 MHz (each about 5 KHz wide). The signal pegs like a strong signal in these places so this could be new birdies. The birdies mentioned in the user manual are not heard at all on my 398. There may be others but I've not gone through every band completely.
When testing I noticed that the speaker needs to be connected if you use stereo headphones or you will loose one channel. That had me going for a while.
These are the dry transfer lettering kits that I have been using since the 70's.
CPU board (back)
CPU board (front)
Analog board (back)
Analog board (front)
Location of connector CN2 containing the mute pin.
Location of connector CN1 which is not used in these mods.
Rotary tuning switch as removed from the CPU board.
Rotary switch exploded view.
Inside the rotary switch showing plastic debris.
Tuning detent location.
Testing the LEDs before reassembly.
Close-up of the dropping resistor which bends under the board for a connection on the solder pad.