Sangean ATS-909/Radio Shack DX-398 FAQ: Modifications

Taking it apart

The following modifications are courtesy of Rick. <rickw999@aol.com> and are reproduced with permission from his web site.

Unplug the power, antenna, headphone and tape connections.
Remove the volume, manual tuning and RF gain knobs by pulling them straight off away from the radio. Be careful not to bend the shafts.
Move the whip antenna out of the clipped holding position so that the small screw can be removed from the antenna compartment. Use a small Phillips head screwdriver for this.
On the back of the radio, there are five holes with arrows pointing towards each screw. Remove these with a medium tip Phillips screwdriver. Put the whip antenna back in its holding slot.
Now is the sticky part: apply pressure to the two radio halves at the top and the bottom so that the tabs inside will unlock. If a tab does not unlock, a small piece of plastic holding the tab will break; it can be glued back in place but the radio can be reassembled without it. Lift up and pull the back of the radio up and toward the top of the radio as you work pulling the bottom half away going left toward the volume knob. This is somewhat awkward but can be done right, if done slowly.
Now, if you push in between the volume and tuning control with light pressure the left side should come loose at the next tab. Now push the back up toward the top of the radio a tab in the top left should come free as well as a tab at the top right of the radio. The last tab is at the center of the external connections for power, headphone, rec and antenna jacks and hinges at this plate.
The back can now be separated from the radio but be careful--the antenna is connected to the back with about 2-1/2" of white wire. The antenna may be disconnected by removing the radio assemble from the front cover and pulling the antenna connector from the board, but be careful with the red and white speaker wires.

Anti-chuffing Modification

Once the back is off, look down on the left side of the board under the tuning knob. Use a magnifier to locate from top to bottom, the surface-mount parts, 224, 334 and 474 and 103 (the 474 and 103 are side by side). Directly below 474 and 103 is a "round" surface mount diode, orange with a black band at the top, soldered vertically onto the board. Next to that diode to the right are 2 leads with solder. The leads are between the round diode and the single leg of a surface mount transistor to the right of the leads.
After the leads have been located, carefully power up the radio using the external power supply. Push the up and down tuning and see if the mute goes away when shorting these 2 leads with a screwdriver and see that the correct leads have been located. If this is correct, solder the 2 leads together to disable the muting. You could also install a micro SPST switch if you like. It's surface-mount, so be careful not to overheat when you solder it.

Be advised that this allows all PLL tuning noises to come through while using the radio; to many, this is preferable to the chuff noises. When the freewheel tuning knob mod is also performed, it works much like an old analog radio and greatly enhances the enjoyment and usability of the radio.

Removing the beep

With the radio still disassembled, carefully pull the radio assembly out of the front half of the radio by pulling it up and out. There are no more tabs or screws in the top half. The speaker wires are red and white and only 3" long, so be careful when you lay the front section of the radio aside.
Now with a small Phillips screwdriver remove 2 small black screws from the display board. 1 screw is near the time set switch and the other is near the speaker cavity. Now, there are 3 tabs on the bottom under the keypad and 2 on top of the display board just above the LCD. Carefully pull the top and bottom tabs back one at a time, lifting the board with each tab gently until the board is free.
The wires to the "beep speaker" are a fine gauge red and black wire soldered to the display near the left side of the LCD. To disable the beep, disconnect the wires or remove the speaker completely, which is held by one small screw.

Removing the detent on the tuning knob

There are 6 solder pads on the tuning knob near the LCD display light at the left and just below the power button. Using solder wick or a desoldering tool, remove all the solder from the 4 inside pads first. Then remove all of the solder, from the 2 outside pads with the "ears", or ground of the tuning control. If the solder has been removed, then the control can now be removed from the board.
With a fine flat blade screwdriver or knife blade, carefully lift the four tabs on the back of the tuning control. Use a pair of needle nose pliers to straighten them up. When taking the control apart, note how it's assembled.
Carefully remove the plastic control housing and post from the metal shell at the same time. Inside the metal shell with "ears" is a copper ring. Remove the ring, after noting how it's assembled. The copper ring has 2 bends and a notch that sticks up on one side on the center of a bend. This, along with the plastic post is what causes the tuning knob to be detented, working in a clicking motion.
Using needle nose pliers, gently flatten the notch that sticks up on one side of the ring without smashing the curves in the ring flat. Then re-assemble the control and try turning it in your fingers, holding the assembly together. Make any necessary adjustments to the control before you re-solder it back into the board. If you need to, check it with the tuning knob on the control first. The stiffness of the control is somewhat controlled by how the knob is pushed up to the surface of the radio.
After the control has been put back together gently flatten the 4 tabs back into place using the needle nose pliers. Re-solder the control into place on the display board. Check your work and power up the radio to check the manual tuning. You should now be able to tune much easily, possibly with only one finger on the tuning knob. This mod works great!

Finer tuning for single sideband signals

This mod adds a much finer tuning resolution for the slow setting on sideband. I can pretty much zero beat every signal. You will lose the tuning range, but will gain a finer resolution.

Referring to the schematic, attach a 4.5K resistor across the left/bottom and the right solder posts on SVR5. SVR5 is the 48 Hz tuning for the sideband.

This modification does not work for all side band operation. If a signal is too far off zero-beat, the signal will drop off. If that happens, just switch to the fast setting.

Improving the AGC

This mod provided courtesy of Ingo Truppel <itruppel@atb-potsdam.de>. Contact him if you have any questions.

Big capacitors are responsible for the slow AGC and they used big capacitors because of the high slope gain of the AGC control circuit.

Now to the changes:

C57 on Pin15 of IC1 (TA7758P) should be changed from 22μF to 10μF

An additional resistor with a value of 18kOhm has to be placed between Pin15 and Pin17 of IC1 (TA7758P)

C11 should be checked--the scheme needs a value of 4.7μF--my radio contained a capacitor with 10μF. 4.7μF is the right value

the upper trace to the + terminal of C11 should be cut, so that it is isolated from the circuit. An additional resistor with a value of 4.7 kOhms has to be inserted connecting the + terminal of C11 to the base of Q53. In other words, use a serial connection of 4.7kOhm and 4.7µF instead of 4.7μF for C11

A further change that should be done is to shunt the resistor R7 (68 Ohm) from the lower point of the Gain poti VR1 to be zero. That gives you a control of the input sensitivity down to zero. This is very helpful to receive strong signals especially if you use an external antenna.

Putting it Back Together

Carefully re-install the LCD display board into the tabs, they should snap into place when the board is lined up. Remember to put the 2 small black screws back that hold the display board. If the speaker wires were removed, reconnect them with the correct polarity and check the antenna connector. Make sure the LCD screen and plastic faceplate inside the front cover are clean.
Lay the radio assembly back into the front cover checking the speaker wires for pinching and the alignment of the time-set and MW step buttons, along with the battery compartment ribbon. When putting the back cover on, the switch covers should line up with the switches and the connector holes should line up with the connectors on the radio before dropping the cover in place as well as all the connectors at the side of the radio. Remember to pull the battery ribbon up through the hole.
Put the four long screws at the back of the radio in place. Then move the antenna out of the clipped holding position so that the small screw can be replaced back into the antenna compartment. Put the antenna back and reinstall the batteries if used.
Check that all the buttons and switches are functioning properly. Reinstall the volume, manual tuning and AM-RF gain knobs. Reposition the tuning knob in or out of the case to adjust the tension to the control. That's it! You now have a new radio.

Restoring 76 MHz FM band

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).

Procedure:

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 the previous section on disassembly 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 metal needed.

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:

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.

Test

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.

Reassemble

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

Other Diode Mods

There are a total of six extra diodes that can be installed, according to a small note on the schematic.

D322  FM down heterodyne
D316  FM step 50 KHz
D310  FM 76-108 MHz                
D314  FM 64-74, 87.5-108 MHz

D311 and D312 are used to control the AM coverage. The factory default is both diodes absent which gives the widest coverage:

LW         MW         SW
153-519    520-1710   1.711-29.999

The only other configuration that gives almost full AM coverage is adding diode D311 which changes this to:

LW         MW         SW
153-519    530-1602   1.603-29.999

So adding diode D311 will shift some freqs from MW into SW. And chop off the lowest 10 kHz of MW coverage. other diode configurations reduce the AM coverage even more.

-- Jim Bowlin

Using a Sony 7600 Speaker

Peter writes, on the 909's speaker:

One thing I noticed when I was pulling apart the 398, though, was that they'd clearly intended to use a much better speaker than the one they ended up using. The 7600 speaker turns out to be (likely) what they were planning - they left enough knockout in the circuit boards for a speaker with a 1 3/4" magnet, which just happens to be the 7600 magnet.

I called sony after figuring that one out and mailordered a replacement speaker for a 7600g; it fits perfectly into the case, and sounds a bit better to my ear than the old one. Later this week, I'll actually take the radio down and listen to it side-by-side against an unmodfied radio.

Oh, why'd I mailorder a speaker? try finding *anyone* who's actually selling a 3" round speaker these days that's thin enough to fit into the case, and yet has a better magnet than the 398....

Call Sony Parts at (800) 488-7669 or (941) 768-7669 (voice). Their fax number is (941) 768-7790.

[Just be careful of the magnetic field. A speaker with a too-large magnet may desensitize the radio, just like with the GE Superadio--DM]

Jim Bowlin's 909/398 amp/speaker match modification

This might be useful to some people and/or aliens:

Locate the power amp ICs: IC4 and IC5, they are pin SIPS located on the main PCB between the speaker hole and the batteries. Then find the combination of C188 and R204, C188 is a small beige surface mount square and R204 is a black surface mount rectangle with "333" on it. They are located next to each other vertically almost between the two ICs and horizontal just to the knob-ward side of the ICs. If the battery compartment is at the "top" then these two components are mounted vertically. The bottom connection is to a wide PC run which is at ground. The other side of the components go to a land that has a nice fat wire sticking out if it which is good for soldering to. If you touch this land with a finger or small wire, you will hear a LOUD 60 Hz hum. Solder a 2.2k resistor between this land and ground. The wide pc runs in this area are grounds. Don't do any soldering with the power on!!!

Next find C148 and R179. They look just like toe other two. They are between the first pair and the ICs. But they are not right next to each other.

Again one side of each goes to ground and the other side goes to a land that causes a LOUD hum when touched. This land also has a fat wire sticking out of it that is good to solder to. Another 2.2k resistor goes between this land and ground.

You may want to roll off the high frequency response. To do this add a cap in parallel with each resistor. A value of 0.005 uF will start the rolloff at about 15KHz. If you use polarized caps, make sure the negative side goes to ground.

IF Clipping Mod

Rick:

I have noticed clipping distortion on both of my 398's, particularily on certain MW stations. I have tried the main amp. & pre-amp reduction mods, but they didn't cure the clipping distortion. (It sounds like the speaker is breaking up.)

Last night I found a simple solution. Connect a 1K ohm resistor between IC1 pin #9? and ground. (I will try to confirm the pin# tonight when I have time to look at it more closely) It's the left side, 2nd from the bottom. IC1 is located up and a little right from the negative battery terminal. It also cured the distortion present when plugging in headphones in the line out jack. I'm not exactly sure what it reduced. I think it has something to do with the IF. Any users out there who could confirm?