WOW! Almost 10 years to the day and now it's time to provide an update to the trusty old ATS-818/DX-390 mods. Thanks for sticking around with me and being very patient as I was away taking care of life's impromptu moments.
It feels like things are winding down a bit and I've been more active in the SWL hobby once again. One of the things that sparked my interest was a great opportunity to pick up a Gundig G5 in excellent condition for $30 late last year. It was one of those situations where someone knew that I was a ham radio operator and said their friend had some old radios from a friend who recently passed. While I do like the size and sensitivity of the G5, it lacks some features on the DX-390 such as an excellent speaker for great sound and tone controls. The G5 goes with me on trips where size and weight are key factors.
Back to the 818/390. This project started out as a means of improvements to an already impressive radio. I liked most of the features on the radio, but wanted to squeeze out all of the potential it had. The next enhancement that I completed on the list was replacing the dim incandescent light and adding on-demand light control. For this task, I researched the radio schematic and attempted to make heads or tails of the circuit. The 3 transistor circuit connects +6v to the lamp when the light button is shorted to ground. Capacitor C414 and C415 play a significant part in this circuit as they are responsible for triggering the transistors. Their values can be substituted with smaller or higher values to change the delay in trigger and duration. I replaced C415 and tested this theory with success. This circuit could also be rewired using the same components with some jumper wires and cutting traces to achieve my goal, but I chose to leave it as-is to avoid risk of permanent damage to the PCB.
Next, I looked at the Toshiba TC4S11F NAND gate (used in other radios) as a solution to my problem. It would work, but what about a 555 IC?
A little more digging and I stumbled across the KY002 Bi-Stable switch. Could it be that easy? Yes! The KY002 or KY002S variants use a trigger circuit to connect vcc to output 1 via a single trigger shorted to ground. The output is toggled on/off each time the momentary button is pressed. The KY002 can handle the voltage and amperage requirements for the light adequately.
Here is the radio schematic and wiring diagram to the KY002. Brown and red wires connecting the existing lamp circuit can be used to drive the KY002 and LED. This bypasses the old light circuit leaving it in place to revert it back to factory as desired. The only parts of the existing circuit in use are the light switch and ground pad of the old lamp.
The existing light switch needs to be connected to V1 of the KY002. There is a convenient thru-hole solder pad available next to R423 (104) that can be used.
Ground for the light switch can be found at the existing lamp location here. Lift up the tab that covers the old lamp. Connect this pad to GND of the KY002. Now is a good time to remove the old lamp and solder the short side of the LED to the same pad that you use for GND. I bent the leads to figure out best placement. You can use the white foam to support the led and a little hot glue to secure it.
I ran all the wires to the back of the board and attached the KY002 using double sided foam tape. Here you can see all 5 wires routed and soldered into place. I used scotch tape to hold down the wires, then reattached the metal RF shield.
The KY002 modules can be found on eBay or other Arduino parts retailers.. I paid about $2 per module.
Update: I had some requests to substitute a 555 timer circuit in place of the KY002. Here is the schematic.
So what's next for the 390? One possibility is an IF-Out mod and create a panadapter for my laptop/SDR dongle.