A fellow Ham recently bought the QYT KT8900, a small cheap chinese radio. After few months he had no sound at all, not even menu beeps. dead.
I decided to ask him for the radio and try using it for my Allstar node, that because my Baofeng 888 power is around 1 Watt and I’ve long coax cable running to the antenna which cause high loss.
After opening it up, I found that the LA4225 audio amp is doing its job and I could see the signal coming from the speaker terminals on my oscilloscope.
Then I tried to solder another speaker and it worked!
PTT and audio
Finding the PTT was easy because it’s coming off the mic RJ45 connector in the front. The Mic input is on pin 3 The PTT is pin 5 The GND is pin 6
I took the audio from the volume knob:
When you open the front panel, you’ll find 3 connections for the volume knob, the right one (close to the plastic case) is what you want.
COR (or COS) was the most difficult to find.
After hours of probing (no datasheet for this radio) I found it on the main IC. When the front panel facing you, it’s the 5th pin from the bottom on the right side of the IC.
It’s the yellow wire on the picture.
The orange wire is a small mod I added for getting discriminator from the radio. It’s not necessary for Allstar.
You can find the mod Here.
The red and black wires coming from the left (over the 1000 cap) are connected to small fan I salvaged from an old graphics card and placed in the groove of the internal speaker.
Now you have PTT, GND, MIC IN from the mic connector. AUDIO OUT from the front panel and COS from the IC.
Connect it to your URI and you’re done.
P.S remember that it’s chinese radio and you don’t want to use it on high power. duty cycle issues.
I decided to leave the mic RJ45 and route wires from the front panel through the mounting hole in the side.
Prepared DB25 cable and now it’s connected.
Next, i’ve added 120mm fan on the side (Sleeve bearing should work better when mounted vertical) and attached the fan thermistor to the chassis of the radio.
I just need to build small rack to hold the radio and the fan steady.