Broken a YYT T668 Talkie

I carry this little talkie back to home from Tokoy. It works while i got from Akihabara. 

Receiving audio companie with motorboating  

It's very small, good toy while riding with familly.  it works initailly and became creayze after awhile. Reviced audio became very flur, sound like low frequncy oscillation. It should be one kind of motoboating. 

It's very easy, i shoud recap it, sound reasonable. (keep reading!)

AF PA enable transistor burned

recaping with MLCC is kind of easy, except it's very small caps and resistors. recaping seems not working very well. I became crazy to suspect it's MC3361 filter,  and quad detector problem. replace all of them, and even the MC3361 IC. 

Very sadly the switch transistor, responsible chip-dsiable the audio PA, is very hot, i Desolder it.


Things became strange

After i do everything, escpecially repalce MC3361, the straning things happens. It always in receiving status, it's always mute-off (signaling received!).  MC3361 did not working! it's broken? this redio use PIN14 as muting siganl, it's always stay in HIGH level. MC3361 broken?

desodering the MC3361, PIN14, to CPU is stil HIGH, CPU broken? 

Oscillator 

while debuging, i found one signal run into oscialltor module, it's very strange. there is no dependence between the oscilator and receive IF system. i then open the oscillator and checking what happens.


the red line marked 3 runing PCB back and go into ocillating module. the maked 3 red line connected MC3361's Quad detecor and other stuff.  I can't figure out why oscillator needs IF system's something.


IF MC3661 Mute detection logic

It's time to checking how MC3361 detecting the RX siganl and given the mute indicator. 
Pin9  is AF output, its send back to PIN10 is another AF filter amplifer,  it amplify AF, output from pin11 then detected by a external diode get the "true or false" RSSI, the mute indicate signal.

I't then became as a DC siganl, send to PIN12, it's a  logical cell with Hysteresi . 

PIN 13 given High(HIGH asserting) indicating siganl Received. PIN14, it's a BJT but use collector as emimiter, get a very low gain amplifer,  given Low (negtive asserting) indicate siganl rxed( Mute OFF).

then it's obviously, CPU is not dead, it's the MC336 did not pull it's PIN14 down while not aserting the mute off. but why?

MC336 SQL siganling Power

I had another working radio.  on board mesuring telling me some exciting news.   the pcb trace whiche marked 3, is rx power supply， RXV, being routing to quad detector's one PIN.

The black block in red square is the quad detecor, it's one trace routing the RX supply to MC3361, it's broken, maybe, it's already broken while i  first time  desoldering it!

Today, I broken a YYT 668, it maybe just need replce the battery!!!!