IC dilemma- Please help on DTMF

[vinodquilon]

Pls find the pdf attachment ('CI-2-DTMF').
After setting the circuit (using mt8870) in differential configuration, when I switch on the circuit LEDs 1,2,3, & 4 are all suddenly ON without detecting ring volt and key press. And remain in the same ON state after pressing * and then decimal keys from the telephone keypad on caller end.
So, I am tempted to test in single-ended configuration (see second attached file 'dtmf'). I manually short R18 to accept remote calls. But same problem arises.

 

[audioguru]

Your circuit is missing protection zener diodes at its input so the 90V ringing or 50V on-hook voltages probably destroyed your IC.

 

[vinodquilon]

Your circuit is missing protection zener diodes at its input so the 90V ringing or 50V on-hook voltages probably destroyed your IC.

But what about the case with differential configuration check. Because of its failure I went for single-ended check.
If any problems in line coupling section does turns on LEDs...

 

[vinodquilon]

DTMF IC dilemma solved

Pls find the pdf attachment ('CI-2-DTMF').
After setting the circuit (using mt8870) in differential configuration, when I switch on the circuit LEDs 1,2,3, & 4 are all suddenly ON without detecting ring volt and key press. And remain in the same ON state after pressing * and then decimal keys from the telephone keypad on caller end.
So, I am tempted to test in single-ended configuration (see second attached file 'dtmf'). I manually short R18 to accept remote calls. But same problem arises.

well done!!!!!!!
Ya, I have replaced with another vendors' IC (MT8870CE). Now it is working fine.But on

power-up all states are HIGH, then on pressing keys it obeys the actual Truth table. I think

it wouldn't be a bottleneck for my project as I feed the DTMF IC outputs to a decoder. I can

utilize the decoder outs in any way as I desired.
Thanks for all support by
Siong Boon
**broken link removed**