Kenwood TK-762HG-1 No Key from Microphone

[vt600c]

TK-762HG-1 will not key from microphone. PTT is being received on pin 26 of IC502, approx 3 volts DC with key open, 0 volts when mic button key is pressed. The RF section is working as the radio produces high and low power when using the KPG-56D SW test mode. D16 switches to TX when running TX test using SW test mode. IC-502 VCC checks good. The SW test mode must not use IC-502 pin 26 (PTT) as the voltage remains approx 3 volts when running the TX test. Radio passes receive tests, programs as expected, all front panel buttons function, the radio just will not key when using the microphone PTT button.
What else should I check?
Thanks.

 

[kd4efm]

the 762 is a 8 channel VHF mobile.
Nothing fancy to prevent the mic jack from keying the radio.
you can short the two pins to see if it keys up without the mic.
Also which mic are you using? KMC-12, 32, 65 (8 pin) or a 6 pin?

 

[vt600c]

Thanks for the reply. The radio mic is an 8 pin. Have tried using a KMC-27B (no keypad on mic) and tried an MC-62 (illuminated keypad lit as well so mic plug does not seem to have any issues.). Also been using a Cat 5 cable to short pin 4 (PTT) to ground (pin 3). Mics have been tested, work with other KW radios. PTT input on IC502 drops from approx 3 volts to 0 volts when using mic or shorting pin 4 to pin 3.

There must be an IC502 output to drive the TX side of the transceiver but I don't know which pin, maybe the XOUT pin 11.

Spent some time this weekend to see if it would key from CN4, pin 7, external PTT (active when low). It still would not key. Changed the freq to the same as the test freq, which keys the radio with full power out, when using the SW test mode. Wanted to be sure there was not a out of band freq check preventing the radio from keying.

I'll check the schematic and trace CN4 pin 7 to see where it goes. The problem appears to be common to both key methods, external via CN4 and front mic jack.

Thanks.

 

[EWC_BDN]

Only 3V? Schematic says 4.85. Looks like it's right out of the a 5V regulator to a diode and 100ohm resistor? is IC12 putting out 5V. I doubt this matters that much. Is somthing on 5C pulling it down?

Do the front programmable keys work?

Usually I was always sus of the ribbon cable connections on these ones. I would always reseed them whenever it did something odd. I can't remember if that ever actually fixed something though....

you can read and write to this radio? (seems like it) That uses the PTT line too like it's the same pin and almost the same circuit. Maybe try a whole new codeplug?

Sorry this is super helpful, just some more things to try. Great move checking with the software test mode on the same frequency though. That's a good idea.