I've got an issue with my Kone Elevator phone. I snapped a photo of the pcb for reference, but the tech said that he can dial out, but can't talk to the operator. They want to replace the phone for 1k, which is insane. I think this is probably an easy fix somewhere, but don't know enough about these phone boards to know where to test.

Do you think it's something as easy as adding a new onboard mic? The mic connection spot is near that big capacitor in the middle. None of the capacitors or anything look burnt, so any advice welcome, thx!

Phone PCB
Phone Front

 

Have you checked the mic itself, looks like it on the opposite side of the board, according to the PCB.
Try injecting a signal into the two mic terminals.
Max.

 

Not yet. i am going back over to take a photo of the other side of the board later and bring my multi meter... will report back.

 

They had the elevator shut down for maintenance, so i finally got a chance to inspect further.
Phone PCB Back
Mic Closeup

I put my multimeter on it and the ohms was reading 2.028 and 2.556. I couldn't tell which was positive or negative, so i just got a reading both ways.

 

Most of those chips can be found on the internet but the chip with the tape over it is likely a UV-erasable microcontroller with proprietary software. Hopefully, the ambient UV didn't leak through the tape after all these years and start fudging up the code.

The voice-switch in lower right corner is available from Taydaelectronics.com for $2.00. I haven't check the rest but I just bought one of the voice chips so that part number looked familiar.

http://www.taydaelectronics.com/mc34118-voice-switch-speakerphone-circuit-pdip-28.html

 

If you have an audio amplifier to hand, try probing around the microphone and ground terminals, to see if you can hear it while speaking, or use an Oscilloscope..


This circuit could be of use....

https://www.google.co.uk/search?q=m...AUICCgC&biw=360&bih=512#imgrc=MCBPw3a2sVm5DM:
..