MC1000 Tone Remote stuck in TX (L3213)

[maxkelley_kc2spy]

Hi all,

Have a customer who has a number of MC1000 tone remotes connected to their MTR2000, and one of them got stuck in TX. We took it back to the shop, took a look at it. Neither of the transmit "switches" (carbon contacts/pads, or whatever) on the front panel are shorted (there is no continuity from one side to the other), and when I put it into button test mode, the transmit LED is at first on, but when I hit the transmit button a few times, it responds to the button rather than being stuck on. When I exit the button test mode, the unit resets and it is stuck in transmit again. I'm not sure what it could be...

Looking at the schematic, all of those buttons are in a button matrix that's "scanned" by a 74HCT273 and 74HCT573, spanning 4 bus lines, if you will. There's various diodes and such between the various buttons that, in essence, tell the "scanbus ICs" which button is pressed. I'm thinking perhaps it's one of the diodes between the switches?

Hope someone can help!
Thanks, Max.

[dag]

Check the TX buttons for high resistance, not a short. Those popple pad drivers generally do not require a near zero resistance to activate. You may even be able to pull off the popple pad to expose the board underneath and clean the contacts with an eraser. Compare the carboned contact on the popple with the other buttons, it may also be time for a new popple pad.