CDM750 Modified

[nycapt7112]

Good evening all, I recently was given a few UHF R split CDM750s from a local radio shop that was closing down (owner was retiring). These seem to be modified for some kind of data, possibly for a cab service. The radios program up and receive fine but will not transmit from the handheld mic through the front mic port. I've opened one up to see what modifications were done under the hood but this is definitely over my head. I've attached some pictures of what I found, I'm hoping someone has seen/done this before and can tell me exactly what was done and how to make the radio function normally again. Thanks in advance!












print screen windows

[kb4mdz]

OK, first look at what pins in the Accessory connector are used; if it's acting as a 'black box', you can still guess what is Mic Audio, PTT, RX Audio, maybe a COR, from the connector. The last shot with the wires soldered to the inside of the accessory connector can help you deduce something, too. Figure out what pin on the mic. jack is being tapped with the white wire, and trace it down. Let us know what you find.

[jackhackett]

From what I can see...
The white wire goes to pin 6 of the mic jack, which is PTT.
Grey goes to the input of a transistor that switches PTT, this point normally connects to pin 6 of the mic jack.
The green wire connects to a point in the microphone circuit.

I would guess that there is a cut trace between mic pin 6 and the point the grey wire is connected to, to allow whatever device was attached to control PTT.
The green wire most likely injects a signal into the mic audio, probably some sort of signalling. You can probably carefully remove it.

Connecting white and grey together should give you PTT from the microphone, or you could find and repair the cut trace.

They robbed a few pins on the accessory jack to bring the wires out to an external unit, if you want it back like stock you may need to replace the jack. If the pins they used weren't anything important for your needs you could just disconnect the wires.

[nycapt7112]

Your advice did it Jack, thank you very much! The radios now work great. Thanks again!