Cobra 142gtl /Tram D300/washington base talkback problem????

[speedracer6g]

Working on this tram d300 solid state radio same as the cobra 142 or washington base. it has talkback but it is not supposed to have talkback. cant find any resistor jumped to add talkback. replced a couple questionable caps in the audio section but i am still getting talkback. anyone ever had this problem???? I read somewhere about some other unidens having this problem on old models. something about he rx audio is not shut off completely. having trouble tracing problem.

Mike

[RedEye]

Check the speaker leads, or insulators around the jacks. I had one doing the same, turned out to be the speaker was grounding completing the circuit just like the resistor mod!

[speedracer6g]

It has the external speaker only. the problem seems to be audio is getting back in the speaker. looking at the schematic the mic should turn the rx audio off when you key up but somehow there must be a leaky cap or something to ground somewhere. it still has a low level rx without the mic hooked up. I tried pulling the rx wire off off the mic jack no luck there. I dont know maybe one of the caps on the speaker jack board is leaking to ground. I feal like i am chasing my tail around on this one. this wouldnt be a problem but he uses this with a d104 lollypop power mic so he gets feedback when he keys up because of the audio on his speaker.

still looking for the culprit,

MIKE

[speedracer6g]

Well hopefully i can save someone alot of headaches. I traced the problem to the headphone jack. I didnt have the insulated washer and was somehow grounding enough to let some audio through the speaker at all times. so could be a bad cap in the mic jack. but i just put a nylon washer behind the jack. works fine now

Mike

[Rick]

It sounds like the speaker is still grounded in TX mode.

The Speaker is always hot in this chassis and the only way it is disconnected is by the keying of the mike, so must have a short from the cold side of the speaker to ground.

Try total isolating speaker from the chassis.

[speedracer6g]

this radio has no internal speaker. But i did find out from looking at the schematic that you are right about the audio. they route the audio though the mic. this unit has low receive with no mic on it also. this led me to beleive there was a short to groung somewhere in the audio distribuiton. went looking for i and well seems to be related to the headphone jack. when i took it out there was a nylone spacer on the jack to isolate it and i think there was a nylone washer missing so i installed one and the audio problem went away. I am not sure if this is the real issue but it got it going for now. I suspect there is a shorted cap in the mic jack or spaker jack board somewhere. what do you tink rick.

thanks for all the help guys,

Mike

[Rick]

This should not be hard to do as it’s not rocket science.
Just disconnect the cold side of the speaker and with a DVM set for a resistance check, put one probe to Ground and the other to where the cold side of the speaker was connected and without the mike connected, then start unsoldering components on the sub board until the problem go away.

[speedracer6g]

yep no problem got er done thanks agian

MIKE