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Cobra 29 Constant Key Up

Chris Lawrence

New Member
Jul 9, 2021
35
21
8
54
Radio keys up constantly (with or without a microphone). Besides that, everything works. A quick check of PTT pin 3 does NOT show a short to ground.

What info can I provide for further help?
 

NZ8N

Sr. Member
Dec 25, 2017
430
680
103
58
 

NZ8N

Sr. Member
Dec 25, 2017
430
680
103
58
I would check C117 first, TR21 is forced into Transmit mode if C117 is shorted, or even partially shorted, just enough to turn TR21 on. I have seen it quite a few times. Rick Jackson of Euroradio has seen it enough to put in the notes along the left side of the schematic.

Cobra 29 LTD Classic C117.jpg

In the Cobra 25 LTD series, C114 is the same capacitor in the same (general) place.
 

Chris Lawrence

New Member
Jul 9, 2021
35
21
8
54
I would check C117 first, TR21 is forced into Transmit mode if C117 is shorted, or even partially shorted, just enough to turn TR21 on. I have seen it quite a few times. Rick Jackson of Euroradio has seen it enough to put in the notes along the left side of the schematic.

View attachment 58890

In the Cobra 25 LTD series, C114 is the same capacitor in the same (general) place.
Thanks. I’ll be checking all these things the first chance I get today. Will let y’all know.
 

Chris Lawrence

New Member
Jul 9, 2021
35
21
8
54
Update: I was in the correct area yesterday as I was trying to trace down the issue. TR21 has 7.24/7.93/8.05 volts to each leg. Unsoldering center leg unkeys the radio.

Today: Replaced C117 w/ new and no change. Lifting r95 also unkeys radio.
 

Chris Lawrence

New Member
Jul 9, 2021
35
21
8
54
I would check C117 first, TR21 is forced into Transmit mode if C117 is shorted, or even partially shorted, just enough to turn TR21 on. I have seen it quite a few times. Rick Jackson of Euroradio has seen it enough to put in the notes along the left side of the schematic.

View attachment 58890

In the Cobra 25 LTD series, C114 is the same capacitor in the same (general) p
Thanks. I’ll be checking all these things the first chance I get today. Will let y’all know.
Thanks. I replaced c117 w/ new. TR21 still receiving ~8vdc
Simply unsoldering the center lead of TR21 so it can't touch the rim of the hole will reveal if it's forcing transmit mode.

73
Thanks Nomad. Middle leg is receiving 8 volts. Desoldering center leg unkeys radio. Trying to trace down the short.
Simply unsoldering the center lead of TR21 so it can't touch the rim of the hole will reveal if it's forcing transmit mode.

73
Thanks Nomad. Middle leg is receiving 8 volts. Desoldering center leg unkeys radio. Trying to trace down the short. Lifting r95 also unkeys radio. I guess it feeds tr21(?)
 

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