COBRA 2000 GTL hung carrier fix

[BBB]

My Cobra 2000 GTL has a hung carrier and I'm wondering if anyone can point me in the right direction as to the cause.

Obviously I first changed the mic and the carrier is still there. Even with the mic disconnected the carrier is still there. The mic jack appears OK. With the mic reconnected and keyed, it modulates properly. Let go of the PTT button on the mic and that 4 watt carrier is still there so I'm thinking something failed in the mic keying circuit. With the speaker connected it lets out a loud feedback howl because I think its acting like it has talkback (it doesn't) It howls FULL volume no matter where the speaker volume is set.

I'm glad it failed when I was keying and not when I was away from the radio because it keeps transmitting the carrier on AM. On SSB it shows some minor transmit to a nearby radio.

I checked the TR41 2SC1419 AM modulator but it was not shorted? Since these radios don't use relays I'm asking if anyone knows what I might look for to get it fixed. Thanks.

 

[BBB]

Found out that if I lifted pin number 5 on IC4 (MB3758 voltage regulator with RX/TX control) the 4 watt carrier stops. So I'm assuming that this pin is what controls the radio's transmit when the PTT mic switch is keyed.

Now I need to find out why this pre IC4 keying circuit is trying to energize (or de-energize) this pin incorrectly.

Anyone know what this pin #5 is looking for in the way of voltage to key the transmitter. Is it 4-6 volts DC because that's what I'm seeing there? If it is maybe I can follow the components back to the mic jack to see what is leaking the DC voltage to cause the voltage regulator with RX/TX control to false trigger the transmitter op:

 

[BBB]

Looked at a few things that are before the IC4 pin 5 keying circuit Checked diode D43 which looks to be in line with a bias resistor R164 that is on Transistor TR35.

Tested the IS1588 diode D43 both ways with the continuity tester and it showed open both ways. If this is a normal diode than leads me to believe it is failed open?

I'm not exactly sure whats going on with this keying circuit but I aim to find out <can not stand it>

 

[BBB]

Triple checked the diode D43 and TR35 and found both were good. Puttin' em back in and checking other components.

Anyone know how this Cobra mic keying circuit works? Should be the same or similar to the older 148's except the 2000 has a 5 pin jack and the 148 has a 4 pin jack? I figure it's a switch closure in the mic's PTT button that sends voltage to pin 5 on IC4?

Lack of voltage present on pin 5 on IC4 = no key, no carrier?
Voltage present on pin 5 on IC4 = key with carrier?

Unfortunately the local CB shop declined to trouble shoot my radio because it was too old so here I am.

 

[BBB]

Alright, I got it

For others that may experience this hung carrier:

There is also a capacitor in parallel with pin 5 for IC4. That electrolytic cap #C133 10uF at 16V was shorted out internally and went to ground so I replaced it and now there is no hung carrier. Radio Keys as it's supposed to.

I'm assuming that since C133 was shorted it faked IC4 into "thinking" that the mic was keyed. Other components in that circuit did not shut down the receive and hence the loud feed back squeal when the capacitor component failed.

 

[unit248]

I'm glad you figured this out. I was going to say it sounded like a problem with the RX/TX switching but since I don't know enough about the Cobra design to offer any useful hints beyond that obvious point, I decided to just save my breath until somebody smarter spoke up. Looks like someone did.

One thing though: the original Cobra 148, Cobra 142 and Cobra 2000 all had the same 5-pin mic connector. It's the newer (Chinese made) Cobra 148s that have 4 pin connector, and it's on the front panel instead of on the left side. I used to own an older 148 in the mid/late 80s and currently have one of the Chinese ones. There's no comparison in terms of quality.

-Bill