Cobra 2000 GTL chassis ground/psu question

[Pollard]

Hello,

When measuring voltage from the negative DC terminal on the back of the radio to the chassis, I’m seeing 13.64v (the same voltage present on the regulated DC output of the PSU.

I don’t have another radio for an example. Is this normal? Something tells me it’s not right. PLL Adjusts and aligns fine, modulation is good. No receive audio or output from final currently. The voltage on chassis is present with an external DC supply as well. It goes away when the fuse is pulled and in AC, so I don’t think it is supply related ...

Thanks for any help.

[Bluerunner]

Sounds like the microphone, mic wiring, CB / PA switch & mic socket are the major suspects.

Check to see if the mic socket & CB / PA switch may be cracked. Check the mic socket & switch solder joints. Most of us make "utilitarian" solder joints so the difference from factory & homemade may be obvious.

I had a used / salvaged radio that gave me headaches similar to your issues, but no spark just receive audio issues. Turns out the previous owner rewired the mic socket to suit whatever mic he had (I didn't get the mic in the deal).

I also came across mods on other radios that used the mic circuit to key an external amps relays. No problems with it, but took a while to figure out what the extra wiring was for.

Also had mic socket pins push halfway out of the mic socket (cracked plastic) on the radio whenever the mic was plugged in. Intermittently caused problems just depending what the pushed back pin bumped against. Unplugging the mic would pull the pin back into place which made it difficult to identify the issue.

Just some suggestions. Good luck finding the problem.

[Pollard]

i found the culprit late last night!

the ceramic washer that insulates TR41 AM power regulator was cracked and allowed the mounting screw to short, shunting the 13.8v DC supply rail to chassis ground. the transistor was fine, though, as it received the voltage it was expecting!

now that I no longer have a supply issue, I need to hunt down the cause of the receive issues. volume knob all the way to the speaker works, and I can see the RF meter bouncing from the locals, but there is a disconnect between RF and the audio section somewhere. back to the blackboard . . .

[Bluerunner]

Glad to see you making progress. Those little gremlins can be pretty elusive. Maybe that is why I'm loosing my hair, from scratching my head too much trying to figure stuff like that out.

[nosaj]

i found the culprit late last night!

the ceramic washer that insulates TR41 AM power regulator was cracked and allowed the mounting screw to short, shunting the 13.8v DC supply rail to chassis ground. the transistor was fine, though, as it received the voltage it was expecting!

now that I no longer have a supply issue, I need to hunt down the cause of the receive issues. volume knob all the way to the speaker works, and I can see the RF meter bouncing from the locals, but there is a disconnect between RF and the audio section somewhere. back to the blackboard . . .

If it were me, I'd probably start at the squelch portion of the circuit.
I have an oscilloscope that outputs ch1 to a bnc which I made a connector with a .001 cap to block DC that then goes to a bench amplifier. In effect a Audio/Visual Signal tracer.
nosaj
N1EBC

[MDYoungblood]

I'm thinking it could be the speaker switch or the jacks in the back might have an issue.

3's

Greg

[Pollard]

i found the culprit late last night!

the ceramic washer that insulates TR41 AM power regulator was cracked and allowed the mounting screw to short, shunting the 13.8v DC supply rail to chassis ground. the transistor was fine, though, as it received the voltage it was expecting!

now that I no longer have a supply issue, I need to hunt down the cause of the receive issues. volume knob all the way to the speaker works, and I can see the RF meter bouncing from the locals, but there is a disconnect between RF and the audio section somewhere. back to the blackboard . . .

If it were me, I'd probably start at the squelch portion of the circuit.
I have an oscilloscope that outputs ch1 to a bnc which I made a connector with a .001 cap to block DC that then goes to a bench amplifier. In effect a Audio/Visual Signal tracer.
nosaj
N1EBC

could you elaborate, please, on how I might do this utilizing an o-scope and a separate signal generator?

thank you! i'm eager to learn. this is a lot different than troubleshooting audio circuitry that i'm used to.

[nosaj]

If it were me, I'd probably start at the squelch portion of the circuit.
I have an oscilloscope that outputs ch1 to a bnc which I made a connector with a .001 cap to block DC that then goes to a bench amplifier. In effect a Audio/Visual Signal tracer.
nosaj
N1EBC

could you elaborate, please, on how I might do this utilizing an o-scope and a separate signal generator?

thank you! i'm eager to learn. this is a lot different than troubleshooting audio circuitry that i'm used to.

Do you have a scope? Mine is a tenma and on the back of the scope is a BNC jack for CH1 output, which you could hook a frequency counter up to. In my case in audio work I connected it to an amplifier. If you were tracing an AM signal you could use an RF probe to demodulate what you were probing.
nosaj
N1EBC