Cobra 18 WX ST II
 

Hello,

I recently purchased a used Cobra 18 WX ST II and connected it to a new
antenna installation. In testing SWR I receive off the chart high SWR. I
suspected a ground short and began to test each component starting at the
antenna. Isolation testing reveals all components are good wrt a short
between hot and ground, except the radio. When testing the radio
(completely unplugged from both power and antenna) I see continuity between
the center pin coax connector and the coax threads or radio case. I expect
there to be no continuity between these (maybe when transmitting or
receiving?).

I contacted Cobra to verify this indicating a problem with the radio, but
they only replied with a quote for evaluation and repair. Before I
consider repair or replacement, can anybody verify that continuity between
these points on an unplugged radio indicates a problem?

Thanks in advance,

Tom

Cobra 18 WX ST II
 

On Tue, 18 Sep 2018 23:39:12 -0000 (UTC)
Tom wrote:

Hello,

I recently purchased a used Cobra 18 WX ST II and connected it to a
new antenna installation. In testing SWR I receive off the chart
high SWR. I suspected a ground short and began to test each
component starting at the antenna. Isolation testing reveals all
components are good wrt a short between hot and ground, except the
radio. When testing the radio (completely unplugged from both power
and antenna) I see continuity between the center pin coax connector
and the coax threads or radio case. I expect there to be no
continuity between these (maybe when transmitting or receiving?).

I contacted Cobra to verify this indicating a problem with the radio,
but they only replied with a quote for evaluation and repair. Before
I consider repair or replacement, can anybody verify that continuity
between these points on an unplugged radio indicates a problem?

Three things spring to mind:

1) Whoever sold you that radio is laughing their ass off because they
knew it was broken to begin with. Sucker!

2) Congratulations, you've diagnosed your own problem but are too much
of a mouthbreather to see it. You'll fit in fine on CB.

3) Cobra think you're a dip****. They're not wrong.

EYE NUB JOO, TOM,

..Nick.

Cobra 18 WX ST II
 

On Tue, 18 Sep 2018 23:39:12 -0000 (UTC), Tom wrote:

Hello,

I recently purchased a used Cobra 18 WX ST II and connected it to a new
antenna installation. In testing SWR I receive off the chart high SWR. I
suspected a ground short and began to test each component starting at the
antenna. Isolation testing reveals all components are good wrt a short
between hot and ground, except the radio. When testing the radio
(completely unplugged from both power and antenna) I see continuity between
the center pin coax connector and the coax threads or radio case. I expect
there to be no continuity between these (maybe when transmitting or
receiving?).

I contacted Cobra to verify this indicating a problem with the radio, but
they only replied with a quote for evaluation and repair. Before I
consider repair or replacement, can anybody verify that continuity between
these points on an unplugged radio indicates a problem?

Thanks in advance,

Tom

There is no way any CB radio should be short at the antenna output.
That model has a 1/2 watt 1k ohm resistor (R83) shunting the output to
ground, check to see if that is burned- could be "blown short"- a 1/2
watt resistor is big and fat and easy to spot.
http://www.cbradio.nl/cobra/Service_...X_STII_EMG.pdf
If you look at the picture of the PCB on page 9 it's right near the
top edge and should be easy to locate. That's the most likely cause of
your symptom, but on the other hand if it's been keyed several times
into a short like that chances are good the final amp is damaged. The
value of that radio used and in good working condition is about as
much as a large 7 topping supreme pizza, (about $70 new, about $20
used and working) if it takes more than 15 or 20 minutes to track down
and fix you'd be better off giving it to the trash collector and
chocking it off to a learning experience.

Cobra 18 WX ST II
 

On 2018-09-22, Douglas D Anderson wrote:
On Tue, 18 Sep 2018 23:39:12 -0000 (UTC), Tom wrote:

I recently purchased a used Cobra 18 WX ST II and connected it to a new
antenna installation. In testing SWR I receive off the chart high SWR. I
suspected a ground short and began to test each component starting at the
antenna. Isolation testing reveals all components are good wrt a short
between hot and ground, except the radio. When testing the radio
(completely unplugged from both power and antenna) I see continuity between
the center pin coax connector and the coax threads or radio case. I expect
there to be no continuity between these (maybe when transmitting or
receiving?).

[snip]

There is no way any CB radio should be short at the antenna output.
That model has a 1/2 watt 1k ohm resistor (R83) shunting the output to
ground, check to see if that is burned- could be "blown short"- a 1/2
watt resistor is big and fat and easy to spot.
http://www.cbradio.nl/cobra/Service_...X_STII_EMG.pdf
If you look at the picture of the PCB on page 9 it's right near the
top edge and should be easy to locate. That's the most likely cause of
your symptom, but on the other hand if it's been keyed several times
into a short like that chances are good the final amp is damaged. The
value of that radio used and in good working condition is about as
much as a large 7 topping supreme pizza, (about $70 new, about $20
used and working) if it takes more than 15 or 20 minutes to track down
and fix you'd be better off giving it to the trash collector and
chocking it off to a learning experience.

Thanks for your reply and the service manual. I took a look at R83 and it
tests ok, however I noticed that L19 is missing its core and appears to be
physically damaged. I suppose it could be the source of a short. I may
try replacing it, although I don't see it noted in the service manual so
I'm not sure how to adjust a new component. Perhaps my best move is to
follow your advice and move on to a new radio!

Thanks again,

Tom