I have a Cobra 148GTL AM/SSB citizen's band radio that appears to be
fairly old (made in Taiwan) and has the following problems:

(1) Despite what LED channel indicator shows, the receiver doesn't
change frequency.

(2) When I push the mic push-to-talk button, no signal is transmitted
and the receive/transmit (RX/TX) light remains green (receive mode).

(3) When I first got the rig, it did transmit and change receive
channels, but on frequencies above the CB band.

I suspect the problem is in the frequency synthesis phase-locked loop
or related circuitry. Did a capacitor failed after being uncharged for
years?

I thought I'd ask newsgroup readers for any insight before I charge in
with my oscilloscope and soldering iron. Perhaps some of you
encountered similar problems with this unit and can suggest a good
starting point diagnosis and repair.

-Dave

wrote:
I have a Cobra 148GTL AM/SSB citizen's band radio that appears to be
fairly old (made in Taiwan) and has the following problems:

(1) Despite what LED channel indicator shows, the receiver doesn't
change frequency.

(2) When I push the mic push-to-talk button, no signal is transmitted
and the receive/transmit (RX/TX) light remains green (receive mode).

(3) When I first got the rig, it did transmit and change receive
channels, but on frequencies above the CB band.

I suspect the problem is in the frequency synthesis phase-locked loop
or related circuitry. Did a capacitor failed after being uncharged for
years?

I thought I'd ask newsgroup readers for any insight before I charge in
with my oscilloscope and soldering iron. Perhaps some of you
encountered similar problems with this unit and can suggest a good
starting point diagnosis and repair.

-Dave

The first thing that I would check would be your B-supplies. They have
an IC that supplies seperate DC sources for receive and transmit. Some
of your issues seem may point to one of those sources being dead. As
for your capacitor comment... electrolytic capacitors these days (and
even in the recent past) have an electrolyte and containment design
that is very reliable. Don't spend too much time thinking along those
lines. Hope that helps...

www.telstar-electronics.com

wrote in message
oups.com...
I have a Cobra 148GTL AM/SSB citizen's band radio that appears to be
fairly old (made in Taiwan) and has the following problems:

(1) Despite what LED channel indicator shows, the receiver doesn't
change frequency.

(2) When I push the mic push-to-talk button, no signal is transmitted
and the receive/transmit (RX/TX) light remains green (receive mode).

(3) When I first got the rig, it did transmit and change receive
channels, but on frequencies above the CB band.

I suspect the problem is in the frequency synthesis phase-locked loop
or related circuitry. Did a capacitor failed after being uncharged for
years?

I thought I'd ask newsgroup readers for any insight before I charge in
with my oscilloscope and soldering iron. Perhaps some of you
encountered similar problems with this unit and can suggest a good
starting point diagnosis and repair.

-Dave


If your interested, I have schematics, and other 148 related info.

--
Owner of Daryl Hunt
Master of Slow Code
Suffer, unto me.
Indulge, my every need.
UV Count 1
Religion is a whore! (Slayer)
Stupid is as stupid does.
ALL HAIL BOOKMAN!!!

On 8 Sep 2006 09:39:07 -0700, wrote in
.com:

I have a Cobra 148GTL AM/SSB citizen's band radio that appears to be
fairly old (made in Taiwan) and has the following problems:

(1) Despite what LED channel indicator shows, the receiver doesn't
change frequency.

(2) When I push the mic push-to-talk button, no signal is transmitted
and the receive/transmit (RX/TX) light remains green (receive mode).

(3) When I first got the rig, it did transmit and change receive
channels, but on frequencies above the CB band.

I suspect the problem is in the frequency synthesis phase-locked loop
or related circuitry. Did a capacitor failed after being uncharged for
years?


Who knows. The radio has been "tweaked" by someone who clearly didn't
know what they were doing. The problem could be just a cold solder
joint, the core in the oscillator coil could be cracked, your PLL chip
could be fried..... it's a big list of possibilities.


I thought I'd ask newsgroup readers for any insight before I charge in
with my oscilloscope and soldering iron. Perhaps some of you
encountered similar problems with this unit and can suggest a good
starting point diagnosis and repair.


You can either spend time time and effort to restore and de-mod the
radio, or take the more practical route and just buy a new one.

Frank Gilliland wrote in
:

On 8 Sep 2006 09:39:07 -0700, wrote in
.com:

I have a Cobra 148GTL AM/SSB citizen's band radio that appears to be
fairly old (made in Taiwan) and has the following problems:

(1) Despite what LED channel indicator shows, the receiver doesn't
change frequency.

(2) When I push the mic push-to-talk button, no signal is transmitted
and the receive/transmit (RX/TX) light remains green (receive mode).

(3) When I first got the rig, it did transmit and change receive
channels, but on frequencies above the CB band.

I suspect the problem is in the frequency synthesis phase-locked loop
or related circuitry. Did a capacitor failed after being uncharged for
years?


Who knows. The radio has been "tweaked" by someone who clearly didn't
know what they were doing.



Now that's a Big Ten-four. We call someone like that a CB'er.

SC

Ok--I solved the problem with the radio!

Yes, it had been modified. Someone grounded pin 10 of the PPL chip and
fiddled with the VCO for above-band operation. I removed the jumper on
pin 10 and then used a frequency counter to realign the VCO. Also, the
output power was a bit high, so I adjusted the modulation circuit to
make it operate at the legal limits for AM and SSB.

Thanks for the advice. It was helpful.

-Dave

On 10 Sep 2006 18:08:57 -0700, wrote:
Ok--I solved the problem with the radio!

Yes, it had been modified. Someone grounded pin 10 of the PPL chip and

What's a "PPL chip?" A popcorn popping loop? Why do I get the feeling
that you're just blowing smoke in an attempt to impress someone?

Also, modifying the freqency or power determining circuits of such
type-accepted equipment automatically makes it illegal to use on CB.
Way to go.

You must be a relative of mine. I love getting in the FCC's face,
because they're afraid of me. Crank your power up to 100 watts, move
to 22 meters, and blast away. Nobody will touch you because I said
so.

On Mon, 11 Sep 2006 04:01:55 -0400, wrote:
On Mon, 11 Sep 2006 02:11:25 +0000 (UTC), Lloyd Daugherty
wrote:

On 10 Sep 2006 18:08:57 -0700, wrote:
Ok--I solved the problem with the radio!

Yes, it had been modified. Someone grounded pin 10 of the PPL chip and

What's a "PPL chip?" A popcorn popping loop? Why do I get the feeling
that you're just blowing smoke in an attempt to impress someone?

dry up "lloyd" since it seems plain you can't repiar a blown fuse why
not shut up

The original poster probably meant a PLL - a phase locked loop -
something you didn't even notice, Expert Boy. But you don't put a
typical, synthesized VFO on a new frequency by simply doing something
to a PLL. You have to modify the divider / multiplier circuitry which
takes its input from one or more crystal controlled oscillators. In
addition, you'd probably need to change one of the crystals if you
were going to go really far out of band. Most CB "modders" don't do
any of the above; they change the operation of a logic circuit
(sometimes by removing a diode, but usually by replacing a chip) so
that certain illegal frequencies can be dialed up in addition to the
legal ones.

The original poster - just like you - didn't seem to know what he was
talking about.

Now go away and flood some more, Murgatroid. It's the only expertise
you have demonstrated in these newsgroups.


----
In message .com,
makes a death threat when he writes "the mostly
ooccasion [sic] where where you might it will be seconds before you
die."

wrote in
ps.com:

Ok--I solved the problem with the radio!

Yes, it had been modified. Someone grounded pin 10 of the PPL chip and
fiddled with the VCO for above-band operation. I removed the jumper on
pin 10 and then used a frequency counter to realign the VCO. Also, the
output power was a bit high, so I adjusted the modulation circuit to
make it operate at the legal limits for AM and SSB.

Thanks for the advice. It was helpful.

-Dave


You're welcome, My pleasure.

There's nothing like the sound of a good loud fart or belch on CB
channel 19 to get the conversations going especially when they're
broadcast over-modulated with echo and punctuated with a roger beep.

SC

Slow Code wrote:
wrote in
ps.com:

Ok--I solved the problem with the radio!

Yes, it had been modified. Someone grounded pin 10 of the PPL chip and
fiddled with the VCO for above-band operation. I removed the jumper on
pin 10 and then used a frequency counter to realign the VCO. Also, the
output power was a bit high, so I adjusted the modulation circuit to
make it operate at the legal limits for AM and SSB.


That would make you virtually unique on the Citizen's Band.

One of the reasons for the desirability of the classic 148GTL
Sideband was the ability to crank up the output power and modulation
depth by simply opening up a couple of pots.

Though this was not the official position of Cobra, I remember
sitting in the service department when one of the techs told caller
after caller how to make this adjustment.

Many that left the building for the personal use of staff had also
been so adjusted.

Then again, the chief engineer had a Galaxy (complete with roger beep
and reverb) with about 100 watts connected to a yagi on the roof at
Cortland St, so liberties with the regs were not uncommon at Cobra.

Later models have been revised so such power adjustments were not
simple to make.

The above band operation is new to me. Although I did work with
someone, some years ago, who had modified an old Midland to work above
the band, fiercely arguing her right to do so.

There was a crackdown in St Louis that year, and it was sternly
explained unto her that she had been misinformed.