Ok guys, put on your thinking caps. This should be fun and a wonderful
learning experience for alot of people, particuarly myself

I've got a Cobra 19XS that I've had for quite some time. It hasn't worked
since it was given to me by my father. I thought it would be fun to try
and get it working again for no other reason than "I've got nothing
better to do". It'll be cool.

The radio lights up, but has no RX or TX. If you crank the volume, you
can hear a very slight hissing sound with your ear up against an external
speaker. REAL quiet. Attemping to transmit illuminates the TX light, but
alas...no carrier. Connecting the RF siggen to the antenna terminal
doesn't help. I'm having trouble tracing the signal back from the antenna
based solely on the schematic I've obtained (purely a skill/experience
issue).

I suspect there's a problem in the PLL synthesizer circuit, as a failure
here would make the radio behave as observed. There could be other things
wrong, I'm sure.

The only data I've been able to obtain on the radio thus far is a
schematic from cbtricks.com. It's helpful, but I don't have the skills
necessary to identify various circuits in the radio or determine proper
test points based on the schematic alone. A service manual would be nice,
but I don't have one. This is where some of you REAL techs might help me
understand things a little better. I can't tell what
voltages/frequencies/etc to examine at what points based soley on the
schematic. Perhaps some of you can? I do have the data on the PLL (Sanyo
LC7131).


I do have some nifty equipment available to help us troubleshoot this at
this very moment on my bench:

1.) 8 digit benchtop frequency counter (10Hz - 2.4Ghz)
2.) BK Precision RF Signal Generator model 2005 (100KHz - 150MHz,
Internal 1KHz audio gen, provisions for external audio gen, variable RF
and Mod levels, provisions for external FREQ. counter). Checked this with
my freq counter, and it's suprisingly stable.
3.) Fluke 8060A DVM. (It's old, but it works!)
4.) Soldering goodies.

The schematic is available he
http://www.cbtricks.com/Cobra/19xs/cobra_19xs_sch.pdf

I don't have a scope available at the moment, though I'll be purchasing
one shortly.

If I NEED any additional equipment to get this radio working and/or
provide feedback to your questions I'll purchase it. I want this to be a
fun learning experience.

That being said, let's start troubleshooting! This is what this group is
all about.

--
Alan Strawinski

http://alan.strawinski.net

Check the crystal and check for cold solders in the VCO area.

You've just determined you have no 10.24 MHz. Make it oscillate and
try it again. I toss Cobra 19's in the trash, working or not.

Radioman wrote in :

You've just determined you have no 10.24 MHz. Make it oscillate and
try it again. I toss Cobra 19's in the trash, working or not.

I tried something else. I probed pin 13 of the PLL (Vss, ground) and found
10.17MHz present. That seems odd to me. Why would I see 10.17MHz THERE of
all places? And why is it 10.17MHz as opposed to 10.25MHz?

As far as making it oscillate goes, I tried injecting a 10.24MHz signal at
the Reference Oscilator Input (RI) and got nothing. There isn't even a
measurable output at the Reference Oscillator Output (RO) pin of the PLL
while I'm injecting one at the RI pin. I would expect there to be one
there, but I could be wrong.

I know that alot of people don't like Cobra 19's, but I still want to get
this thing working. I'm not sure why people don't like them, they're
perfectly fine radios. I don't want to give up.

--

Alan Strawinski

http://alan.strawinski.net

Alan Strawinski wrote:
Radioman wrote in :


You've just determined you have no 10.24 MHz. Make it oscillate and
try it again. I toss Cobra 19's in the trash, working or not.


I tried something else. I probed pin 13 of the PLL (Vss, ground) and found
10.17MHz present. That seems odd to me. Why would I see 10.17MHz THERE of
all places? And why is it 10.17MHz as opposed to 10.25MHz?

As far as making it oscillate goes, I tried injecting a 10.24MHz signal at
the Reference Oscilator Input (RI) and got nothing. There isn't even a
measurable output at the Reference Oscillator Output (RO) pin of the PLL
while I'm injecting one at the RI pin. I would expect there to be one
there, but I could be wrong.

I know that alot of people don't like Cobra 19's, but I still want to get
this thing working. I'm not sure why people don't like them, they're
perfectly fine radios. I don't want to give up.

lift pin 13 and see if the pll is supplying the freq....

Alan Strawinski wrote in message ...
Ok guys, put on your thinking caps. This should be fun and a wonderful
learning experience for alot of people, particuarly myself

I've got a Cobra 19XS that I've had for quite some time. It hasn't worked
since it was given to me by my father. I thought it would be fun to try
and get it working again for no other reason than "I've got nothing
better to do". It'll be cool.

The radio lights up, but has no RX or TX. If you crank the volume, you
can hear a very slight hissing sound with your ear up against an external
speaker. REAL quiet. Attemping to transmit illuminates the TX light, but
alas...no carrier. Connecting the RF siggen to the antenna terminal
doesn't help. I'm having trouble tracing the signal back from the antenna
based solely on the schematic I've obtained (purely a skill/experience
issue).

I suspect there's a problem in the PLL synthesizer circuit, as a failure
here would make the radio behave as observed. There could be other things
wrong, I'm sure.

I would also go for that area.

First, check that the supply is getting to the PLL on pin 18. Regulator
circuits have been known to go down.

Check the voltage going from the PLL to the VCO, and the VCO frequency.
The voltage should change with the channels, but should usually remain
between about 0v8 and 4v5.
If the frequencies and voltages are correct, then there is not a problem
with the PLL.

If the voltage is high, the PLL is seeing a low VCO frequency. If the
voltage is low, the PLL is seeing a high VCO frequency. If what it sees is
correct, then the PLL is most likely working... has the VCO core been messed
with?

If what it sees is not correct, first check the reference (10.24MHz). Also,
check that the VCO signal is getting back to the PLL, on pin 19.

If the frequency is a standard bench one, with probe input, then it will
most likely be good enough to test these signals without problems. The
digital meter should be fine for the voltage readings, better than an
analogue one.

If you are planning to get more involved in the tech side, the 'scope should
prove very useful.


Regards,

Peter.

Thanks for all the replies guys. Turned out to be a bad crystal. I replaced
t with one out of a parts radio, and voila! Works like a champ.

That was fun. I've got a few other busted radios lying around here that I'd
love to attack as soon as I get proper service data for them. I just bought
these at the flea market yesterday. I fully expected that none of them
would work, which is what I wanted. If any of them did work, I wouldn't
have much fun fixing them If anything, I've got tons of parts now. I
paid 5 bucks (total) for:

Montgomery Ward 775 (40ch)
Pace 2300 (23ch)
Realistic TRC-24c (23ch)

All are mobile, and AM only. I'm getting to love fixing this stuff. What a
hobby.



--

Alan Strawinski

http://alan.strawinski.net

Radioman wrote in :

You asked on Wed 11:18 PM. I gave you the solution on Thu 12:31 AM.
It took you until Sun 5:37 PM to fix it? :

Yup. I work on it when I can. It's a hobby, not a profession. I've been
extremely busy with various work related studies, so the CB radios take
the backseat sometimes. Nonetheless, it's really fun working on them when
I can. I don't see how the time makes any difference. If I was charging
somebody by the hour to repair the radio, that would be a different
matter altogether. I'm a network engineer, not a radio tech. If I was
helping a friend by giving him advice on routing, I wouldn't bust his
balls because it took him 3 days to configure access lists on a Cisco
router, even if I could do it in a couple of minutes. I would say "Hey
man, that's great. Good job". If he was a well compensated coworker, I'd
read him the riot act.


Have fun fixing the rest of them. Bad caps and cold solder joints
are about to found.

I bet you're right! I haven't powered any of the radios up yet, but I
did pull a couple apart. One has two completely blown caps, and another
has a charred pcb near the power connector and a couple of missing
diodes. This should be fun. There's just something really cool about
fixing radios others have discarded. For me it's very challenging, much
like a good puzzle. When it ceases to be a challenge, I'll either look
for REALLY busted radios, or perhaps try my hand at repairing other types
of radios. CB's are kinda fun because I can pick them up all day long at
the flea market for a couple of bucks. I've also wanted to understand
them (and radio in general) since I was a kid. Lately, I decided that I
should quit wondering and start learning. I'm having a blast so far.

If/when I get really good (and fast) at it, I was thinking of fixing
other people's radios around here at no charge in my spare time. It's a
hobby. There really aren't many reputable shops around here. The couple
that I'm aware of are always backed up and charge ridiculous rates.
Naturally, there is no shortage of guys working out of vans and camper
shells willing to turn screwdrivers and "tweak and peak" radios. Every
one that I ever ran into seems to have no clue as to what he's actually
doing. That's fine, until they start charging guys like my dad 50 bucks
(true story) to nock the carrier power on an otherwise good radio down to
two watts, and turning the modulation up high enough to make my meter go
"ding" when it smacks the right side of the case. (OK, I'm exaggerating a
little) And don't get me started on how he managed to turn a once
spectacular old Royce into an off frequency splatterbox. Nope. I'd like
to someday be able to do a fantastic job free of charge. What better
source of broken radios is there than people bringing them to me?
Ahhh...someday when I'm better at it and much faster...

Incidentally, now that the Cobra is fixed, I'm thinking of giving it back
to my dad. I don't think he ever thought it would work again. Neat huh?

Thanks again for the help man, I appreciate it.


--
Alan Strawinski

http://alan.strawinski.net