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.

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