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.