"N_Cook" wrote in message
...
Chuck wrote in message
...
On Sun, 6 Feb 2011 19:20:39 -0600, "Mark Zacharias"
wrote:

"N_Cook" wrote in message
...
Owner likes the 2 SW bands, other than find a broken one or related
model
also with the same tuner TUX-020 , any ideas?
Courtesy of some Danish and Polish threads someone found out that the
80
pin
Philips tuner IC TEA6842H is likely the same as the one badged Sony
8-759-653-23 in this tuner. All DC at the tuner 19 pins agree with
Sony
Schema and rails, Xtal lines etc agree with the TEA... datasheet.
There
is
local Xtal osc, serial data and clock line signals on engaging "SEEK"
,
front display shows changing f but the TEA pin 42 tuning V stays at
0.045V.
Nothing amiss DVMing around the variable cap diode , pushing IC pins
if
loose, and no bad ESR local caps. Whatever occured happened in a
garage
during a cold spell of weather, vehicle rarely used.




Try bringing in an external tuning voltage. If by varying this voltage
you
can get an IF output from the front end (and receive a channel at that
point...) then you know the front end transistors etc are OK and you
either
have a bad PLL chip or no oscillator sample going to the PLL chip.

Mark Z.


Just one more thing to check. Once you tune in a station, remove the
tuning voltage source. If the station stays tuned for a few seconds,
then you'll know the problem isn't caused by leaky varicaps. If the
frequency immediately drifts, the varicaps need to be replaced. Chuck

Incidently the radio data system works back to the display so even less
likely corrupted serial link going the other way.
Cutting my applied tuning voltage , the station goes off tune immediately.
I've not managed to get my head around the control system - a chicken and
egg situation ? Could a leaky varicap lead ,around the houses , to failure
to have an applied tuning voltage through the chain of VCO / PPL etc ?




I vote for the PLL chip. If the varicap were leaky, you'd see some voltage
there as you tried to tune; I've seen this just limit the tuned frequency to
say 95.1. If the cap were very leaky, even applying an external voltage
wouldn't get you a station. The varicap bleeds off quickly mainly because
it's capacitance is very low to start with. Most PLL chips have a couple
buffer transistors between the tuning output of the chip and the actual
front end. You can look there, but my guess is you'll see nothing.

Mark Z.