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Posted by Mark on 12/15/06 02:05
Notes on Accurian ADR 3223 DVR.
This unit is a clone of some popular Lite-on units. My brand new unit
had a bad video hum associated with the built in NTSC tuner. I took it
back and got a new unit and the new one had the exact same hum. The
hum was associated with the video and was worst with white scenes. I
examined the built in NTSC mono video tuner. It uses a Sanyo LA75676
video demodulator chip. The data sheet for the chip indicates that Pin
2 is the power supply filter bypass pin for the audio FM detector and
Sanyo recommends a 1uF cap to ground. The tuner had a small chip cap
connected to pin 2. It may be 1 uF, I could not tell. I added a 68uF
filter cap to ground and the hum is greatly attenuated. The hum is
not caused by the 60Hz power line but is crosstalk from the video via
the internal power supply in the LA75676 demodulator chip.
(Note there can be many causes of hum. This particular hum can be
identifed because it changes with the video. Very white scenes cause
more hum, dark scenes cuase less hum. If your hum is not like this,
then it is a different problem.)
The built in tuner is a small sub assembly that can be easily unplugged
from the main board by removing the back panel. Note the screws are
of different types, keep track of where they go when you take it apart.
The LA75676 can be accessed by removing the shield cover on the bottom
side of the tuner can. The cover cannot be completely removed but can
be bent back a little enough to get in and tack solder a small wire to
pin 2 of the IC. Then a capacitor is added outside the shield can
attached to the small wire. This is a small 24 pin SOIC. Don't try
this unless you are good at soldering small SMT type parts.
Fee free to forward/post this info to however needs it.
Mark
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