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NEC TV N3456 reprogram presets without remote control!

 
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vk6fh



Joined: 21 Sep 2007
Posts: 1
Location: Perth Western Australia

PostPosted: Fri Sep 21, 2007 12:09 pm    Post subject: NEC TV N3456 reprogram presets without remote control! Reply with quote

With regard to the NEC brand color TV model N3456 and perhaps other similar models.
Reprogramming of station presets is only possible by using the hand held remote-control, and if this has been misplaced or non-functioning the set becomes essentially unusable!

This http://tech.yahoo.com/qa/1006041711890 is typical.

However the circuitry between the front panel push buttons and IC101 (multi-purpose custom controller IC) has been traced out and if you feel technically
competent the addition of a simple 4 switch external push button keypad, will allow you to program the preset channels (without the remote-control!)

Even though no schematic diagram was readily available for the NEC color TV in question, the screen printing on the foil side of the PCB reveals the function of each of the pins of the custom IC which controls the operation of the set. (IC101)
Pin 27 labeled key 1 (pin 28 labeled key 2) are of interest.
Key 1 connects to essentially a series resistor chain weighted 1 2 4 8
i.e. 3K3, 3K3, 5K6 and 12K, and the key-switches which short each node to ground. (See diagram)
Upon investigation of circuitry from pin 27 shows only one switch (of a possible 5) called Picture (brightsess/contrast/color/sharpness)
Therefore a duplication of the switch/resistor chain from Key 1 circuitry onto the Key 2 pin of Ic101 produced some good desired results.
I ran a pair of wires from Vss pin 21 (gnd connection) and pin 27 (key 2) to an external 4 switch keypad .
So, if you are skilful with the soldering iron you can affix run a pair of wires out through one of the vent slots in the rear of the TV casing.
Summarized in the schematic pushing preset and combinations of other keys will allow user programming of the preset channel positions.

External keypad can be removed once programming has been performed.
So no need to throw out those sets that have lost their preset information
And the remote control is nowhere to be found.
I (at this stage) dont know why the stored channel preset info becomes volatile perhaps if the Set remaining unplugged for an extended period of time causes the memory contents to be lost.
Anyway why not give it a try it certainly worked for me.

Circuit diagram

http://www.members.westnet.com.au/impeh/NECn3456ColorTV.jpg

I cannot vouch for the absolute accuracy of the above, any errors or omissions
Will be corrected as they become obvious ( I promise!)
Your comments welcome (through this forum)
Frank Hughes VK6FH Sept 2007
_________________
No responsibility accepted for any unseen errors or inaccuracies. Corrections will follow !!
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