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Using Osc

Replacing components (coil/xtal/resonator)

  Coils look like chrome cubes, and they have 6-7 solder points. Two points are wide tabs from the chrome cube, and they go to GND. This is a shield to "protect" the signal. There are two traces which each have two connection points. Sometimes there is a single contact point which does not connect to anything on the pcb.

  To remove the coil I destroy it. I cruch the cube with pliers a little until I can reflow the solder on the tabs and remove the cube. Then I cruch the coil itself, and remove the legs from traces. I find this method easier than desoldering 6-7 solder points, and I do not plan on reusing the coil. Plus trying to desolder that many solder points could damage the IC the traces connect to.

  Xtals are skinny cylinders with two legs. Simply flow solder and remove.

  Resonators can have 2 or 3 legs. They are bulbous or in a square/rectangular package. Simply flow solder and remove.

  The next bit is common to dealing with all of these components:
    Two traces come off the IC. The component (mentioned above) connects to both traces. Each traces goes to GND through a small ceramic cap. Remove both of these caps.
    The two pins on the IC are OSC In and OSC OUT, also called OSC1 and OSC2. I have read in several datasheets that an external osc can be used on OSC IN while OSC OUT goes to GND or is unconnected. So by removing all the other components you can now add your variable osc.
Below is a pinout for the main chip in V-tech Alphabet Desk.
Notice pins 7 & 8, and then refer to the diagram below the pinout.




This circuit shows how the xtal, coil, or resonator would be hooked up. Inside the chip is inside the thicker U-shape. So if you remove the external components you can hookup your own osc. Now xtal, coil, or resonator are not "directional", one side IN other side OUT. What does have a direction is the NAND gate in the chip. INIT and OSC1 are both inputs. CLK and OSC2 are the same line and the Output. The Output feeds back through the component of choice. That with the 2 caps is what sets the frequency of the osc. The 1M res connected between Output and Input makes the NAND act like a 10x amplifier. INIT is held HIGH so that when OSC1 is HIGH Output is LOW and when OSC1 is LOW Output is High, hence oscillating HIGH-LOW. If INIT is brought LOW the entire circuit resets (got that bit of info from datasheet).

    The freq the removed components operated at will determine which osc is the best to use. I find 555s are not up to the job, especially when in the 2Mhz+ ranges. Sometimes a 2 inv will do, and the LTC1799 will always work (1k-33Mhz) but again is a pain because of size.

  





cappy@sailormouth.org