LF: Epson HX-20 service info

Tony Duell ard at p850ug1.demon.co.uk
Sun Jul 3 15:41:02 CDT 2005


> 
> Hi Tony,
> 
> >Now, I would start by checking that LCD supply voltage signal. It starts
> >on the main board where IC 2C (4049) acts as an oscillator and driver to
> >operate a voltage doubler, You should see about 7V (I think) on the +ve
> >end of C1 or C6. This then goes up to the keyboard, to a 2 transistor 
> >voltage controller circuit there. Check for an adjustable voltage (set 
> >with the thumbwheel on the side of the machine) at the emitter of Q2 on 
> >the keyboard. This voltage, BTW, is fed back to the mainboard to power 
> >some of the ICs that connect to the LCD driver chips.
> 
> Thanks for the useful information.
> 
> I am seeing only about 3.5v on C1/C6 ... and it has about a 1v DC
> square(ish) wave superimposed on it. I see the same voltage/waveform at

I suspect thost capacitors have dried up. There's no way there should be 
that sort of ripple on the +ve side of those capacitors.

> I see lots of very healty looking 5v square waves coming out of IC2C,
> however I am having a great deal of difficulty trying to trace the circuit
> on the board. Can you give me a bit more of the circuit?  Which pins
> coming out of IC2C are driving the power supply, and what is the circuit
> between these and C1?


             1000pF                                               VL
    820k      C31                                                 ---
     R78       ||                                                  |
 +--\/\/\---+--||----+         |\                 +----------------+
 |          |  ||    |      +--| >o-----+         |                | 
 |          /        |      |  |/ 2C/c  |         |           C12 ===
 |          \ R74    |      |           |         |          47uF ---
 |          / 390k   |      |  |\       |         V  D6            |
 |          \        |      +-=| >o-----+        --- 1S2075       --- 
 |          |        |      |  |/ 2C/b  |   10u   |               ///
 |  |\      | |\     |      |           |   C2    |
 +--| >o----+-| >o---+------+  |\       |  | []   |         +---VLD     
    |/        |/            +--| >o-----+--| []---+--->|----+--------+
  2C/d        2C/e          |  |/ 2C/a  |  | []       D7    |        |
                            |           |           1S2075  |        |
                            |  |\       |                   |        |
                            +--| >o-----+               C1 ===   C6 ===
                               |/ 2C/f  |             10uF --- 10uF ---
                                        |                   |        |
                                  D5   ---                 ---      ---
                              1S2705    ^                  ///      ///  
                                        |
                                       ---
                                       ///

That's an ASCII-art version of the appropriate bit of the schematic. Section
d )Input on 10, output on 10) and e (input on 11, output on 12) of IC 2C form
the oscillator, the other 4 inverters are paralleled up as the driver.

Hope that helps.

-tony



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