3 Jul
2005
3 Jul
'05
5:30 a.m.
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
the transistors, and on the voltage input to the LCD. The thumbwheel
does not affect it a whole lot - it mostly clips the tops of the square
wave.
Unplugging the LCD module makes little difference, so it's not a loading
issue (at least not from the module).
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?
Regards,
Dave
--
dave04a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.parse.com/~ddunfield/museum/index.html
3 Jul
3 Jul
1:41 p.m.
Hi Tony,
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.
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 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
7114
days inactive
7114
days old
1 comments
2 participants
participants (2)
-
ard＠p850ug1.demon.co.uk -
dave04a＠dunfield.com