Epson HX-20 repair details - more info - test-drb@ccmp.vtda.org

11 Nov 2002

...

Here, basically, is how the power switch should turn on the PSU.

Turning on the power switch on the keyboard assembly brings pin 4 of IC 
5D low. Thus pin 6 goes high (check both of these).
Pin 6 of IC 5D is connected to pin 9 of the same chip. Pin 10 (other 
input of that gate) should also be high -- it comes from IC2B in the 
power supply area, and shuts the machine down if the battery is too low 
(I think).
So pin 8 of IC5D goes low. This is connected to pin 4 of IC5F. The 
output, pin 4, of this gate therefore goes high. This brings pin 2 of 
IC7E high, which makes pin 15 low, turning on Q8.

I would now check all the pins I've mentioned. Something must be wrong in 
that area.

 As an aside, for those who maybe wondering why I'm bothering - I work at      

That question never occurred to me. The HX20 is a computer, so of course 
it should be repaired.

   If any
one would be willing to supply photocopies of the relevant bits 
 of the manual (schematic in
 particular) I would of course pay towards copying/postage/time.     

I will see what I can do...

Thanks for the above information - I have only just had chance to act on 
it so here goes.
If I am reading the board correctly IC5D is a 4000 CMOS 2x3 input nor 
and a not gate.
Checking round the pins, the inputs and
outputs are logically consistent but pin 4 does not go low when the 
switch is turned on. Suggesting
that some thing is wrong else where on the keyboard board perhaps. ( 
Also if it is a 4000, then
pin 9 and 10 are on separate gates which doesn't agree with the above - 
maybe you could confirm
that it is a 4000 )

However, the power supply voltage to the CMOS chips is very low - 
bewteen 2.9 and 3.2
depending upon how long it has been plugged in which is right at the low 
end of the operating range.

Unless I am mistaken the supply for a whole set of chips (presumably 
those that run all the time
even when the machine is "off") is provided by Q10. If this is so, Q10 
has 4.9V on the C (presumably the
main supply) and 3.3 V on the base but only 2.9 V on the emitter which 
seems to be the
chip supply.

Is this inference correct and if so are the voltages right ?

Again, thanks for all your help,

Matthew Clemence