Epson HX-20 repair details - more info

From: M.Clemence <M.Clemence_at_exeter.ac.uk>
Date: Mon Nov 11 08:01:20 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
Received on Mon Nov 11 2002 - 08:01:20 GMT