Sharp PC-1600 keyboard issues

[Josh Dersch]

On 2/7/2015 3:37 PM, Josh Dersch wrote:
> On 2/7/2015 12:37 PM, tony duell wrote:
>>> I'm more concerned about disconnecting the membrane from the PCB (the
>>> PCB is on pg. 41/42, the membrane on 45/46 -- you can see on the lower
>>> edge the connections made between the two) as I'm not sure how the two
>>> are held together -- there's a metal bracket on top that sandwiches the
>>> two together (and is screwed into the "front" of the machine -- see
>>> parts 20/21 on the exploded disassembly).  I'm afraid if I take the two
>>> apart I may never get them back together making good contact again.
>>> Have you dealt with anything like this? Anything I should be concerned
>>> about?
>> I wouldn't separate them unless you have to, You may well find they are
>> lightly glued or even soldered, if so, do not separate them at the start. Of course
>> if the membrane comes off when you undo the screws then it is intended that
>> it is separated during servicing, so no worries.
>>
>> When you have the PCB out, then do a continuity check on the signal in question,
>> Even if you find it open, make sure the 'open' is at the junction of membrane and
>> PCB, not an open track on one or other.
>>
>> -tony
>>
>
> OK, I probed things out and there are two discoveries to report:
>
> 1. Continuity of KIN3 is good from the keyboard PCB to IC1 (the 
> SC7852) in 98.  That's good!
> 2. KIN3 and KIN4 (KIN4 is used for "SHIFT") appear to be shorted 
> together somewhere.  That's bad, but explains some of the bizarre 
> behavior...
>
> I'll have to do a bit more poking around to see if I can figure out 
> where this is happening.  The membrane is well attached to the PCB 
> after removing the boards from the housing, so I assume they're glued 
> and/or soldered together...
>
> Thanks,
> - Josh
>

Found a tiny solder bridge on the keyboard PCB just below where the 
membrane is attached, between KIN3 and KIN4; I removed it and now 
everything is working properly, except the SML key still puts the 
machine into some manner of Japanese input mode rather than lowercase 
(which doesn't really work well since this machine doesn't have the 
Kanji character sets in ROM...).  Not sure what the cause of that is, 
but it's a pretty minor issue all things considered.

Thanks for the input here!
- Josh