Apple IIc Repeating Space Character

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Apple IIc Repeating Space Character

Hi,

I've seen a few articles on keyboard issues on the IIc but none that match the issue that I'm experiencing. 

 

I have a IIc in good condition. Upon power up I get the usual "Check disk drive" message or, if a disk is in the drive, the disk loads. It is a UK model ROM 255 variant, so not the Alps keyboard.

 

In the first scenario above if I drop to the BASIC prompt by CTRL-reboot, I get a continuous stream of space characters that eventually hits the end of the line buffer, beeps and starts doing the sames on a new line. Pressing any of the other chacter keys (letters, numbers, punctuation) don't seem to do anythind, but I can soft and hard reset it using the CTRL, open Apple and rest key combinations. If I toggle the keyboard button then the space character changes to an "I" but pressing any character key doesn't chnage this. However, keys such as CTRL and shhift display other letters (can't remember what exactly shift displays but something like a "G").

 

If I boot off a disk (for example Apple Adventure!), once loaded I get the same problem - a constant stream of space characters.

 

From reading around the forum and other sites, this is possible as simple as a broken key switch or more serious. How could I check the keyswicth armed only with a multimeter with continuity function? The keyswitch has four contacts, two diagonally oppostie the other, but I'm not sure why four are needed, so don't know how to see if the switch is broken. 

 

Can anyone help shed some light on what the issue may be and how I can start eliminating potential issues? I class myself as 'dangerous' with a soldering iron, but basic tests I can probably handle.

 

As an aside, can anyone recommend UK-based  companies/people who can offer repairs (as I can see this might eventually be what's needed), but my usual go-to person up in Tynemouth is now doing less repairs and more retro development. 

Any pointers gratefully accepted.

 

Thanks,

Dave

 

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Hi, Just an update after some

Hi,

 

Just an update after some testing. 

 

As I'm not sure why there are four pins to each switch I thought I'd use the continuity function to see if I could trace the inputs on the ribbon cable to the four solder pads of the space bar switch. 

 

The space bar has two pins connected to ribbon cable pin 4 and two to pin 7. So when tsting continuity there is no connectivity between pins 4 and 9 when the space bar is not pressed, but there is when it is pressed. For comparison I tested this on two other keys (one conencted to pins 1 and 7; one to pins 2 and 9) and they exhibited the same results.

 

So, I'm wondering if this is a broken space key at all as this limited testing suggests that it is OK. So, UB16 seems so be the next in line. Could failure of this chip result in these symptoms? Anyone know of a good source of ones for the IIc (I can seem them on the net but they seem to be generic ones). Am I heading in the wrong direction on this?

 

Thanks,

Dave

 

 

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Now I'm back at home I've had

Now I'm back at home I've had some time to do some more digging and have a look at the AY-5-3600.

 

It would appear that the voltage rails are good (pin 30 shows a little under +5V and pin 27 a little under -12V). I thought that I'd see if the multimeter could see anything on pin 16  which, if I've read up on this correctly, should only be high when a key is pressed. It gives a constant voltage of around 4.9V so I suspect that this is the issue - a dodgy keyboard decoder chip.

 

Can anyone recommend a reliable place to source a good one? I've heard that the Chinese one are a bit of a crap shoot as to whether to get a fake one or not.

 

Thanks,

Dave

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I've just received a couple

I've just received a couple of the generic AY-5-3600 chips from China (ordered as soon as I suspected the chip) and can confirm that they are not usable, simply preventing the machine from booting. Presumably this is because the vendor-specific functions are not programmed in, or simply because they're fake.

 

The search for an Appple variant continues, but these appear to be hard to come by.

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