Apple II Plus modifier key doesn’t work

Question

I received my Apple II Plus today from eBay. I powered up my computer without adding anything into it (it didn’t came with any card either). It started up in the ROM Applesoft BASIC just fine. However, I found one problem on this machine. All modifier keys doesn’t seems to work. For example, I want to enter the quotation mark, it should be shift+2 on this machine, but both shift keys don’t work (2 works, it writes 2 on the screen just as if the Shift isn’t been pressed). Control+G should create a beep, but the control key doesn’t seems to work either as it just enters a G on the screen. The speaker definitely works as the power up sound do work. Reset and esc doesn’t do anything either.

I’m thinking it might be a keyboard problem, but it is weird that it’s only the modifier keys that don’t work though.

Where should I look next?

P.S. the return and repeat key does work on this machine.

Update: I found a switch in the outer side of the Keyboard circuit board labeled S1. After flipping that switch it appears that the reset key started working. Not sure what that do though, might be related to this problem.

Update 2: I forgot to mention that the power LED also doesn't work on this machine. After taking apart the keyboard, I found out that the 3 keys that don't work are on the same wire as well as the power LED, maybe something broke there?

Answer 1

Since the Apple II/II+ keyboard is based on a hardware encoder (AY-5-3600 (*1)), thus handling of Shift and CTRL is done by this chip. So any failure to recognize either modifier is most likely due the encoder or the keys themself.

Having said that, there is a possible single point of failure for both modifiers: A 7400 on the little board below the keyboard. Two of its gates are used to invert and discriminate the modifiers, as they are not part of the matrix (like with a PC keyboard), but seperate inputs to the encoder. So if that IC is bad - or just out of place, both keys might work as if always enabled, or never - depending on the failure.

First iteration (*2) here would be checking in- and output side of the IC at position U2 (*3). Pin #8 is the output signal for Shift and should go high when pressed, while pin #9 (shortened with #10) is the input from the key switch(es). Normaly at +5V, but going down to (almost) 0V when either Shift is pressed. Vor CRTL pin #11 is output (going high when pressed) and #12/#13 is input.

So when the signal seams fine after the gate, then the keyboard encoder might be dead. Bad luck as the are not realy common anymore. If not working there, but fine before the gate, it's the gate, and if not working there, it must be the keys.

There is another possible single point of failure, the connector between the two boards (PCB with keys and interface), but to affect all modifiers it would involve two connection pins (#3 & #24) broken on different ends of the SIL connector And at the same time no other damaged. Rather hard to imagine - and I can't see any hint on your picture.

---

*1 - That's for the more common two pice keyboard like you have. If it's a real old II without a little secondary board, it's a different chip. MM-something, I don't remember.

*2 - AFAIR it's direct soldered, without a socket. If it's socketed, the very first step would be just pressing it back into - giving every socketed IC a firm wellcome (press) is always a great idea when revitalizing an old machine.

*3 - just checked the schematics.