Apple II clone Unitron Keyboard issue...

[waltermixxx]

it appears to be auto repeating characters as soon as you press the key.
if I tap any key, it will generate 5 letters... if i hold it down it will just keep generating that character.

the issue is definately the keyboard... and the strobe seems to be producing 66.5 us pulses each time a character is produced...
so if i tap the A key, i will get 5 A's on the screen, and the scope shows 5 66.5 us pulses. I checked the REP key, and it's not shorted.


it isnt keyboard bounce as it does it equally with any key pressed... so it appears to be in the "strobe" section of the keyboard?
this keyboard is the classic clone keyboard with an eprom on the board and a few support chips...

Picture is here... it has a 74LS273, 2716eprom, 74LS123 on one side, and the other side of the keyboard has: 74LS42, 74LS93, NE556.

Does anyone have a schematic for this or a similar clone keyboard? or any suggestions?

Cheers.

 

[gertk]

I would check the capacitors first near the LS123 and the NE556 since these are both time related components.

My guess is that there is a timer circuit which waits for a specific time on any key down and then enables the repeat circuit.

Since the delay before repeat is relatively large, there is probably an electrolytic capacitor in that part which has gone bad.

 

[waltermixxx]

I would check the capacitors first near the LS123 and the NE556 since these are both time related components.

My guess is that there is a timer circuit which waits for a specific time on any key down and then enables the repeat circuit.

Since the delay before repeat is relatively large, there is probably an electrolytic capacitor in that part which has gone bad.

there is no delay when you press a key before the repeat starts, it starts repeating right away, Capacitors, that was my first thought, but there are no electrolytic caps on the board, only 1 tantalum and it seems fine, not shorted...
i checked with a meter... the rest are pretty much ceramic caps... and they all look ok?

thanks for your reply by the way... every little bit helps.

 

[amauget]

Hi,
Based only on the datasheets, I will try to replace the LS123 and the 556.

 

[David_M]

I dont have a circuit or exactly that keyboard but I know how the circuit works. I have a similar keyboard that doesn't use a ne556 and uses a different chip as the data latch.
So educated guess is the ne556 is used to provide a delay before auto repeat kicks in, possibly used to keep half of the 74ls123 from repeating.

Based on what I have here, half of the 74ls123 provides a clock to increment the 74ls93 (counter) the counter is used to index into the eprom and also drives the 1 of 10 decoder (74ls42).
Keys are placed between the outputs of the decoder and seven of the address lines of the eprom. The the data from the eprom is arranged 7 bits for the decoded key and one bit to trigger the strobe circuit and latch the eprom data into the 74ls273.
Any address input to the eprom that doesn't correspond to a key being pressed will not have that 8th bit set.
The other half of the 74ls123 is used to output a strobe pulse of a specific length, the ne556 will be used to prevent the 74ls123 from retriggering.

So my guess would be a problem with the ne556 not working properly.
Monolithic ceramic caps do sometimes go faulty with age and sometimes resistors die so if it's not the ne556 replace the caps and meter the resistors attached to the ne556. They are cheap so personally I'd just replace them along with the ones connected to the 74ls123 for good measure.

 

[waltermixxx]

thanks folks. I will replace the 556 first, test, then the 123, then test, then the resisters and caps near the 556.
i appreciate the help. I may not get a chance to try this until after VCFMW, but if I do, i will report back and if I get it working I will bring my Briel Apple I replica with me to VCFMW as well.

cheers.

 

[waltermixxx]

Well looks like she's be coming to VCFMW after all. I replaced and socketed the 74LS123, and then re-flowed the 556 and the keyboard appears to be working fine now. So thanks folks... just wondering though, if anyone has a generic "apple ii clone" keyboard schematic? I suspect they are all pretty much the same:

NE556 to generate keyboard repeat, and the clock for the keyboard scanning
74LS42 and 74LS93 along with the eprom for scanning the keyboard...
the 74LS123 one shot and the 74LS273 latch to get the data out.

anyone with a schematic, it would be great if you could post a copy?

Cheers and thanks everyone for your comments and wisdom.

 

[David_M]

Good to hears it working again

I do have one schematic for a clone keyboard here but it uses a MM5740-aap keyboard encoder chip. To my knowledge there are only 3 or 4 basic designs and a lot of variations based on those designs so a schematic for them will generally be of limited use.
It seems like every manufacturer copied from each other and added their own little twist to the copy. Fortunately the keyboard electronics rarely go faulty, it's usually bad keys that are a problem.

 

[waltermixxx]

okie doke, thank you for checking

Cheers.