Apple IIe keyboard decoder gone bad?

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Joined: Aug 24 2011
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I purchased a IIe several months back and all was well except for the keyboard missing a couple of keys. I finally got my hands on an excellent quality replacement keyboard and I replaced the broken keyboard with the new one. I don't fire up the machine that much as I'm waiting on my CFFA3000 order to arrive before I dive in too much. Before I replaced the keyboard, the encoder/decoder (AY-5-3600-PRO) was functioning perfectly. After replacing the keyboard I powered on the IIe to make sure all keys on the keyboard functioned properly...which they did but to my dismay all of the keys repeated for a short amount of time. The IIe wasn't registering the key release quickly enough it seemed. I can get only one character for a key, but I have to be REALLY fast in pressing that key. I usually get 8-12 repeats of that character before it starts.

Oddly enough, the spacebar doesn't work either. The behavior exists for both of the keyboards I have (new one and the old one with broken key studs).

I've taken out the AY-5-3600-PRO with an extractor (even filed the contacts a little so new metal is showing) and reseated a couple of different times but it didn't make any difference. Everything is making it into the socket. For kicks I also reseated the IOU. The circuits on the main board are good as well (nothing has come lose under the decoder socket).

I figure this is a textbook example of a AY-5-3600-PRO in need of replacement but before I went to that trouble I figured I'd get a second opinion. The price of IIe's on eBay are astronomical at the moment else I would just get a POS one as a spare for this chip. A few months ago I could have gotten one for a song.

I do have a multi-meter laying around, but I'm not sure how I would go about verifying that the decoder is definitely bad.

FWIW, I did search around for a while to find a similar thread, but I didn't see one here or in the Apple II forum. Maybe one exists somewhere, but then again, I want confirmation for my issue independently anyway.

Thanks!

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speedyG's picture
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Joined: Nov 16 2011
Posts: 1436
Re: Apple IIe keyboard decoder gone bad?

Just stay a few minutes more online... O.K. ?
I´ll just checkout the circuitplan and immediatly turn back and edit this post with more information...
for the moment just one point: this does not sound to me like a bad decoder chip... the kaypress is normally stored in a so called schmitt-trigger that is released by a strobe signal and this mistake seems to me to be related with the strobe or with the timed flipflop. )I´ll be back within the next 10 minutes....

first advice: search asimov website for the "SAMs Computerfacts IIe" that contains the circuitplans of the IIe
- the related pages start at page 20 !

the circuits around the Keyboard ROM area partially relevant to this Strobe signal... In that plans they are markes AKD and KSTRB for acknowledge Keyboard ( meaning: hey a key has been pressed ) and Keyboard Strobe ( meaning : O.K. i have picked up the key ) and there is also a ceramic-capacitor marked C70 with the value of 47 pF - it is also responsible for the internal timing of the decoder (close to the notched side of the keyboard decoderchip ) and in small part also the IC UC12 is relevant too ( its a 74LS251 which is important for driving some of the adresslines to the keyboardROM ).

So before struggling for a sparepart-board
my first technical advice would be to checkout the ceramic-capasitor C70 for damage or bad solderingpoint... next check also seating and pins of the keyboard ROM and make sure that it´s the correct version ( some old boards contain a mix of different versions of ROMs enhanced and not enhanced mixed up together - several of my ealier threads / replies treat this topic and can be searched in my profile history )
and then i would checkout the mentioned 74LSS251.... only if all three trackups fail ....
i realy would think about a search for another board.....

The AKD ends up at the IOU-chip ( pin 31 ) and the KSTRB ends up at the pin 32 of the IOU-chip.
sincerely yours
speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Joined: Aug 24 2011
Posts: 4
Re: Apple IIe keyboard decoder gone bad?

I had already reseated the keyboard ROM but I did it again for kicks and cleaned the contacts, but that made no difference. I did a few things with the C70 capacitor though. Bridging it just causes an infinite space...but when I placed a smaller spare capacitor that I had on hand (I didn't have an exact match), I was able to greatly reduce the # of times each key repeated. Not being an electrical engineer or anything, I'm not sure what significance that has if any.

How would I check that capacitor with a multi-meter to see if it is damaged or not (visually it looks fine)?

Thanks!

speedyG's picture
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Joined: Nov 16 2011
Posts: 1436
Re: Apple IIe keyboard decoder gone bad?

hello dyelton,
the fact that a change also affected the amount of repeatings shows that C70 is your trouble. The trouble is - that with a simple multimeter you will not be able to check the required specs of the capacitor ( multimeter only shows a basic "go no go" test but not the capacity ! ). Such a ceramic capacitor is very cheap ( in usual electronic-shop less than a Dollar... ) it is a very familiar part - that even every TV-Set repair-store has in stock for repairs.....
as explained that capacitor is responsible for a inner timing circle of the decoder-chip ( the so called RC-chain of the debouncing-circle - a RC-chain works for timing in that way that a capacitor becomes loaded with current within a defined period of time by electricity passing a resistor ) and within this specified time the number of impulses given from a key trigger only one impulse at the output of the chip.... if the timing is wrong then the debouncing effect is damaged and the chip generates for every "micro-contact" a impuls .... thats what you face as repeatings....
debouncing could also be viewed in wikipadia for explanation....
replace with new one with the defined value
sincerely speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Joined: Aug 24 2011
Posts: 4
Re: Apple IIe keyboard decoder gone bad?

Interestingly enough in the midst of all of this, the system is now behaving even more strangely. The spacebar seems to repeat forever (not the keyboard as I can replicate with a known good keyboard). After this happened I replaced both capacitors at C70 and C71 but it made no difference. I actually have an AY-5-3600-PRO coming to me as well as an extra IIe logic board to have on hand. I suppose my last step would be to replace the decoder per my original assumption.

I haven't been able to find anything online about the space key repeating infinitely. Oddly enough, special keys seem to work just fine (e.g. Ctrl + Reset for a new line or Ctrl + Closed Apple + Reset for system test).

Does this sound like a decoder issue now?

speedyG's picture
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Joined: Nov 16 2011
Posts: 1436
Re: Apple IIe keyboard decoder gone bad?

hello dyelton,
it sounds more like a fault of the encoder (i.E. chances raise that you had the correct idea ) , but still there is also a chance that the trouble is located before the decoder.... if now the the bouncing of the single keys has stopped and now the effect is "reduced" to the spacebar to not be bouncing - but hangup - the better choice to view would be the lines of the keyboardmatrix on the PCB of the keyboard itself or the cable from the keyboard to the mainboard.....
or the connector to the chip socket ( with the chip extracted ) - with a multimeter to measure ohm ( resistance ) you could measure ( after the keyboard is unplugged from the mainboard ) the matrixlines from the keyswitch of the spacebar itself to the connector of the keyboard that is plugged into the connector of the mainboard and check out for mistake if the switch or shortcut with other lines of the matrix ot the switch itself...... ( no resistance if key is pressed - high resistance if key is not pressed )......probably cleaning the switch itself and resoldering it (as well as resoldering very shortly the connectorpoints - to make sure that you don´t fight a bad soldering-point ) would be now the probably best choice for next tasks and the checking the matrix-lines....
viewing the case in entire it could be a case where 2 mistakes joined together and one is shutoff now and one is remaining....
did you get the circuitplan from asimov as explained earlier ? this is now realy important for the execution of this tasks....

http://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Books/Sams%20ComputerFacts%20-%20Apple%20IIe.pdf

beware loading this document might take some time because the size is 65 MB......

alternate is loading from:

http://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Books/

only mainboard is:

http://mirrors.apple2.org.za/Apple%20II%20Documentation%20Project/Computers/Apple%20II/Apple%20IIe/Schematics/

sincerely
speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....

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Joined: Aug 24 2011
Posts: 4
Re: Apple IIe keyboard decoder gone bad?

The decoder chip is the confirmed culprit here. Last night I received my replacement AY-5-3600-PRO and everything works as expected now. I suppose in addition to the new decoder chip, I also have the two capacitors that were replaced prior to replacing the decoder chip. I appreciate your assistance with this issue!

speedyG's picture
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Joined: Nov 16 2011
Posts: 1436
Re: Apple IIe keyboard decoder gone bad?

hello dyelton,
i´m happy to hear that the keyboard is up again and doing it´s duty.... hope you picked up the shematics just for the shelf in case of any future problems .....
and happy typing....
sincerely speedyG

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In early days I had a lot of money but no time - now I have no money but a lot of time....
the second part includes less friends but a lot more joy on life....