A2000 R914 resistor (keyboard problem)

[duga]

Hi

So, I have this A2000 with a sporadically working keyboard. Some keys never work, some work sporadically. Caps lock blinks once at power up.

My theory right now is that the R914 is the cause of this issue, with the help of http://home.arcor.de/kickstart/TKA/Tutorials/troubleshooter/trouble2.html and other sites.

Before the resistor I have 4.97 Volt, after the resistor I have between 1-3 Volt as it differs a lot.

Problem: I only have 0.25 W resistors but no 0.5 W resistors. Any suggestions for a replacement resistor? It should be 1 Ohm, 0.5 Watt according to the mentioned site but as I said I only have 0.25 Watt resistors.

[duga]

No offense, but this seems like a no brainer.. Just buy the correct resistor and replace it
they should be available at any electronics supply store.

If you are in the usa, try mouser,digikey or such.

Here in Sweden what you find on a Sunday (at Kjell & Co or Clas Ohlson) is mostly 0.25W resistors in big packs. I already have that.


I changed the capacitor C912 as I had one with the right specs, didn't make any difference.

[duga]

If you have two 2ohm resistors you can attach them in parallel, That will result in a 1ohm/0.5w resistor. If you have the physical space for it, and the soldering skills of course.

Thanks, but I only have 0.25W.

[duga]

Aha. No, nothing smaller than 7.68 Ohm 0.25 Watt here unfortunately.

[duga]

I don't have another keyboard to test with nor another A2000/A3000 to test this keyboard on.

I measured it while running, both with and without the keyboard. I don't get what you mean by "turn the computer off, and measure it with a multimeter".


I should probably mention that (of course) there has been a battery leakage. Battery is removed. Have seen much worse damage than this though.

[duga]

There's a switch in the back of the computer. Turn that to the *off* position. Unplug the keyboard. Also unplug the power cord from the system. Open the system. Locate the resistor. Switch the multimeter to "Ohm", written as a big omega. Connect one wire of the multimeter to one end of the resistor, connect the other wire to the other end. What do you read?




Then apparently there is more damage than you assume. Really. That's a number one reason why strange things happen. Just changing the resitor will not provide much. Keyboard power and wiring is pretty much in the same position as the real-time clock, namely in the front-right position of the board.

In that case, remove the board completely and clean carefully, inspect all wires. It wouldn't surprise me the least if some of the connections seem to look good at first inspection, but fail to connect properly because they were corroded by the battery.

Thanks. It reads, after "jumping" for a while, 14 M ohms. Yes, 14 M ohms. Not 14 K ohms or 14 ohms.

[duga]

That's more like the resistance of your body, from one hand to another. With that resistance, the keyboard would be completely inoperable, not "half way broken". Hence, your meter did not make contact with the resistor properly. It *does* supply *some* current to the keyboard, after all, and for that, 14 M Ohms is way, way, way, way too large.

I guess the layer of battery acid on the resistor legs doesn't help. I measured other resistors and they show the correct values.

Expecting new components in a few days.


Edit: in the meantime I removed one 470 ohm resistor that shouldn't exist on a rev 4.5 board (originally rev 4.3) as it is already on the motherboard as R1000 from rev 4.3 and up. Source: https://ftp.fau.de/aminet/docs/hard/Amigafix.txt . Not related to the keyboard issue of course.

[duga]

Fault most probably found. It's on the keyboard. On the circuit board there's a black dot which I presume is short circuiting four lanes. That explains why some keys are working and others not.

The model is:
Mitsumi
KKQ-E96YC
PCB A-2000 (E)

[duga]

"black dot" removed, problably didn't cause any short circuiting. At least the fault is still the same.

Working buttons:
q, w, z, 7, 4, 5, 6, 1, most of the other buttons on the Numpad and a few more.


Can anyone confirm if the 16 leg chip on the circuit board should be 74LS123 as mentioned on http://home.arcor.de/kickstart/TKA/Tutorials/troubleshooter/trouble2.html or HD74HC123AP as on my board? Can they be considered to be the same type of 74 chip?

The mentioned chip + the big MOS chip might have been changed on my board.

[duga]

It was given to me recently.