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Old 11 July 2018, 20:09   #1
Mekanikal
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Amiga 600 keyboard problem

Hi!

I have a problem with my amiga 600. I received this amiga a couple of months ago and it looked to be in quite a bad shape. The capacitors had leaked and there was battery acid on the motherboard from a memory expansion. Cleaned it well and made a recap. The motherboard looks fine now and except for some traces from U13 to the keyboard connector which I will check for continuity everything looks like new.

The amiga works perfect. Boots up fine every time except for a problem with the keyboard. If I make a cold start after a couple of hours the keyboard works fine most of the time. If I then make a warm reboot the caps lock with light and start flashing rapidly. After about a minute it will flash more infrequent and slower and then stop. If I make a warm reboot the caps lock will stay lit and if I make a warm reboot again the caps lock will turn off and the keyboard works perfect.

If I make a warm reboot again it will stop working and the next reboot will work. Every second time it will work. All the keys will work and no problem at all.

Where should I start looking? Traces from U13 to keyboard connector? Some problem with the reset function? Should I check the keyboard for bad membrane?

If I hadn't recapped I would have suspected that but since the computer works perfect there must be something with the keyboard circuit?

Happy for some help with this.

/Lars-Erik
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Old 11 July 2018, 23:23   #2
Mekanikal
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I have verified all traces from u13 to cn13 and u36 and all were fine. The keyboard membrane did not have any broken traces either.
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Old 12 July 2018, 12:30   #3
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The keyboard membrane and connector aren't likely to cause that sort of behaviour, it does sound like it's connected to the reset circuit. I would look at the reset lines and make sure they're giving nice clean signals, they should be 5V during normal use and drop to 0V when the reset is taking place, though it can be very quick so a multimeter might not register it.

I would also check for corrosion around the capacitors, oscillator and other parts nearby U13, just in case it's getting a bad clock signal. And check the connections to the CIA U7 as that handles the input from the keyboard controller. But it's not a fault I've seen before so I can't say for definite where the fault might lie...

Anyway, if you haven't already looked at it, http://www.amigapcb.org/ now has a full PCB layout of the Rev. 2 A600, which might help with the troubleshooting.
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Old 12 July 2018, 19:07   #4
Mekanikal
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Thank you, Daedalus! Just a quick update. I found out that the keyboard actually doesn't work on the first cold boot. If I type in workbench the keyboard will lag a lot and repeat the keystrokes multiple times. On the next warm boot it will start to blink. I have tried to count the number of flashes and it seems to be 3 flashes on every "blink". They will be slower after maybe 2 minutes and then stop. First 3 rapid flashes and after about 2 minuter 3 slow flashes. 3 flashes indicates "The watchdog timer test failed". Is this same for the Amiga 600?

The corrosion on the U13 pins are rather massive but they all seemed to have ok traces. I will try to measure the reset lines and the connections to U7.
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Old 12 July 2018, 20:14   #5
wiz12
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If you don't have any luck fixing the membrane you could replace it with a new one.
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Old 12 July 2018, 22:25   #6
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The membrane is fine. All the keys are working.
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Old 13 July 2018, 01:38   #7
Mekanikal
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Both the KB_DATA and KB_CLOCK are fine to U7. I checked traces to the nearby oscillator and it looks fine to. Checked traces to R622 and it also measured 100 ohm. The Amiga PCB Explorer is the best page on the internet by far .

I checked both VCC and RST on U13. They were at 4.82v. Is that too low?

There is no corrosion around U36.

The reset signal was always 4.82 both when the keyboard flashes and when it works. I just measured with my multimeter.
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