Started by wyldcat34, 13:32, 07 June 13
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Quote from: wyldcat34 on 13:32, 07 June 13Turn on the 464 and immediately the following pops up:98]PIYTWQ followed by reoccuring arrows (right) til I hit break
Quote from: Bryce on 14:34, 07 June 13However, the characters you've listed aren't all on the same row or column as most keyboard failures tend to display which is quite strange.
Quote from: Bryce on 14:34, 07 June 13Hi wyldcat34, There are several parts other than the membrane that are involved with keyboard input. IC107 (8255 also known as the PIO) produces a four bit scan code, IC101 (74LS145) decodes this into 9 scan lines. The return signals are then read by the AY-3-8912 which is also used to produce sound on the CPC and sends the results back to the CPU via the 8255 again. There are no pull-up resistors involved, so the problem can only be one of these three ICs.You've said you've already cleaned the membrane and tried other keyboards without success (@Robcfg = Read his entire post before suggesting he does stuff he has done already ). The next thing to test is what happens when you turn on the CPC with no keyboard attached? If it is still producing the series of characters, then the AY is usually damaged. However, the characters you've listed aren't all on the same row or column as most keyboard failures tend to display which is quite strange. So I have the feeling that the 74LS145 might be the issue. The other strange thing is that the series stops when you press break. This would suggest that an IC is unstable.So here's my suggestions:1 - Check what happens when you turn on the CPC with no keyboard attached.2 - If the problem still occurs with the keyboard disconnected, swap the AY-3-8912.3 - Check that there's no loose wire / screw or anything under the PCB that is causing a short circuit.4 - Check the capacitors associated with the three ICs. These are the small brownish discs at the end of each IC: C128 for the 74LS145 C124 for the AY-3-8912 C119 for the 8255 If any of these are chipped, broken or damaged, this could be the problem.5 - If you still haven't found the problem, swap the 74LS145.Bryce.
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