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Old 16 June 2021, 16:14   #1
Screechstar
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A600 red screen and reset not working

Hello,

I wonder if anyone here could point me in the right direction... as an enthusiastic amateur, I'm stuck! Thank you!

Issue:
1. Red screen prior to the reset part of boot cycle, then completes its reset and all is ok… boots WB and runs games etc. with no issue, except…
2. Soft-reboot Ctrl-A-A is not working

Checked:
1. Amiga test kit results
- CIA timing - ok
- Keyboard - all keys ok
- Memory test - ok
- Video & sound - ok
- Floppy - ok
- Pressing ‘help’ to reset within test kit works (but Ctrl-A-A doesn’t)
- When testing the keyboard, pressing Ctrl-A-A shows raw key codes 63 66 78 78 then it pauses for a few seconds then shows bf fd fe. The 78’s are the reset codes.
2. Kickstart - tested two, the original 37.350 and a 37.300. Same issue.
3. With or without CF harddrive - same issue & floppies always load fine too.
4. PSU is a meanwell RT-50B. Works fine on another A600 I have.
5. Recently “professionally” re-capped - it's a new motherboard to me, there’s minor general corrosion with matt, dark grey solder here and there, but no obvious damage or trace issues.
6. Worked ok when first set-up - booted fine 3 or 4 times, played games for a hour or so, then noticed that Ctrl-A-A didn’t work. Power cycle resulted in seeing the current red screen issue.
7. Alternative keyboard from another A600 - same issue.
8. Multi-key presses work ok in WB (A+Arrow moves the mouse, A+E open ‘execute’ etc.)
9. I’ve checked the schematics and traced all connections in the _KB_reset circuits, from the keyboard membrane connector, though the chips, resistors, capacitors and transistors, to the _KB_reset input into Gayle. All connections seem ok, between themselves and to VCC and ground too. All the resistors rate correctly.
10. There is a single caps lock LED flash on boot, before the reset within the boot cycle. I think that may be normal though?


It's kind of a similar issue to the youtuber GadgetUK164 where a transistor ground was not connected in the reset circuit, but all my connections seem seem good! I'm lost as what to check next.

Last edited by Screechstar; 17 June 2021 at 00:14.
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Old 16 June 2021, 16:30   #2
indigolemon
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Possible damage in the reset circuit?

Have a look at this old thread - sounds like a similar issue: https://eab.abime.net/showthread.php?t=47624
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Old 16 June 2021, 16:36   #3
Screechstar
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Thanks - I'll take a look through this (I missed this one so thank you!)

EDIT: just checked that post and the problems look really similar - they had trace damage to a transistor in the KB_reset circuit, but all my traces and connections seem to check out ok.

It's making me think a component may be bust but wouldn't know where to start!

Last edited by Screechstar; 17 June 2021 at 00:18.
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Old 17 June 2021, 00:03   #4
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Just found those keyboard codes that Test Kit show after pressing Ctrl-A-A.
It’s part of the keyboard self test after start-up, starting with ‘initiate power-up key stream’ = FD, followed by ‘terminate power-up key stream’ = FE.

The keyboard is trying to reset… but not the Amiga?

Last edited by Screechstar; 17 June 2021 at 00:18.
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Old 17 June 2021, 15:17   #5
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Update: Think it's the 555 IC, U14 that's duff.

Thanks for that link Indigolemon. It stated which pins on the U36 should be high or low, depending if Ctrl-A-A is being pressed or not. Put the voltmeter to use and checked each pin and chip U36 is fine.

Following on to the 555 chip, my voltmeter shows that the U36 input to this, on pin 2 of the 555, is working... the input is high and gets pulled low by pressing Ctrl-A-A. However the pin 3 output does nothing - it should change, to cause the reset.

The 555 connects to a transistor Q511 and if I short to ground pin 3 or 1 (which should send the signal to Gayle to initiate a reset), the Amiga resets as normal.

So I'm convinced it's the 555 that's had better days.

Last edited by Screechstar; Yesterday at 17:55.
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Old 18 June 2021, 11:05   #6
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Great work! And glad the thread helped
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Old Yesterday, 17:53   #7
Screechstar
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Solved: Just to confirm… ordered some Texas ne555d chips from RS and they arrived today. Only using a soldering iron, so added a blob of solder on each side of the chip to get some thermal mass and, working quickly between each solder blob with the iron, they stay liquid long enough to really easily remove the tiny chip.

New chip soldered in place and reset works and no red screen on boot! Totally chuffed!!!
Thanks again for your help indigolemon!
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