25 October 2019, 23:13 | #1 |
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Amiga 600, kept in reset
Hi
I'm troubleshooting an Amiga 600 board which I recapped. There where capacitor leaks in several places and some corrosion damage, so the possibility is that my problem is due a short or open that I haven't found to fix. The problem is that the board is kept in reset when powered. I'll try to summarise what I have measured on the board:
The low level on _RST seems a bit unstable. Sometimes it's rising slowly, and on occasion it seems to go over the high level of _RST, because I have seen the computer get to light grey screen, dark grey screen, red screen or green screen sometimes , when I touch randomly on the board or put a probe tip on _RST. I have also seen flickering between these different coloured screens. I'm thinking that Gayle should drive _RST up, with _KB_RESET being high, but that something is driving it the other way. I'm considering to start lifting chip legs , connected to _RST, to narrow down the faulty candidates, but I'm not super happy about doing that, with the risk of damaging the chips. So does anyone have any idea about other things I could do/check? Is there anything other than _KB_RESET that can get GAYLE to hold _RST low? |
26 October 2019, 00:58 | #2 |
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The CPU itself can assert _RST (and _HLT).
Have you checked the 555 timer (U14) and associated caps and resistors - C612/R612, and C611/R611? If these are faulty they can keep _KB_RESET low. |
26 October 2019, 06:41 | #3 |
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_KB_RESET is high, so there shouldn’t be a problem with the 555 circuit.
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26 October 2019, 09:40 | #4 |
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What revision is the A600? And what colour screen do you get?
Does the reset circuit assert _KB_RESET briefly on boot? And what happens when you do a keyboard reset with CTRL-Amiga-Amiga? You could also try to manually pulling _RST high by shorting it to VCC briefly and see what happens. If something is shorting _RST to ground then it will get hot - so try to fing any unusually hot ICs or components. |
26 October 2019, 12:58 | #5 | |
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Quote:
I don't have a keyboard, just running the bare motherboard, but I guess I could do a soft reset by grounding the appropriate pins, the ones going into the NAND gate that controls _KB_RESET. I am getting some heat from chips, I don't know if it is normal levels though. Nothing burning hot, I would guess around 40-45 degrees Celsius. I will do like this: next week I'll take the board to work, there I can measure _KB_RESET and _RESET with a digital scope, during start and soft reset (only have an analog scope at home). I can also take a picture of the board with our IR-camera, to see if anything stands out. A thought though, wouldn't it be odd if a chip with a _RST input failed shorted to ground? I mean, most chips don't have drivers on the _RST line, only inputs, and inputs usually fail as high impedance, or am I wrong? Another thing, I measured resistance between _RST and GND when the board was unpowered. It was off the scale high, so there are no passives that are shorting the line. |
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26 October 2019, 14:38 | #6 |
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If you believe that no passives are shorting to ground, then the only thing I would suspect is the CPU, as this is the only other thing that can assert _RST. The CPU must have an issue, or it detects an issue - i.e. some issue with data and address signals to the memory chips, and to/from Gayle and Agnus.
I would also suspect U32, the 74HCT244 chip. That might be the easiest first thing to replace. Last edited by solarmon; 26 October 2019 at 14:48. |
26 October 2019, 15:28 | #7 | |
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Quote:
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26 October 2019, 17:12 | #8 | |
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Quote:
https://www.amigawiki.org/dnl/schematics/A600_R1.5.pdf |
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27 October 2019, 07:32 | #9 | |
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Quote:
Is there any detailed description on how Gayle works? I haven't found one. |
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27 October 2019, 18:19 | #10 | |
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Quote:
Unfortunately this is the closest I can find so far: http://eab.abime.net/showthread.php?t=35282 |
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27 October 2019, 18:50 | #11 | |
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Quote:
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29 October 2019, 05:40 | #12 |
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Ok, I did some measurements at work. I'll adress them in a list:
So, it would be interesting to figure out where that square wave is generated. Also, I tried doing a keyboard reset, but it didn't do anything new, just the same thing as when _KB_RESET was asserted/deasserted at power on. Last edited by hadoque; 29 October 2019 at 06:35. |
29 October 2019, 06:30 | #13 |
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IR-image can be compared to thermal footprint of Amiga 600 from another thread: http://eab.abime.net/showthread.php?t=93782
Looks very similar to me. |
29 October 2019, 08:50 | #14 |
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29 October 2019, 09:30 | #15 |
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30 October 2019, 20:58 | #16 |
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Ok, so now I have tried forcing _RST high. The board then goes through the black, grey, white boot process and then it’s stuck on white. I’m starting to suspect the CPU, am I on the right track?
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31 October 2019, 10:28 | #17 |
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From your heat map, it looks like the CPU is the hottest. So I would still suspect this.
Was the A600 working before the recap? Was there any cap leakage damage that could have caused corrosion that could break a trace or cause a short? This could also happen underneath components which is not visible to the eye until you remove it. What colour screen do you get when _RST is kept low? You should really connect a keyboard and see whether CTL-A-A does anything and whether there are any LED flashes on the keyboard that could help troubleshoot. |
31 October 2019, 10:53 | #18 |
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The computer had not been started for 6 years, and it had several places with corrosion, so it’s not impossible that there is damage causing these problems, though I’m really not seeing symptoms associated with the _RST problem that indicates a corrosion problem on traces, possibly that corrosion has damaged chips.
I did not start the computer before recapping, because of the corrosion, I didn’t want to ruin it with a short or similar on power on. The screen is stuck on white when I keep _RST high, when _RST is kept low, screen stays black. I don’t really need a keyboard to do a soft reset, I just trigger the 555, and that doesn’t give any other behavior. I have wired up LEDs to the LED output connector, so I can see if anything would be blinking, and I only get a steady power on light. I guess removing the CPU is inevitable, any other ideas of tests I can do before that? |
31 October 2019, 11:46 | #19 |
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I was suggesting the keyboard so that you can check whether the keyboard MPU (U13) is working OK, along with the other components that lead to _KB_RESET.
I've drawn up this diagram to summarise what components are associated with _KB_RESET and _RST: This is obviously already detailed in the schematics, but I wanted a summarised view that showed everything, and in a form I could understand and annotate. So it may or may not help you at this stage. You can see all the components that are connected to _RST, each one of these could be holding down _RST, for whatever reasons. Or it could be a _RST trace that is shorted to ground. So I would use http://www.amigapcb.org/ to check those specific traces - that they have continuity and are not shorted to ground. |
31 October 2019, 14:58 | #20 |
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The image is really low resolution, can you post one with higher? I can’t make out the text.
I don’t think there’s a short to ground, since _RST to GND is high resistance when power is off, but I will check against PCB layout to see if I can get any other clue. I didn’t know about amigapcb.org. |
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