28 May 2018, 19:51 | #1 |
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Black screen A2000 (rev 4/german)
Het guys,
I’ve just begun unearthing my old Amiga collection that’s been sittning in storage for the last +20 years. And I guess some of it Will need minor/major overhauling. I’ll start with trying to get the A2000 up and running and move on to the other hardware later One of the machines is an old A2000 (rev 4) with the CPU-slot 1Mb ram expansion and two floppies, original kbd and mouse. It also has some sort of joystick/mouse split adapter with a switch and a 1084 RGB->SCART (seeming to lack some color signals, the image is quite desaturated, but that’s for another thread). While demoing a game of Rodland for the kids I flipped the switch on the joystick/mouse-adapter to enable the second joystick on the mouseport and... BOOM. Sceen turned black I kinda forgot, after +20 years, that the splitbox wasn’t supposed to be used hot swap-style. So, I suppose I’ve fried something while flipping the switch. The machine starts, power led is on solid, caps lock works on kbd, ctrl+A+A seems to reset the machine, but floppies aren’t scanned and the screen stays black. What to do?! I can’t stand to bin the system, I know it’s the sucky german motherboard, but the machine is in excellent condition and I was either going to ATX/Vampire it or sell it, but neither will happen when it’s in this state. Help a guy out here? Best regards Christoffer, Sweden |
29 May 2018, 12:16 | #2 |
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This A2K can be fixed, so please do not bin it! Black screen usually means NO CPU detected. If the keyboard is responding this is a good sign. Check the CAPS LOCK LED is that blinking when powering up? If so check the blinks against the following:
When caps shows 1 Blink this could be ROM checksum failure. This has to be continuous as the system LED does blink once on first power up, but then stays off. If it continues to blink once, then ROM checksum has probably failed. 2 Blinks usually means RAM test failed. Watchdog timer failure is down to 3 blinks. Finally, 4 Blinks could be a short exists. Although the above is usually associated with the A500, the A2K is similar. Switching the switch on that joystick adapter could be like connecting and/or disconnecting a device from the rear ports, when power is ON. When doing so with the DE9 ports, you can fry the CIA ICs. Although this would display a light green screen rather than black. (Power should always be OFF when connecting and/or disconnecting any device to the rear, of any Amiga computer.) Did you removed the evil battery off that motherboard? This can also cause issues specially if the unit has been stored for long periods of time, without power. Check for leakage around that area, it can reach the 68K and cause black screen error code. Try disconnecting everything and start with just the bare motherboard then work from there. It is a process of elimination, sourcing the offending item and replacing if necessary. Last edited by MigaTech; 29 May 2018 at 12:29. Reason: Text positioning. |
29 May 2018, 13:05 | #3 | |||||||
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I would also investigate the power supply. I'm not sure if the original A2000 has the same joystick protecting fuses that the B2000 has, so it's possible that the switch, through some fault, has shorted out the 5V rail and left you with an out of spec or unstable power supply. It could still be causing a partial short if it's still connected, dragging the 5V rail down too low for the machine to start properly. Quote:
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29 May 2018, 13:25 | #4 | |
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Thank you for your adding a very detailed break down of my assistance to the OP. I really appreciate it, as I would not like to offer incorrect information that may lead to more issues, for the OP. Although you was quite fair with me in respect to certain information offered. Does this apply to both or just one, at any give time of failure? |
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29 May 2018, 13:36 | #5 | |
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However, on the off chance that there was a total failure of the CIA, swapping the two CIAs would allow the system to boot. It's worth trying anyway. |
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29 May 2018, 21:16 | #6 | ||
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30 May 2018, 09:31 | #7 |
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Wow! This is amazing feedback and it makes me feel Very enthusiastic! I’m even starting to recall some of my lost Amiga-skills just reading your posts.
I’ll try pulling all extra hardware and evaluate the power led. AFAIK the led did dim/brighten, that’s how I came to the conclusion that the machine could intercept a ctrl+A+A. I looked at the battery, and indeed it’s an original, but encased Varta without and apearent signs of leakage(?!) - but I’ll drill down deeper and remove that badboy. Surely I could find suitable CR2032 replacements somewhere? (My A4000 is also in need of a battery swap, it’s begun to look just a bit ”hairy”, but not overrun or spread). The PSU is marked with Amiga 2500(?), P/N 380708-01, 220w and is an absolute Beast. I’ll check the fuse, sounds like a likely culprit due to a potential short. I’d like to replace it with a silent ATX. Has anyone tried this? Stable power is always nice and a silent psu would be nice on the ears. |
30 May 2018, 09:52 | #8 |
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If the LED is starting dim and going bright, then that's something. It does also mean that the reset sequence is completing, which in turn means that the PSU is at least mostly working.
There are battery replacements available that use CR2032 cells, such as this one. If it's encased and hasn't leaked, that's probably not the root cause however. Still, might be worth changing (and the one in the A4000 definitely should be changed as a matter of urgency). That is indeed a noisy beast of a PSU, and would be ready for refurbishment even if everything was running fine. I'm not sure if a full ATX supply would fit, and even if it did, you might have trouble meeting the minimum load requirements. I would check out a small form factor ATX PSU instead, these will be smaller, quieter and better suited to running low loads like an Amiga. Ian Stedman has some useful ATX adaptors available that might make life a bit easier if you're going down that route. |
30 May 2018, 10:51 | #9 |
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I’ll have a look at the battery situation asap, especially on the 4000. I have a feeling that machine it’s about to deserve it’s own thread soon enough.
Any way, status update for 2000. I’ve pulled the RAM expansion, inspected the MB and the PSU - and nothing at least looks out of place, loose or damaged. No bulky caps either, from what I can see. Can’t find a dedicated 5v fuse in the PSU, is there even one? The mains fuse is ofc still intact. I double checked the LED, and I was mistaken. It doesn’t brighten. It’s the Caps-key that blinks on ctrl+A+A. And sure enough, after 20-30 keystrokes the buffer fills and the caps key becomes irresponsive. It does however seem to clear the keyboard buffer when ctrl+A+A. Is that a keyboard thing, or does that mean that at least part of the reset cycle runs? I also noted that the PSU makes a sort of ”electronic / arc-like” noise, apart from the buzzing fan that is. I can’t recall what a fresh PSU would sound like anyway, but would an ATX-adapter and an alternative PSU be a way forward? Is there some intuitive way to test the PSU? I have a bunch of ”modern” ATX PSU’s that I could perhaps load down with, harddrives, would that be enough for testing purposes? Or does the minimum load have to be drawn from the ATX-connector? Eliminating a possibly damaged PSU just seems like a reasonable next step. Removing/swapping the CIA’s also seem, perhaps, easy enough. I’m a bit reluctant to start bending and removing those old chips |
30 May 2018, 11:41 | #10 |
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Before you remove any of those ICs try reseating them. Over time they work loose and this can cause sudden failures and/or non booting systems. Just push down evenly on each chip on the motherboard and then try the system again. It might just be the problem, certainly worth a try.
@Daedalus, Your thoughts on the following please. Black screen code can also mean unspecified ROM error often a result of CIA error, if I am not mistaken. Also if the power LED is dim does that refer to a possible Buster error? The OP stated that the CAP LOCK LED can be depressed more than 10 times, which indicates the CPU is servicing the CIA interrupt requests. Do you think changing setting on J300 might have any effect on this system? He would have to change it when using a modern ATX anyways, right? I also thought that maybe the OP could try a different video output, although if there are no drive clicks, would it be worth bothering? Last edited by MigaTech; 30 May 2018 at 11:52. Reason: Text update. |
30 May 2018, 12:27 | #11 | ||||||||||||
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- Bad/no CPU - Bad/no ROM - Bad/no Buster - Bad/no Denise - Bad/no Gary - Bad/no Video Hybrid - Bad/no bus buffer chips - Bad/no connections to any of these (sockets, vias etc.) Quote:
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30 May 2018, 13:24 | #12 | ||||
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What about resistors do you think any of them might be down? |
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30 May 2018, 13:44 | #13 | ||
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- Check the PSU - Check the reset lines - Check the clocks And if all of them are ok - Check the ROM overlay signal from the CIA. |
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30 May 2018, 16:59 | #14 |
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Ok, I’ve got multimeters and osciloscopes (albeight, also vintage electronics) and can manage reasonable soldering if required.
So, let’s get cracking here... I’ll rip out the PSU tonight and try to figure out if the voltages are stable. If I can get my scope up and running I’ll also evaluate pin 14/TICK. As far as replacement PSU’s go, It would seem that the Rev A / 4.0 motherboard lacks the J300-switch. but the adapters from http://www.ianstedman.co.uk/Amiga/de...x_adaptor.html feature a TICK-generator, so that should solve the TICK-issue. I’ll also reseat/nudge all chips. How would I go about checking the reset lines? Thanks for all the invaluable help! // christoffer |
30 May 2018, 17:28 | #15 |
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Ah, having a scope makes things much more interesting! The reset lines are active low, and are held low briefly as the machine starts before going high. This should let the CPU run, but if the reset line is held low, the CPU cannot start. That can be caused by a failure in the reset circuitry itself, or in one of the chips controlling the reset lines (there are typically 2 or 3 separate reset lines in an Amiga that feed into each other: CPU reset (goes to CPU and custom chips), keyboard reset (comes from keyboard controller and is buffered to the CPU reset), and peripheral reset (also buffered from the KBReset line). I don't have the German A2000 schematics so I can't tell you all that much about the specifics, or even if it uses the same scheme, but most Amigas have reset circuits that are based on a delay timer on boot, and some also use a voltage detection circuit to hold the reset line low when the power supply is out of spec. This makes sure the PSU is stable before letting the machine boot, so PSU regulation or noise issues could cause the reset line to be held low indefinitely.
The Tick signal should be a 50Hz square wave (assuming European 50Hz mains power), and of course if the scope is up to it, the Amiga's clock signals could also be checked to ensure they're present. The B2000 schematics can be used as a vague guide regarding pinouts for the CPU and other chips, so I would have a look at those. Just don't depend too heavily on them as there are significant differences from your machine. |
31 May 2018, 15:12 | #16 |
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You can just add a wire between the TICK pin on the power connector (or Pin19 of the CIA that does the printer port) and the VSYNC pin of the Agnus.
edit: looking at the schematics and the motherboard pic on BBOAH, the A2000A has a tick jumper as well. It is J34. You should cut the trace that connects pins 1 and 2 of this jumper, solder a piece of pin strip on there and place your short circuit block on pins 2-3 to get vsync tick. Last edited by Jope; 31 May 2018 at 15:28. |
04 June 2018, 00:22 | #17 | |
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I haven’t had the time to fetch and hook up my scope from the music studio yet, and I’m a bit afraid that it might be suffering from similar issues as the Amiga (old caps and whatnot). Short OT: I’m considering getting a small and portable scope for these kind of simple analytics. Do you have an opinion on for example the OpenScope? I have however measured the PSU connector with the following results: Pin 1: 5,2V (yellow) Pin 2: 5,2V (yellow) Pin 3: 5,2V (yellow) Pin 4: 5,2V (yellow) Pin 5: Ground (blue) Pin 6: Ground (blue) Pin 7: Ground (blue) Pin 8: Ground (blue) Pin 9: 12,54V (orange) Pin 10: - Pin 11: -11,96V (red) Pin 12: 11,27V (green) - isn’t this supposed to be +5V ? Pin 13: -4,95V (white) Pin 14: TICK unconfirmed at this time All measurements were done without any load on the PSU. I’ve understod that this might be an issue? If so I’ll hook up some hardware and redo the test. From what I can see, the negative pins seem to be a bit lacking, but perhaps within reasonable margins? That +5V USER pin though seems to be way out of spec at 11,27V - what is even a ”5V USER” line and how does it differ from the regular 5V lines? I can’t seem to find any information on it. Could I perhaps just cut it and use power from one of the other 5V lines, or from a separate power source just to confirm the culprit? The search continues! |
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05 June 2018, 15:11 | #18 | |
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Pin 12 is plus 5v and is used to supply the joystick and mouse ports. You could use another 5v supply feed, only make sure ratings are similar to Pin 12. |
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