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Old 04 December 2009, 17:34   #1
absence
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Broken A500

There's an old A500 in my parents' attic that doesn't work. Had half forgotten about it until they wanted to clean up and throw some junk, including the computer, which made me wonder if there's anything to do about it.

It's a PAL OCS rev. 5 with Kickstart 1.3, official A501 memory expansion, and a (very unofficial ) 2 MB chip memory upgrade. Despite falling on the floor once (sending the Action Replay III flying across half the room), and an attempt to insert a Kickstart 2.0 chip from an A600 (red error screen, flashing LED), it worked perfectly fine until the chip memory upgrade.

The upgrade was performed by a random electronics technician without Amiga knowledge. It's a board with a 2 MB Agnus and 1 MB memory which fits in the Agnus slot on the motherboard. There's also a wire with a hook to be attached to a pin on the Gary chip. Finally, there was a jumper to be soldered.

Since then the Amiga always booted in 60 Hz screen mode with fewer lines (NTSC except for the PAL colour clock). I didn't worry too much about it, as I could switch back to 50 Hz with the Action Replay. After a while the computer started to act weird though, hanging when powered on. If I just left it on for a few minutes, a keyboard reset would eventually make it boot properly, as if it needed time to warm up. Once it had booted properly, there were no further problems until it was switched off.

I'm not sure if this was simply a bad connection in the Agnus slot or Gary hook, or if something else had broken very slightly during the chip memory upgrade. It was "stable" (if you can call it that) for several following years. Then all of a sudden it was dead one day I switched it on. No LEDs active, freeze button on Action Replay did nothing, black screen (but possibly Caps Lock lit as usual during keyboard reset, can't remember). Tried a lot of stuff, and once got a green (or perhaps blue) error screen with flashing LED. I figured I'd get it fixed "some day" and stowed it away in the attic.

One thing I didn't consider all those years ago is the possibility of clock battery leakage. I only read about it recently, and it had me thinking. The Amiga died shortly after I got a PC, which meant the Amiga saw a lot less use than it had. Disuse (along with old age, of course) is often the cause of battery leakage, but since the battery is on the A501 expansion, which is encapsulated in metal for shielding, it's possible that a leak would go unnoticed when I opened the computer.

This was 10-15 years ago, and if the battery hadn't leaked by then, it certainly has now. This means I can't tell what the state of the battery was when the Amiga died. My question is, what is the most likely cause of death? If it's battery leakage, a replacement for the trapdoor expansion could possibly bring the computer back to its feet, but I don't want to waste time and money tracking down a working replacement if it's not a likely cause of the problem. So what do you think?
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Old 04 December 2009, 17:44   #2
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Pretty sure you could get it working again. First thing to try is to remove the A501.

Sounds like it is an NTSC MegAChip rather than a PAL one and is probably slightly incompatible. Make a note of the number on the top of the chip and I can advise which version 8375 Agnus to replace it with. Pop out the chip with a PLCC extractor and plop in the replacement.

If you're lucky maybe you can do a swap with an NTSC Amiga user.
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Old 04 December 2009, 18:27   #3
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Quote:
Originally Posted by alexh View Post
Sounds like it is an NTSC MegAChip rather than a PAL one and is probably slightly incompatible. Make a note of the number on the top of the chip and I can advise which version 8375 Agnus to replace it with. Pop out the chip with a PLCC extractor and plop in the replacement.
My parents live far away, so I can't find out right now. Wikipedia mentions two versions of 8372 that seem to support 2 MB and both PAL and NTSC. Come to think of it, there was mention of isolating a pin on the Agnus connector for PAL on the instruction sheet that came with the upgrade kit, but I had some trouble with it and left it connected as it wasn't a problem. The upgrade kit was bought in Europe, so it would be strange for it to be an NTSC chip.
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If you're lucky maybe you can do a swap with an NTSC Amiga user.
Not sure if I understand you right. An NTSC user with a PAL Agnus sounds a bit unlikely.

At any rate, PAL/NTSC is the least of my worries. But do you think it's likely that the cause of death is battery leakage in the A501? You say the first thing to try is to remove the A501, but IIRC an A500 with the chip memory modification won't boot without a trap door memory expansion. I'd have to replace it, which is what I'm wondering if is worthwhile.
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Old 04 December 2009, 19:05   #4
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I was quoted on that Wiki page

I would have thought it would have been very unlikely to be an 8372, most MegaChips (2MB Agnus upgrades) seem to be 8375 based which are either PAL or NTSC not both. But it is not impossible, there are some early (pre 8375) 2Mbyte Agnus upgrades that use A3000's 8372. I'd double check when you get it if I were you. I don't think you can rely on the fact it was bought in Europe not to be NTSC but you are right, makes it much less likely.

They made very few NTSC A600's or A500+'s and I swapped my NTSC 8735 agnus this year for a PAL one removed from an old A600.

Battery leakage will release fumes which can rust the entire A500 motherboard and not limit the damage to the A501

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IIRC an A500 with the chip memory modification won't boot without a trap door memory expansion.
I was not aware of that. A replacement will be very cheap. £5-10 off ebay.
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Old 04 December 2009, 19:16   #5
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Battery leakage will release fumes which can rust the entire A500 motherboard and not limit the damage to the A501
Ouch! I'll take a look when I have the chance. Guess there's not much point in pursuing this further until then. Thanks for the info.
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Old 04 December 2009, 19:22   #6
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The main acid damage will be localised to the A501 though, just everything else will be a nice shade of green (copper oxide)
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Old 04 December 2009, 19:24   #7
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Quote:
Originally Posted by alexh View Post
I would have thought it would have been very unlikely to be an 8372, most MegaChips (2MB Agnus upgrades) seem to be 8375 based which are either PAL or NTSC not both. But it is not impossible, there are some early (pre 8375) 2Mbyte Agnus upgrades that use A3000's 8372. I'd double check when you get it if I were you. I don't think you can rely on the fact it was bought in Europe not to be NTSC but you are right, makes it much less likely.
By the way, do the 8375s support switching between PAL and NTSC in software, or are they hardwired to one of them?
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Old 04 December 2009, 21:33   #8
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60Hz mode in software is still available to a PAL 8375 Agnus.
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Old 22 December 2009, 00:20   #9
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Right, now I've dug the A500 out of the attic. I've unplugged the A501 expansion, but it still won't boot. I haven't bothered with the monitor yet, so I don't know if there's anything on the screen, but I doubt it. The power LED never comes on or flashes (as it does on those colour code error screens). The drive doesn't start to click, but when I hold down Ctrl+A+A, the caps lock lamp is lit (which I seem to recall is normal). This is the same way it acted when it was packed away, and at that point the screen was just black.

I haven't opened the A501 to look at the clock battery yet. How do I go about that without damaging it, in case it miraclously isn't broken? Do I have to unsolder the shield?

The mother board looks good, no visible corrosion on the traces. None of the chips go warm (I seem to recall chips immediately going warm means they're broken). As for the Agnus on the chip memory expansion board, it says CSG 318069-03 8375B 5292 20. Is that PAL or NTSC? It doesn't really matter as far as making the Amiga work goes, but interesting to know either way.

Is there anything I can check to find out what the problem is? Does anyone know if an A500 revision 5 with chip memory upgrade (JP2 jumper "switched") will boot at all without trapdoor memory expansion?

If it doesn't, how is the computer supposed to act? I.e. does it show an error screen or just play dead?

Also, there's a wire from the chip memory expansion board which is supposed to be "hooked" on one of Gary's pins. Which pin is it, and what happens if it's not attached?

Some chip memory upgrade documents mention bending Gary pin 32. This hasn't been done, and it used to work fine without. What does the bending (or lack thereof) result in?
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Old 22 December 2009, 02:03   #10
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Quote:
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The Agnus on the chip memory expansion board, it says CSG 318069-03 8375B. Is that PAL or NTSC?
Unknown. The current Agnus records (which I compiled) do not include an 8375 318069-03. That is not to say it does not exist.

There is an 8372B 318069-03 Agnus. It is a dual capability (both NTSC & PAL) Agnus. It's mode is selected via a pin. (A feature not present in an 8375 Agnus)

Are you sure it says 8375B and not 8372B?

Photo?

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Also, there's a wire from the chip memory expansion board which is supposed to be "hooked" on one of Gary's pins. Which pin is it?
Pin 36 of Gary or Pin 48 of 68000

Last edited by alexh; 22 December 2009 at 02:11.
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Old 22 December 2009, 02:28   #11
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Photo?
Sure, see attachment.
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Pin 36 of Gary or Pin 48 of 68000
Thanks! I even found the documentation for the upgrade: http://amiga.resource.cx/1000er/1000...egagnus50d.lha The board was manufactured by an USA company, so it's not unlikely that the Agnus is NTSC.

About pin 32 not being bent, I eventually noticed the trace is cut on the motherboard instead. Seems to be for hiding the A501 from the Gary so it isn't configured as "fast" memory.

Not much closer to finding out what's actually wrong and if it can be easily fixed though. :/
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Old 22 December 2009, 10:43   #12
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Apparently I already discovered this Agnus but non of the sites ever updated their lists based on my findings

http://eab.abime.net/showthread.php?t=21961
http://www.amiga.org/forums/showthread.php?t=20895

Last edited by alexh; 22 December 2009 at 10:48.
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Old 22 December 2009, 12:55   #13
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Quote:
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Apparently I already discovered this Agnus but non of the sites ever updated their lists based on my findings
I was a bit quick when I said it wasn't unlikely an NTSC Agnus. Found the documentation that came with the chip memory upgrade kit, which consisted of a print-out of the document in the archive I linked to, as well as a Norwegian document written by the company I ordered the kit from. The latter explains that Agnus pin 41 must be isolated for the Amiga to boot in PAL, suggesting a hybrid PAL/NTSC chip. I do remember fiddling with this, but I also remember that eventually I removed the isolation. I'm not sure if it was because I couldn't get it working, or if it was to troubleshoot when the Amiga started acting crazy and needed that "warming up" I mentioned earlier. I can try to isolate the pin again if I get the Amiga booting. Oh, and I found the original Agnus as well. You seem interested in details on them, so here's what it says in case it's any use: MOS 8371 4188 21

But Agnuses aside, I can't do much without this thing booting, so if anyone can assist to that end, please HELP!
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Old 22 December 2009, 13:36   #14
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Agnus pin 41 must be isolated for the Amiga to boot in PAL, suggesting a hybrid PAL/NTSC chip.
Not 100% sure but I believe this applies to an upgrade which uses an 8372 Agnus. All other 8375 Agnus chips are not dual. But perhaps this -03 variant is the exception.
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Old 22 December 2009, 21:14   #15
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If you remove the A501 and your Amiga is setup to use 1MB Chip it will not boot. Get another 512k RAM card off ebay or something to see if it will work.
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Old 23 December 2009, 00:00   #16
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If you remove the A501 and your Amiga is setup to use 1MB Chip it will not boot. Get another 512k RAM card off ebay or something to see if it will work.
That's what I was hoping, but now that I've opened the A501 it doesn't seem to be severely damaged, so the problem is likely elsewhere. Still it might be interesting to know, what does happen if you boot a chip memory upgraded A500 without trapdoor memory expansion? Will there be a colour code on the screen and flashing power LED, or will it seem completely dead? Or something else?
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Old 23 December 2009, 23:24   #17
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hm, if I recall correctly it does boot, only with 512K chip ofc..
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Old 24 December 2009, 03:23   #18
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If you remove the A501 and your Amiga is setup to use 1MB Chip it will not boot.
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hm, if I recall correctly it does boot, only with 512K chip ofc..

Anyone know for sure?
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Old 31 December 2009, 15:52   #19
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Using a multimeter I've found that the wire with the hook from the chip memory expansion board to the Gary chip is broken. As it connects the A20 line between Agnus and the rest of the Amiga, there's no wonder it doesn't work. Still, is going completely dead the "right" way to not work in this case, or should there have been an error screen? I'm asking because I don't know if this is the only fault.

I'd rather not solder a new wire directly to the chip, and have seen that some 2 MB chip memory expansions use a socket to be placed between the mother board socket and the chip instead. Can these be bought separately? They seem like a much better option than the flaky hook.

Finally, out of curiosity, couldn't a 2 MB chip memory expansion board theoretically be designed to not require any modification of the mother board by using modified sockets for both the CPU and Gary chips? A wire from the CPU socket to the expansion board could bypass JP2 alltogether, and the gary socket could connect A20 and isolate the EXRAM line. Would it work? If it would, why do these upgrades require modifications? Seems a bit shoddy to me.
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Old 09 October 2011, 20:12   #20
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My, my, how time flies. Eventually got around to soldering the A20 line to the Gary chip, and verified that there is connection between the 68k's A20 pin and the memory/Agnus card. But it still doesn't work.

Did some measuring and found that there is a 5V difference between GND and VCC for most chips, but the ROM had somewhere around 1-3V (forgot to make a note ...). I assume that's bad news. Anyone know how to troubleshoot further?
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