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Old 04 February 2015, 07:21   #1

Join Date: Dec 2014
Location: Montreal
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Amiga 500 + slow to chip conversion green screen

Hello cute and furry creatures,

I just bought an Amiga 500 which boots up to a green screen while the power led blink about 10 times before rebooting (*). This is a very classic problem but the originality here lies is that one of the previous owners did modify the motherboard to transform the slow RAM into chip RAM: the appropriate traces are cut and some wires are run from the cut traces and the chip/slow jumper (forgot its number) to a switch at the rear end of the machine. I guess that this past owner wanted to be able to switch between slow and chip RAM at will to be able to play games which expected to see slow RAM.

The computer also initially hosted an A501 expansion which I removed before powering it up.

I am not yet open to the idea to diagnosing faulty memory chips because of this chip/slow setup and also because the first time I booted the machine it went like this:

At first power up:
- green screen with blinking power led, reboot
- green screen with blinking power led, then after a few seconds a kickstart 2.0 insert floppy screen appeared (I didn't try to insert a bootable disk)

Then I turned off the machine:
- flipped the back slow/chip switch hoping it would fix the green screen issue
- turned on the machine
- screen with blinking power led and reboot into a green screen which endlessly rebooted into the same

After that, flipping the switch (with the power off) on or off didn't change the result: I now only obtain a green screen which reboots onto itself endlessly after about 10 power led blinking cycles.

After plugging in a working A501 expansion I get quite different results:
- first a green screen (still blinking led) followed by a grey screen which then reboots to green and cycles to green endlessly
Turning off the machine and flipping the switch:
- yellow screen (blinking led), rebooting onto itself

I also noticed that manipulating the wires slightly can cause either:
- instant reboot of the Amiga
- switch from green screen to yellow then reboot
- switch from yellow to gray then reboot

I hope that the green screen is caused by faulty wiring in the slow/chip switching setup and not by actually faulty memory and the fact that the machine did actually reach the kickstart screen on first power up seems to point in that direction as well but I am interested in hearing opinions about the issue before I start mapping up the modifications properly then checking all wires for proper continuity.

Thanks in advance!

(*) Of course when the seller offered me to test the Amiga when I went to pick it up I declined... Wish I hadn't since that would have allowed me to negotiate a much lower price. Live and learn.

Last edited by Nekoniaow; 04 February 2015 at 07:22. Reason: incorrectly wrote the A500 was new :D
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Old 05 February 2015, 07:20   #2

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Damn, 50+ views and not a single opinion.

I tried to boot the Amiga again tonight and initially got the usual green/yellow screens reboot dance but something changed after I plugged in the A501, did a few tests, then removed it: after switching the slow/chip switch and powering on the computer it finally started to show the usual white/grey blank screens followed by the regular magenta KS 2.0 boot screen. After that all subsequent power-off/power-on cycles showed the regular boot cycle: no yellow nor green to be seen.

The A500 was able to boot the Workbench disk without issue, a few games I tried ended up crashing during loading but it might just be that they needed 1M while there were only 512k present. (It's been so long since I last played Amiga games that I can't recall which are KS2.0/512k incompatible anymore. )

I'm not too sure how to explain this but this seems compatible with my initial impression that the green/yellow screens do not necessarily indicate a chip memory issue but are a side effect something else, possibly bad connections either in the slow/chip-RAM switch system or on the chips sockets.

As I mentioned in my initial post, I would be very happy to hear your opinions and/or suggestions about this issue so do not hesitate! Thanks.
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Old 05 February 2015, 10:14   #3
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fascinating story
what revision of the mobo is this?
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Old 05 February 2015, 10:25   #4
Toni Wilen
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Sounds like unreliable connection in Agnus socket.
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Old 05 February 2015, 11:11   #5
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My pal got green screens on a A500 with a sloppy Agnus contact. He used a wood stick to push the chip down when the error popped up...
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Old 05 February 2015, 11:33   #6
roy bates
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if you wasn't so far away i would offer to replace the offending socket if that was the case.

do you know which revision the board is?
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Old 05 February 2015, 20:27   #7

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Originally Posted by roy bates View Post
if you wasn't so far away i would offer to replace the offending socket if that was the case.

do you know which revision the board is?
Sorry for not including that information from the start, I should have thought about it.

From the picture I have on my phone this is a revision 5.
I'll double check tonight if there are any minor versions numbers, it's not entirely clear from the picture.

Thanks for the proposal, I probably will have to buy an overpriced one on EBay.
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Old 05 February 2015, 22:18   #8
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Originally Posted by Nekoniaow View Post
Damn, 50+ views and not a single opinion.


more details please...

Last edited by pandy71; 05 February 2015 at 22:29.
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Old 06 February 2015, 07:04   #9

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Thanks for the links.
I had found them previously and I think the slow/chip modification was done correctly albeit very sloppily.

I haven't had time to trace and list where all the wires connect but it seems functional since the creature now boots without a hitch. After I can finally figure out where the green/yellow dance came from I will very likely revert it then redo it properly this time.

As for the details: it's indeed a Revision 5 from 87/88 (no minor revision letter) with all chipRAM slots populated (512k total), and Agnus is a 8372A.
I now only get the green/yellow reboot dance on one of the two positions of the chip/slow switch so I presume some of the wires are not exactly connecting too well (or maybe it does require the A501 to be plugged in? I haven't checked the schematics yet). That's not exactly surprising seeing how the guy glued the switch into the hole he made at the back of the machine. (*)

I will write down the schematics of the mod and take pictures this week end. Thanks for the feedback!

(*) I actually labelled the A501 with a post-it written "Filthy": it's covered with greasy fingers marks (yes, the internal expansion!). I also found a not entirely empty potato chip aluminium bag inside the box where all the floppy disks were!
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