Amiga 500 green screen and problematic A501

[Solo761]

I guess this has been asked many times before and I've read probably all of them but maybe I've missed something .

I have one A500 rev 5, it was pretty messed up when I got it, considered dead. I've cleaned it, mostly gunk and dust, it had A501 expansion that had battery leakage, but luckily it was contained to said expansion. Unfortunately it doesn't work. It gives out green screen which should mean bad RAM (or Agnus problem).

I've poped Agnus out, one leg was suspicios so I cleaned it on both sides (chip and socket), but still green screen. Neither of memory chips (16x 41256 chips) are hot to the thouch. I got some new 41256 chips and piggybacked them one by one, but every time I turned it on it was green screen. Tried even piggybacking all I could (I have 10 new 41256 chips) but it didn't help. Last thing I tried was to use Agnus from one rev 6 motherboard that works fine. Unfortunately, still no go. Motherboard looks fine otherwise. No corrosion or cut traces (at least at first glance).

I've read somewhere that amiga goes in sequence when checking memory chips. And only if first few are defective it will return green screen. If some other is problematic it will "only" have reduced memory. If that's correct does that mean that if I replace first few chips with new ones it could work?



Other question is related to A501 that was inside. I've opened and cleaned it. At first it crashed Amiga (working one) if it was connected. Desoldered battery, corrosion was bad, but not terrible. So I desoldered 4 memory chips (ones that battery leaked on, they had almost black legs and their soldering was really dull), cleaned PCB, fixed two lines that were broken and soldered sockets back. It doesn't crash amiga anymore but it also doesn't register as memory. Only works as RTC :/. Last thing I've did was to replace capacitors but it didn't help. I've tried with both old and those new memory chips. A501 is old revision, with 16 chips, they should also be 41256, although they have different writing, 43256 I think, I'll have to look when I get home.

Any ideas what could be wrong with it?

thanks

[DH]

Green screen = loose memory board, SIMM, or a loose RAM chip on the memory board

Here's the self test codes besed on a standard 68000 A500
http://www.amigahistory.co.uk/selftest.html

[roy bates]

the green screen relates to a memory problem,have you tried it without the 501?

piggybacking the memory wont help you.

a green screen can be the support logic on the board as well around the data path.
gary or agnus.

and no,if its the memory it will be a green screen no matter where it is on the motherboard.

im curious as to what revision the board is,im guessing a 3 or 5.
as you mention trying a revision 6 agnus in it,not sure about that one.

[Solo761]

@DH, yeah, I've already checked all that (at least simple methods).


@roy bates

A501 is separate "problem", it isn't connected to motherboard (originally was but there's not point in trying to solve two problems at the same time).

Board is rev5. I've tried exchanging both Agnus and Gary. I used working ones from rev 6 motherboard. But even with Agnus and Gary that are tested OK (from rev6 MB) it wouldn't work. Gary from defective MB is fine, I tried it on working board (rev6) and it powered up fine. Haven't tried Agnus in rev6 MB, one from rev5 is 8371 (OCS chipset), rev6 has 8372A and I'm not sure if it'll work with this older one.

Only ideas I have left is to desolder all memory chips and replace them with sockets so I could swap them easier and test them that way. Or getting new PLCC socket and replacing one for Agnus. Although I checked continuity and those pins I tested were fine.

[kipper2k]

green screen is a common problem on the a500+ normally caused by the battery corrosion. It is not the RAM that is normally to blame, it could also be the supporting logic chips as previously mentioned.. U10, U11, U12 and U13. Without the use of a scope its a case of removing chips, socketing and replacing. If i remember right, only a bad ram chip in the lower area of memory will cause a green screen ?

[zipper]

A contact problem on Agnus can cause it, too.

[Solo761]

@kipper2k

Yes, but this isn't A500+, this is rev5 motherboard, it didn't have battery (except in it's A501) which I disconnected because it has it's own problems .

Would an oscilloscope like this one help?. I've ordered one and it should be here in few days according to tracking. It's not much, but OK for learning and beginners fun.

Yes, I think I've read that somewhere. So for starters Maybe I could try desoldering only first few?

@zipper

I've cleaned agnus pins and also tried to clean the socket. I've pulled every pin a bit out so it could make better contact with pins, but it didn't change things :/.

[roy bates]

to be perfectly honst i wouldnt bother messing with the agnus socket to clean it ,i would just replace it completely.

that scope isent much good.

just because it is not a 500+ wont mean the data path can't be broken in some way or have failed part's,the machine is old.

if your going to desolder the memory to swap them out do them all,the sockets are very cheap.

[Coltch]

The A501 that was installed in an A500 had its battery innards all over the place, it even came through to the pins on the motherboard. I have a green screen problem in this 500 which makes me think the battery has killed/damaged something in the motherboard (reseated chips etc)

[Solo761]

Originally Posted by roy bates

to be perfectly honst i wouldnt bother messing with the agnus socket to clean it ,i would just replace it completely.

that scope isent much good.

just because it is not a 500+ wont mean the data path can't be broken in some way or have failed part's,the machine is old.

if your going to desolder the memory to swap them out do them all,the sockets are very cheap.

Socket wasn't dirty or rusted, I did it just to be safe. Only one leg wasn't shiny. Replacing isn't all that easy as there are a lot of legs on that socket. I only have regular soldering iron and one desoldering iron that has sucker integrated. It works, but to desolder something like PLCC socket it would take literaly hours.

It's hard to desolder even regular DIP chips. That's why I shy from it, not because sockets are expensive . Two days ago I desoldered ram chips on one ram expansion and put sockets there. When I got it battery had already leaked so I wanted to clean it as best as I could so I had to remove them. Those 5 chips (4 ram and RTC) took hour and a half to desolder :/. I have to come up with better method. Either heat gun solder or solder pot. I tried looking for IC desoldering tips for my soldering iron but haven't found any that would fit :/.

Main problem of this machine is that it was stored in some shed (I guess), it was really dirty when I got it. Funny thing was when I washed the case it wasn't yellow underneath all those dirt. At least some use for all that dirt and grit . Motherboard looked surprisingly clean, no visible damage, scratches or like. It seems I'll have to check every signal line.

Originally Posted by Coltch

The A501 that was installed in an A500 had its battery innards all over the place, it even came through to the pins on the motherboard. I have a green screen problem in this 500 which makes me think the battery has killed/damaged something in the motherboard (reseated chips etc)

A501 in this one has leaked but surprisingly not much. Probably the way it was stored (or more precisely thrown away :P) prevented the "juice" from going to mobo. I have one other A500 whose expansion battery also leaked, it didn't get to the motherboard, but got to pins on expansion socket and they had corrosion. This one has shiny pins so it seems they remained dry.



I guess this motherboard will require playing with and desoldering components one by one until (if) it eventually works. Oh well...