Difference between A500 models

[Oldsmobile_Mike]

Maybe a rev. 6A board inside?  What color are the power lights?  Usually red is an older 5 board, green is a newer 6 board, but with Commodore who the he#$ knows, really?  Best to open it up & check!  

[Oldsmobile_Mike]

Excellent!  That's the best one!  You should have the 2MB Agnus in there and can easily add the extra chips to the motherboard and trapdoor expansion to bring it up to full ECS, A500+ capability.  Lucky you!  

[Oldsmobile_Mike]

I've tried using a cleaning disk (not one with the alcohol fluid, but a dry one that doesn't spin) but that did not work.
Strange thing is: both diskdrives do the same, shouldn't I look somewhere else for the drive error?

About the Agnus, I'll look later today what it says.
@Mike: what trapdoor expansion was in the A500+ originally? I've played on one about 19 years ago, a friend of mine had one, but I've never opened his A500+

A500+ originally had 1MB of chip memory on the motherboard with an optional 1MB chip in the trapdoor (A500+ boards came without the battery-backed clock since A500+ already had a clock on the motherboard).  When Commodore stock dried up on the original A500's they started selling a lot of A500 systems with A500+ motherboards, minus some of the extra chips.  Full A500+ would have had ECS Agnus & Denise, 2.04 ROM, and the 1MB on the board.  From the photo you posted it looks like you'd just need to solder on the extra chips to bring yours up to capacity; you can probably find those on a dead A500+ board (lots out there, killed by battery leakage) or remove from any A501-style memory expansion board.

Re: the floppy drive, try swapping out Paula and the CIA's, perhaps?

[Oldsmobile_Mike]

I've got it running now. The head was stuck because of old grease, or at least that's my best guess. Put some new grease on, cleaned the heads and now it's running perfectly.
I put a KCS power board in the trap door: also works (or at least the 512K expansion works). Without it it's just another Amiga 500 with 512K, but the keyboard itself is an A500+ as well: it has two blank keys on the US layout.
One on the right of the left shift and one left to the bottom of the Return key.

Excellent!  Keyboard is just a C= cost-cutting measure, it's an international keyboard, the blank keys would have different symbols on them in some foreign markets.  I've seen them in A1200's and various other systems over the years.  :p

[Oldsmobile_Mike]

Check this auction for pics of full A500+ motherboard and memory expansion.  There's usually a lot of the 1MB trapdoor cards on ebay if you poke around.  

http://www.ebay.com/itm/Amiga-500-Plus-system-w-2mb-chip-ram-expansion-68010-Full-ECS-KS-3-1-PS-/151071809969?pt=US_Vintage_Computers_Mainframes&hash=item232c94e1b1

[Oldsmobile_Mike]

I edited my post, attached a picture of the Agnus chip now

Yes, that is the correct chip, supports up to 2MB of chip RAM.  :-)

[Oldsmobile_Mike]

SysInfo is a horrible, horrible program.   Your best bet is to either crack her open or use something more updated like WhichAmiga to detect what you've got inside.  Also there is no ECS Paula, C= never updated the old gal, even A1200/4000 have the same Paula.  Not sure about installing the chip in the socket minus one pin (you mean one row of pins?).  I'd wait for someone more knowledgeable to come along, or maybe just get a correct 3.1 chip off ebay?

Oh, here you go.  Probably way more information than you possibly need, here:

https://en.wikipedia.org/wiki/Amiga_Enhanced_Chip_Set

http://en.wikipedia.org/wiki/Amiga_500_Plus