A 1MB Amiga 500 Without a Trapdoor Card? Here's how...
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- Опубліковано 26 лип 2024
- For this you need an Amiga 500 with a revision 6A motherboard, and an 8372A Fat Agnus chip, plus four 256Kx4 DRAMs (81C4256A - I salvaged mine from a RAM card that had been battery leak damaged)
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Watch the first video in this A500 series: • Amiga 500 "Power Light...
Blog: timsretrocorner.com
Music: A Night of Dizzy Spells by Eric Skiff - ericskiff.com/music
00:00 - Introduction
01:16 - Harvest chips from the dead board
02:08 - Disassemble the Amiga (again!)
03:23 - Explainey Bit
04:59 - Add Sockets to the Board
06:14 - Add the Capacitors
06:50 - Cut the Jumpers (JP2 & JP7a)
08:58 - Put the Chips In
15:47 - Reassembly - Checking the fit - Наука та технологія
Well done and very clear. I really need to pull my 500 out of mothballs. I may have some 256k chips from an old PC laying around.
Good piece of process of illumination Tim. Hussar!
I did this trick on my A500...
You can't do it with a rev 3 of course (different sized RAM chips).... but a Rev 5a can literally be turned into an A500+ with this mod + a 1mb expansion in the trapdoor provided you do have a 2mb Agnus installed.
I found it better to use a MegaChip 500/2000 (DKB made three different ones) with the 8372b installed.
There is a way to use an AdRAM or Baseboard (these are 4mb boards made by ICD and Expansion Systems respectively)... but it involves running a wire or clip here and there... plus If I recall one of their PAL chips on each board has to be "reprogrammed" to address 1mb on each board correctly.
That works best if the boards' autoconfig is turned off and then you still get 1mb of chip RAM from them (then you know the chip redux worked!)
So then its a matter of running a re-written AdRAM or Addmem program to get the remaining 3 mb of SLOW RAM added.
Works great in a pinch if you really need RAM but slow RAM sucked so it wasn't needed unless... you needed it LoL.
Still, even with that mod/hack, you lose a full 1mb if you "don't" install a MegaChip since that board comes with 1mb on it.
So the ultimate trick would be to do this mod like you did....
then get a Megachip 500/2000... then... use an AdRAM or Baseboard (with the 2mb daughterboard) and disable autoconfig... and when you run the AdRAM program or the Baseboards addmem program you get a whopping 6mb of Slow RAM added.
:D
I figured this mess out right when I got my A3000...
So when I started using the 3000... I drifted away from the 500's and ended up selling them all off eventually.
Most RAM I ever had in my A500 with autoconfig was 14 mb ..
Addmem with the Baseboard could have skyrocketed it to 20mb but the RAM would have had to have been remapped to the "uppder bound" 8mb space and not the same address space as the Dataflyer 8mb RAM...
4mb on the M-tec I had was 32-bit and had an extra digit or two in its address ... causing it to be above and beyond that space.
But it was possible to hit 14mb of fast/Slow RAM with a 68000 as long as you know how to address it and make sure the computer has the correct address space.
Of course 6mb wasn't auto'd...
Which was smart.. considering if you boot from any game floppy or WB floppy you'd hit an error if the RAM tried to access the same address space.
Dunno why companies didn't clue into this early on... things could have gotten quite interesting had they actually put a PAL chip to auto sense.. and then re-address RAM chips if conflicts were detected.
WORST hack advice I ever got was being told I had to "disable" that RAM you added in your video (bank 0 has 512k, bank 1 is 512k and is the RAM you add in the sockets)... just to install an AdRAM 540 in the trap door slot!
AdRAM (and Baseboard) allow for 512k of CHIP Ram in one of its banks... I absolutely did not want to that. Felt like a waste of sockets...
So that is what motivated me to find a way to make it all work.
Had 1.5 mb of CHIP RAM when I did... (using a 2mb Agnus in the mobo socket)
Then figured out that another bank had to be "joined" with the Baseboard's 512k to create "bank 2" for the A500. Baseboard's "sub banks 4 and 5 if I recall correctly.
Changing that ... effectively hooked me up.
Too bad I didn't have enough RAM chips to fill out the rest of the board.. or have the daughter card.
you should explore the "Addmem" command for RAM that isn't autoconfigured.
The one thing I didn't get to try... was in my brother's A2000... he had a 2091 with 2mb.. and I plopped in the Dataflyer 8mb board...
10 mb of 16 bit RAM didn't work...
Then years later... I realized... one little line (or make an executable and stick it in WBstartup)... could have moved either the 8mb or the 2mb into the upper "16mb space".
Sigh....
Anyways.. it's something I'd like to see you try in a video.
You learn that trick you will solve problems for A2091 users everywhere (that card is an asshole!, heh)
Back in the day we would basically do the same on a rev 5 board by piggybacking ram and running some bodge wires. IIRC you just had to bend up one pin on the piggy backed chip and run it to the correct spot.
Really interesting video, I've never thought about using RAM from an expansion card before! Your soldering is a lot better than mine! 😂
Thanks. If only my UNsoldering was as good...! Of course, I wouldn't recommend butchering a RAM card that was viable, but something that's destined for scrap anyway, what's to lose?
@@TimsRetroCorner yes, definitely nothing to lose if it's destined for the bin anyway! I've subbed 👍
Good video , I'm of to ebay now and look for a cheap ram expansion
Yes you could use the broken leg chip. With a bit of skill you could atach a makeshift leg.
Only if its worth it 👍
Done it before and agnus makeshift legs etx 😂
might be worth buying a diagrom to check them properly.
Un'ottimo lavoro,fatto anche bene.con il settaggio che hai fatto ai 2 jumper,il tuo A500 continuerà a vedere anche l'espansione ram slow
Thanks :)
Nice, but would you get an extra 0.5mb of slow ram if you put in a trapdoor card? For total of 1.5mb?
I did wonder this, and next time I have a trapdoor RAM card, I'm going to try it. I suspect the answer is no. (for one thing this mod required setting a link that disables the trapdoor RAM. But if that's on a switch...)
@@TimsRetroCorner Yes, i think installing a switch that makes it reversible would be best. Good to keep compatibility for stuff that dosnt like more than 512kb...
if you wanted to save the chips, you should have desoldered them the same as soldering: go to opposite pins.
I haven't heard of that as a desoldering technique. What benefit does it give? You've still got to desolder all the pins before you can pull it.
Probably the RAM expansion board was fine. If you add new RAM chips to it and replace the battery the RTC will work and you can get another 512KB of SLOW RAM.
That's something I definitely want to explore: whether the trapdoor is still accessible to more RAM, or if it's an either/or situation.
512KB slow ram card can be configured as chip ram from the jumpers.
@@valenrn8657 Nope! after the 512KB are added, the AGNUS cannot address anymore chip ram. Any extra RAM added to the expansion slot will become bogo RAM.
Unless you get the unabtanium A3000 AGNUS.
Used to be unobtanium was hard to get... now it's everywhere!
I did the similar thing to my A500 but I didn't use sockets because I was worry that it will be too high and the chips will be touching a keyboard. Now I see that I was mistaken.
It's a close call. There's probably only a millimetre - max - at the lowest point on the keyboard. It could very easily have gone the wrong way. When I socketed the chips in my 1571 disk drive, I found there wasn't space also to add heat sinks. (I actually added them and had to take them off!)