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Old 02 August 2020, 18:12   #1
turrican9
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Ordered a replica A500 ++ motherboard and have a few questions.

So I finally ordered a purple A500 ++ Replica motherboard.

I already have a working A500 plus motherboard. It's not in perfect condition but I have repaired it and will not use it as a donor board.

I've been looking closely at a Rev 6A motherboard I have and compared it to A500 Plus motherboard. Seems to me I can donate at least 95% (Maybe more) of the components. Been studying hi-res pictures of 6A and A500 ++ replica boards.

So far I've found a few logic chips which are not the same. I have ordered new ones.

Then I noticed I had overlooked U36. It's a 74LS05 in the A500 Plus board and it's a74LS38N in the 6A board. What does this chip do, and are these two interchangeable? So can I use the 74LS38N in the A500 plus board?

Of course I know Agnus is not compatible between the boards and I have ordered a new U34 (is 74F373 in the A500 plus board and is 74F244 in the 6A board. Maybe they are interchangeable?)

Have also ordered a new U33 as it's not the same chips. A500 plus board has a 16 pin 74F258 and 6A board has a 14 pin 74F04N. Area below it is also one of the few areas which are sligthly different between the boards.

I have also ordered a few EMI filter caps as I noticed the 500 ++ board had one more 10nf VS the 6A board. Most of the components is pretty much identical appart from a few areas on the boards a little bit different. Looks to me that 95% of the components with the same number on the boards are identical. Filters, decoupling caps, resistors and so on.

Other than that I have plenty of resistors and so on. I plan on donating most of the small components along with all the big components from the 6A board.

I'm not sure I will bother with the whole clock circuitry. We'll see how it goes when I start to get all the parts. This will be a long project.

I have finally ordered a desoldering station as it's hopeless to use solder suckers on these boards.

Oh, and another question, I plan on donating the 4 RAM chips from the 6A board aswell, and also 4 RAM chips from an old 512K memory expansion. Should be fine? Of course I will put in the extra decoupling caps and everything.

Edit, almost forgot the resistor packs. They look to have the same PCB name between the boards and about the same placements but I wonder if resistor packs are the same sizes between the two boards. I will have to measure them and compare to my working A500 Plus motherboard when I start the project I guess.

Last edited by turrican9; 02 August 2020 at 18:26.
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Old 02 August 2020, 19:51   #2
wazza
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I built one of these a few months ago. My donor board was an older A500 board (I forget which revision, but it had KS 1.2, so pretty old). It must have been stored with a RAM expansion with a leaky battery as the board was pretty corroded, so I decided to sacrifice the board for a new build. I went into it with the intention of reusing all of the 74 logic ICs but desoldering from the board was such a painful experience that I gave up on that idea and just ordered new ones from Mouser.

I managed to find a spare 8375 2MB Agnus. The second hand prices of those have recently started to get ridiculous, presumably due to the number of people now building these boards, I expect. It is possible to use an 8372 1MB Agnus in this board using the following adapter: https://www.tindie.com/products/bobs...apter-by-liv2/

If you have not already found it, I recommend the Retro Tinkering channel on Discord which is run by Rob Taylor who makes the boards. There are many members in the chat who have already built them and will be able to help you.
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Old 02 August 2020, 20:06   #3
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Quote:
Originally Posted by wazza View Post
I built one of these a few months ago. My donor board was an older A500 board (I forget which revision, but it had KS 1.2, so pretty old). It must have been stored with a RAM expansion with a leaky battery as the board was pretty corroded, so I decided to sacrifice the board for a new build. I went into it with the intention of reusing all of the 74 logic ICs but desoldering from the board was such a painful experience that I gave up on that idea and just ordered new ones from Mouser.

I managed to find a spare 8375 2MB Agnus. The second hand prices of those have recently started to get ridiculous, presumably due to the number of people now building these boards, I expect. It is possible to use an 8372 1MB Agnus in this board using the following adapter: https://www.tindie.com/products/bobs...apter-by-liv2/

If you have not already found it, I recommend the Retro Tinkering channel on Discord which is run by Rob Taylor who makes the boards. There are many members in the chat who have already built them and will be able to help you.
Thanks. I didn't know about that adapter to use 8372A in A500 Plus boards. Very cool. Will take a closer look at this one.

I was actually planning on desoldering an 8375 from a less than perfect A600 board I have. Should be the same 8375 as far as I can see?

Hopefully the desoldering station I've ordered will make it easier to desolder from my 6A board. The board is less than perfect and I plan on taking most of the parts from it, so does not matter if I ruin the PCB more.

My working A500 plus was built around a motherboard I bought from ebay that of course came with no chips. And the seller had drilled holes under the Agnus socket to pry out the Agnus. It had some acid damage aswell. I sourced an NTSC 8375 Agnus from icomp for that one. I noticed that even these NTSC 8375s are not listed on the icomp site anymore. anyway, I repaired the acid damage and the system is fully working. Using it together with my ACA 500 + ACA 1221. So the NTSC thing is no problem as I configured the ACA 500 to boot as PAL.

Edit: With that Agnus adapter board you will be limited to 1MB chip because of 8372A? If so, it's not that interesting

Last edited by turrican9; 02 August 2020 at 20:12.
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Old 02 August 2020, 20:16   #4
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I forgot to say that there are a few other bits you'll need to save from your A500 board. The video hybrid, if you can get it de-soldered; I couldn't manage, it seemed to draw too much heat and I ended up snapping a couple of pins off. There are replacements available though on that previous link I posted. There is also a ferrite bead that you'll need to keep.

The BOM is pretty helpful in that respect: https://docs.google.com/spreadsheets...haY/edit#gid=0
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Old 02 August 2020, 20:22   #5
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Originally Posted by wazza View Post
I forgot to say that there are a few other bits you'll need to save from your A500 board. The video hybrid, if you can get it de-soldered; I couldn't manage, it seemed to draw too much heat and I ended up snapping a couple of pins off. There are replacements available though on that previous link I posted. There is also a ferrite bead that you'll need to keep.

The BOM is pretty helpful in that respect: https://docs.google.com/spreadsheets...haY/edit#gid=0
yeah, I've looked very closely on this. As I've said, planning on taking most of the parts from the 6A board.

Did you have a proper desoldering station or did you try desoldering with a solder braid or a simple solder sucker?

I know about the huge groundplanes on the A500 boards. They suck insane amounts of heat. But hopefully a proper desoldering station will make it much easier. Should have bought one years ago

BTW: I would be willing to cut the PCB to pieces to be able to unsolder parts as worst case scenario. I'm not gonna save that 6A PCB anyway. And cutting it to pieces will remove the huge groundplanes
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Old 02 August 2020, 20:35   #6
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I did use a de-soldering station. I have one of the generic chinese ones, ZD915 type. I did manage to salvage all of the connectors using that, but it was hard work. The de-soldering station is usually pretty good and I've worked on other computers OK without damaging anything, but with the A500 board it just seemed that nothing would shift some of the solder. Ground plane aside, I think perhaps some of the solder in my board was contaminated.
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Old 03 August 2020, 06:40   #7
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So what does it mean when the multiplex or logic chips have an 'N' at the end of the code? Like U34 is a 74373. But I've noticed these chips usually have an 'N' at the end. Looking at ebay there are several types. Like just straight up 74373 or 74373N or 74373PC. What's the difference? What does these letters at the end mean? I've also seen 74373PC variant used in A500 plus boards.
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