exxos's Atari, Amiga & retro forum

Well, my replacement IC's arrived as did my 40 pin sockets, but not my 48 pin sockets :(

I did drop in a replacement Gary and Paula but no change. I doubt Denise is an issue, so getting worried about Fat Agnus now... They aren't cheap.

So, I will get the sockets replaced next (I can do the CIA sockets now) and see if that makes a difference, if not then Agnus is in the picture. Everything started going a bit wonky when I removed Agnus from its socket, and though I have a new socket in there already for Agnus I'm a bit concerned.

One thing though. I replaced IC34 &35 fairly early in the process despite the originals testing fine, but I just touched IC35 and it was burning hot. So I pulled it and tested it and it was dead. So I pulled IC34 and that is part dead.

Now although they are 74F244's I just dropped a set of 74LS244s in and the Paula tests and also Denise tests now pass without the machine crashing, and same with the original IC's, but I've left them out on account of the legs being trimmed very short. I'm sure I tested both ICs before I put them in, but there you go, these things sometimes die. I wouldn't be surprised if they are fake.

It would however account for the instabilities I've seen if both IC34 & 35 were misbehaving.

However, as steps go, it's a good one as it seems to have stopped some of the other strange unstable stuff happening, so I can get back to fixing the FDD issue.

They made millions if not tens of millions of 8372 1MB Fatter Agnus chips and after CSG closed they had large stocks (of 8375 1MB). But no one wanted them because under normal conditions Agnus chips never break. A 1MB Agnus should be available relatively cheap. (Although I couldn't find one for less than £30)

It is the 8375 2MB Fatter Agnus chips which are extremely sought after. Almost all the CSG stock went into building MultiMega 2MB ChipRAM upgrades for older Amigas. They now regularly fetch £50+ if you can't get a 1MB one consider a 2MB one and a MultiMega adapter.

alexh wrote: 10 Feb 2025 15:13 They made millions if not tens of millions of 8372 1MB Fatter Agnus chips and after CSG closed they had large stocks (of 8375 1MB). But no one wanted them because under normal conditions Agnus chips never break. A 1MB Agnus should be available relatively cheap. (Although I couldn't find one for less than £30)

It is the 8375 2MB Fatter Agnus chips which are extremely sought after. Almost all the CSG stock went into building MultiMega 2MB ChipRAM upgrades for older Amigas. They now regularly fetch £50+ if you can't get a 1MB one consider a 2MB one and a MultiMega adapter.

Mine is an 8732A, cheapest I can find is £34 in the UK (We found the same one), an 8375 is at least double, though an 8371 comes in about £15.

EDIT: I ordered the 8732A. I like to have spares and I can always sell it on later if I want to.

9ns average delay for 74LS
6ns average delay for 74F

So if you have memory issues with the onboard ram/trapdoor expansion you should exchange those.

What are the problems you are having now?
Still the floppy motor that will not turn on?
You can change the CIA's around without any change?

Maximilian wrote: 10 Feb 2025 18:46 9ns average delay for 74LS
6ns average delay for 74F

So if you have memory issues with the onboard ram/trapdoor expansion you should exchange those.

What are the problems you are having now?
Still the floppy motor that will not turn on?
You can change the CIA's around without any change?

I need to go back and review where I'm at, so I'll let you know. I just downloaded the service manual so I will probably also work through the FDD section of that.

Just use https://www.amigapcb.org/ and select the A500 Rev8 for the floppy.
On the Rev 8 the memory is different and there is a real time clock option, but the rest should be 99% the same.

U36 is an Nand gate set up as to invert some signals from Gary to the floppy drive.
Are these ïnverting the signals?, do they change while accessing the floppy?
Input 2 - output 3 mtrx
Input 5 - output 6 mtr0

OK, so I'm back to booting from floppy now.

I noticed that U8, a replacement CIA was pretty hot, so I pulled it and decided to replace the socket at the same time. I dropped the original U8 back in and lo and behold I'm booting again.

What was weird with the replacement is that the Gotek went into direct access mode, but at least I can boot again after swapping back.

So it looks like one of my replacement CIAs was bad, I guess it's one of those things when buying replacements that are system pulls. Ho hum...

So, I also noticed that if I use 74LS244's at U34 & 35 rather than 74F244's it won't boot with the trapdoor RAM plugged in, so the 74Fs are back in place and I can boot with the trapdoor RAM installed. I still can't see the RAM, but the RTC is still visible to the system.

Booting to Amiga test kit and Denise and Paula check out fine. At lunchtime I will replace the socket for U7 and I'm hoping the 48 pin sockets will arrive today and those will get replaced too.

So that leaves me back where I started essentially. Bootable but no sign of the trapdoor RAM. Given Gary and Paula are replacements I can only think that the last possibility is Agnus and I have a replacement coming.

If it isn't Agnus then I'm not entirely sure what's next.

Finally got to test this:


So I have 8.5Mb now.

If I can get the trapdoor working then I'll have 8MB fast and 1MB chip.

Needs to work out the IDE adaptor as well.

Progress in the right direction I think.

Unless you change the jumper on the motherboard the trapdoor RAM won't be Chip RAM but "Slow" RAM.