exxos's Atari, Amiga & retro forum

And we're back in action.

And with an unusual problem.

It seems that CAS is high all of the time on U44 to U47 on my Mega, hence only seeing the 2mb of ram when using the SMT MMU. Plug the old MMU back in and its back to 4mb right away. Continuity to pins 17 and 18 from the ram chips is fine. I'm just wondering if it's anything to do with the type of ram that I'm using. I've got the SOJ20 to 20pin DIL adaptors with 514400 ram installed.

Edit 1

Just looking at the ST schematic in comparison to what I have and the resistors on the schematic for the CAS lines show 68ohm. I'm measuring 35ohm on all 4 connections. Now to try and track down a C103277 Rev 2.2 schematic

Edit 2

And 33ohm is what its supposed to have. on /WE /RAS /CAS etc. /OE is low on all ram chips and /WE is high, so at the mo I have no idea. when I plug the PLCC MMU back in it works straight away at 4mb. This is also the second SMT MMU i've used and its exactly the same.

@exxos, when you checked your MMU, was it on a 4mb ST?

PhilC wrote: 03 Apr 2019 17:49 @exxos, when you checked your MMU, was it on a 4mb ST?

It was a 1MB machine. But I will try and get it tested on my 4MB one tomorrow.

exxos wrote: 03 Apr 2019 20:01

PhilC wrote: 03 Apr 2019 17:49 @exxos, when you checked your MMU, was it on a 4mb ST?

It was a 1MB machine. But I will try and get it tested on my 4MB one tomorrow.

Can you also try booting a floppy with it please? My Gotek wont boot with that MMU installed but once again, put the old one back in and it boots.

This Mega has had all the floppy patches btw.

PhilC wrote: 03 Apr 2019 20:19 Can you also try booting a floppy with it please? My Gotek wont boot with that MMU installed but once again, put the old one back in and it boots.

This Mega has had all the floppy patches btw.

I always use floppy.. So its working fine :) In fact I have been testing 1772 chips most of yesterday, so formatting 1.44 floppies loads all day. No issues.

It might be my Ram playing up AGAIN. I'll have a look at this over the next few days but I think my Ram was ok, until I did the usual playing around to double check all the connections for/OE etc, so would still be cool if you can rule out the 4mb thing. At least I'll know I have to find ANOTHER day joint in my Ram circuit then.

Old Ataris. Who in their right mind would have one!

Just got home, tried my 4MB machine and it sees and tests 4MB fine. So the SMT MMU board is good.

Ok thanks. More repairs needed.

I think I may just dump the ram and fit a 4mb upgrade, bypass all the old stuff.

Will try it in my Alpha board later on but I'm quite sure it works.

Not bragging but I did some of my best SMT work when I soldered the replacement chip in.

Then time to fix the Mega, put a TF534 in it and leave it alone :D

Amusingly my Mega now works again, thanks to a dodgy pin on a socket and with the original MMU I see 4mb. Still only see 2mb with the new one.

I'll dig out the STF Remake and try it on there. Going to leave this poor old fragile Mega alone now (at least until I get my pak back or put a TF534in it )

Think that MEGA is very poorly!