exxos's Atari, Amiga & retro forum

Yesterday, while playing with a Spectre GCR on the newly built H5CB, I actually read the manual and something popped up: it’s mentioned that some Mega ST might have problems with their 74LS373, mostly the one using SGS versions. These 373 seems to be too weak and cause issues when adding extensions on the bus.

It got me thinking about the issues I faced with the ST536 in a Mega 4 and also all the recurrent remarks from Exxos about the instability of the Mega ST line.

What would be a good replacement for these 74LS373 nowadays? 74F373,74AS373, others? I would like to change these as a preventive maintenance on my Mega STs.

My Mega ST, which works fine with the ST536 has the standard 74LS373s fitted but has had the oscillator changed for the one used in the H5 and uses the exxos ram replacement. This Mega will run for hours now without issue.

I'd just replace the 373s for some other 74LS373s if you really must change them. No doubt there will be a few that will disagree, probably from Germany.

IIRC, the Pak 030 needed faster ST ram to work reliably, I had 120ns ram which it didn't like, so changed those for 70ns chips and it was all good.

Not talking specifically about the 373, but there do seem to be variations amongst 74 series IC's. Obviously I know there a lot of variations in 74 series logic, such as F, LS, HC, S etc.

The example I'd give is my OSI 600D replica. Once I got it running my video was 'fuzzy' and almost kind of twinkled, but the machine ran fine otherwise. Turned out to be a 74LS04. I swapped in different brands, but no change, so I dropped in a 74S04 and that worked fine. Perhaps different timings with newer ICs? I doubt I'll ever know.

The BOM called for an LS part, but it didn't work. That being said I used a 74LS06 when trying to repair an Atari 1050, but it wouldn't play nicely and only worked with the original LS06. I guess I was lucky that wasn't the issue.

So what you're saying wouldn't surprise me at all.

PhilC wrote: 19 Apr 2025 10:22 IIRC, the Pak 030 needed faster ST ram to work reliably, I had 120ns ram which it didn't like, so changed those for 70ns chips and it was all good.

Same. My first STacy memory board had 120ns RAM in it and I changed it out for faster RAM and my Pak 68/3 board was much happier. :)

In my tests, the F series generally makes things worse. I've always advocated sticking to the LS series. Unstable bus stems from pull-ups. Atari themselves started to move away from 10k to 4.7k. but the mega has issues with the 16mhz clock... Lots of variations there on series resistors across revisions etc. The issues on all the types of motherboards and mixes of chips makes a huge headache. I stopped investigating issues after a while. Was just to many for one person to take on and keep up with. Again it's the reason I build the H4/H5 series as I was spending 90% of the time chasing faults then developing stuff.

Yup, faced all that issues when I was trying to run exxos' 16MHz BOOSTER & TOS206 DECODER in my Mega ST...

Never succeeded to make it work in that machine though.

olivier.jan wrote: 19 Apr 2025 10:12 What would be a good replacement for these 74LS373 nowadays? 74F373,74AS373, others?

Usually it is not a good idea to replace components with much faster parts unless you know exactly what are you doing. As I keep saying, faster is not always better.

If you insist in using a "modern" replacement, probably 74ACT would be the best choice. Using slower parts like 74HCT is not too good either.

rubber_jonnie wrote: 19 Apr 2025 10:30 The example I'd give is my OSI 600D replica. Once I got it running my video was 'fuzzy' and almost kind of twinkled, but the machine ran fine otherwise. Turned out to be a 74LS04. I swapped in different brands, but no change, so I dropped in a 74S04 and that worked fine. Perhaps different timings with newer ICs? I doubt I'll ever know.

I have had some issues with replacing IC's with the same type/family that should work, tested ok, but actually didn't.
Watch out for fakes.. even the simple 74xx and 40xx types can be off..

With every issue you solve by replacing parts with unofficial alternates you will create 10 more.
Read the technical manuals for fixes, or better check out the mandatory fixes here on the Exxos forum.

The resistors seem to be an issue on the Atari's if you try to soup the up..