exxos's Atari, Amiga & retro forum

PhilC wrote: 07 Apr 2026 08:29 @stephen_usher my Lynx doesn't have composite out but the RF looks so much better than that. I use it with RGB-Scart and then through my OSC.

Btw, text will only be green on black if you enter the command "TEXT." Otherwise it's White on black and s...l...o...w...

At the moment I only have one 74LS166 fitted on the green channel, so everything's green or black. :-) I forgot to order three of the chips in my Mouser order. The extra chips are in the post. I hoped that they'd arrive on Saturday but there was no post.

I think I may give up on trying to get colour composite working. I do have RGB out.

The chips arrived today. They were awaiting me on the door mat when I got home late this evening.

There's good news and bad news:

The good news is that the keyboard works and I can run BASIC programs.


The bad news is, not for very long.

If I leave the machine idle at the BASIC prompt for more than about 15 seconds the machine will either crash immediately or reset or crash if I hit the return key.

If I type in a counting FOR loop program and run it then it will run and continue to work until it ends or I hit escape. Then the machine will crash.

It feels very much like a RAM refresh issue on main RAM. If the RAM locations are being accessed then they are refreshed by that activity. If not then the contents fades away.

Anyway, no time to test this tonight, so I'll have a look in the morning.

stephen_usher wrote: 08 Apr 2026 00:05 The chips arrived today. They were awaiting me on the door mat when I got home late this evening.

There's good news and bad news:

The good news is that the keyboard works and I can run BASIC programs.


IMG_5035.jpeg


The bad news is, not for very long.

If I leave the machine idle at the BASIC prompt for more than about 15 seconds the machine will either crash immediately or reset or crash if I hit the return key.

If I type in a counting FOR loop program and run it then it will run and continue to work until it ends or I hit escape. Then the machine will crash.

It feels very much like a RAM refresh issue on main RAM. If the RAM locations are being accessed then they are refreshed by that activity. If not then the contents fades away.

Anyway, no time to test this tonight, so I'll have a look in the morning.

Well that's good progress IMHO 😁

Now onto the phase of post build troubleshooting, otherwise known as the swearing at it a lot phase 😁

:lol:

The first thing I'm going to do is swap the chips in the lower and upper banks of memory to try to rule out a bad chip. It's quick and easy.

Some major progress this morning.

I tried swapping the RAM between the upper (video) and lower banks: No change.
I tried swapping the lower RAM with different 4164 RAM (mix of 150ns and 120ns): No change.
I tried swapping out the CPU with one marked as being faster in case it was a clock rate issue: No change.

All this RAM was TI stuff so usually bulletproof.

My theory at this point was that the RAM was too fast and the system wasn't refreshing quickly enough.

/RAS and /CAS on the chips all looked good. Better on the upper, video, RAM but sufficient on the lower.

I was about to give up but found a cache of 80ns 41256 chips. I wondered if the problem would be worse with these chips...


Note that they are not TI chips...

Well, erm, the machine works now and is stable. No crashing or reboots. I've had it running for-next loops for half an hour now.

I still have TI chips in the upper RAM but that seems fine.

Interesting.

What speed RAM is specified for the original?

rubber_jonnie wrote: 08 Apr 2026 12:35 Interesting.

What speed RAM is specified for the original?

From photos 200ns Hitachi RAM.

stephen_usher wrote: 08 Apr 2026 13:18

rubber_jonnie wrote: 08 Apr 2026 12:35 Interesting.

What speed RAM is specified for the original?

From photos 200ns Hitachi RAM.

All a bit peculiar then...

I'm now using TM4164-15 chips in my "ram" bank without issue, so it's odd you're having issues with them but I'm running at 4Mhz right now.

PhilC wrote: 08 Apr 2026 13:59 I'm now using TM4164-15 chips in my "ram" bank without issue, so it's odd you're having issues with them but I'm running at 4Mhz right now.

But you have an original machine with the "correct" combination of 74xx, 74Sxx and 74LSxx parts, which have different timings. Mine is all 74LSxx which will skew things slightly.