exxos's Atari, Amiga & retro forum

stephen_usher wrote: 08 Apr 2026 14:41

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.

Oh I understand that, I'm just surprised you've had to use such different / faster ram.

I think that I'm going to have to make up a little PCB for the composite video based upon this circuit: https://share.google/uJ3lG3TKsVCklSXEi

PhilC wrote: 08 Apr 2026 14:47 Oh I understand that, I'm just surprised you've had to use such different / faster ram.

So am I! :-)

Well, it's not completely fixed.

After running a nested set of for-next loops for a couple of hours the program was corrupt.


This is going to be a devil to diagnose as it's obviously timing and just one the margins of working, possibly within a nanosecond or so.

Ordered 74Sxx versions of all the chips on the original which were the S variants. They're all in the RAM and I/O signalling/switching circuits. Given that these are parts which can be up to an order of magnitude quicker than the LS parts it's likely that these are timing critical.

stephen_usher wrote: 08 Apr 2026 18:24 Ordered 74Sxx versions of all the chips on the original which were the S variants. They're all in the RAM and I/O signalling/switching circuits. Given that these are parts which can be up to an order of magnitude quicker than the LS parts it's likely that these are timing critical.

I had a bunch of issues with my OSI600D, swapping around for different versions of ICs made a big difference.

Seeing as I can't get further with the electronics until the 74S series chips arrive next week I've been working on designing a 3D printed case.

So far I've got the lower half done, other than a minor miscalculation for the DIN port position. I can fix this in Fusion but the current print is good enough for now. I'm using 1mm brass rod to help join the two halves.


I've also been designing an upper case and the first draught is printing now. Though as I've effectively run out of grey filament it's going to have to be brown for the time being. The print is going to take over 10 hour and use a quarter of a kilogram of filament. It's got the same port issue as the lower one as I didn't notice this until after I'd started the print whilst I was putting the lower half together.



The one electronic thing I did do yesterday was change back to using a 5V input and installing a DC-DC converter board to generate the 12V. These things are so cheap! I bought 10 of the boards for ~£7 off Amazon. I had to add a 100uF cap on the output to suppress the switching spikes but other than that they do a good job. Unfortunately the "3A" 5V PSU I bought at the same time isn't well regulated. About 5.7V unloaded but as soon as its connected to the computer, which is drawing about 1.2A, the voltage drops to about 4.8V. At least it's clean with imperceptible ripple.

Nice neat solution there Stephen. Did you copy the original case or design your own?

PhilC wrote: 10 Apr 2026 16:42 Nice neat solution there Stephen. Did you copy the original case or design your own?

I'm using photographs for reference plus the PCBs then designing the case to be as close to the original as I can.

stephen_usher wrote: 10 Apr 2026 17:09

PhilC wrote: 10 Apr 2026 16:42 Nice neat solution there Stephen. Did you copy the original case or design your own?

I'm using photographs for reference plus the PCBs then designing the case to be as close to the original as I can.

Sounds good. There's someone selling replacement case badges on eBay if you've not seen it already?