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I tried swapping the IC15 and IC16. I’m still getting the ’(’ character.

One thing I should mention. It looks like the cursor is blinking on the row just above the ’Zmil’, roughly in the center of the screen. This was even before the RAM swap. I don’t know if it actually is the cursor, since typing doesn’t actually change anything on the screen, but it looks and flashes like a 6502 cursor does.

I’ll probably try to source a batch of 15 ns Fujitsus.

protek wrote: ↑Thu May 07, 2020 6:06 pm I’ll probably try to source a batch of 15 ns Fujitsus.

They are actually 150ns (yes, marked as xx64-15) and this the recommended type DRAM's.
Else there still IC20 which can be put on socket and changed .

^Hahah, yes, 150 ns. Silly me.

I just might get the IC20 changed and socketed. It should probably be an easier job than the hex inverter was as I cleaned my desoldering gun of the clog. Nevertheless, it might be too tight a spot to use my preferred method, combination of desoldering gun and a hot air rework station.

protek wrote: ↑Thu May 07, 2020 6:06 pm I tried swapping the IC15 and IC16. I’m still getting the ’(’ character.

I’ll probably try to source a batch of 15 ns Fujitsus.

Blazing fast. Much better than the 150ms ones suggested up thread somewhere

Swapping IC 15/16 over, and no change to the character? I was expecting the character to change if the DRAM was marginal (problem moves with the chip).

So there is still something odd about data line D3, causing trouble on the ULA's first read attempt.

It was me... I wrote ms insread of ns! Shame on me!

mikeb wrote: ↑Fri May 08, 2020 3:52 pm Swapping IC 15/16 over, and no change to the character? I was expecting the character to change if the DRAM was marginal (problem moves with the chip).

So there is still something odd about data line D3, causing trouble on the ULA's first read attempt.

Would an oscilloscope shed more light into this? I’ve got a pocket sized $10 oscilloscope from China but I don’t know if it’s actually good for anything.

I just realized that the garbled screen is trying to tell the amount of free ram too.

J. M[ NZMM
BYTES FREE

protek wrote: ↑Fri May 08, 2020 5:36 pm Would an oscilloscope shed more light into this? I’ve got a pocket sized $10 oscilloscope from China but I don’t know if it’s actually good for anything.

It might be hard to pick out anything other than fairly obvious problems -- the activity on the data lines will be fast and furious, and varying.

Comparisons between D3 (suspect) and its neighbours (side by side if you have two channels!) may show something. Or triggering off the 1MHz clock to try and keep it somewhat stable!

An idea of what the waveforms look like on a real Oric can be found in P10,11,13 (measured),75 (simulated) of http://oric.signal11.org.uk/files/pub/u ... Schems.pdf

The bit marked "ULA READ 1" is what's going wrong for your machine -- the 6502 access (first) is ok, and the "font lookup/ULA READ 2" to get a bit-pattern for each character (last) is ok too.

Wow, you’ve really done an amazing job deciphering the inner workings of the ULA, Mike!

Yeah, my el cheapo oscilloscope is hardly up to the task. It doesn’t even come with a proper probe.

Anyhow, I went and ordered a set of Fujitsu 8264 150 ns RAMs. I guess I just want to rule out that it isn’t the Korean 4164 being inadequate in general. I’ll try and see if that makes any difference and report back.

Incredible !

I’m still waiting for the new RAM chips to arrive, but I should be getting them tomorrow or the day after. I really hope the new RAM will solve the issue, because I really don’t know what else to try.

Well, no luck with the 150 ns Fujitsus either. One thing I should mention is that, what looks like a cursor, appears to be blinking maybe a 10 seconds from boot-up, before it freezes.

Otherwise, I’m really running out of ideas.

I’ve been going through the steps to find the cause so far, and gone once more through the schematics. There are two things left that affect the ULA which I have not yet tried. There is still one original multiplexer that is soldered onto the motherboard and there is the initialization fix which involves soldering couple of resistors between pins on the ULA and the hex inverter. As I don’t have the resistors but I do have a spare multiplexer and a socket, I’m going to try to swap the multiplexer first and then to see if I still need to try the initialization fix. If those do not help, I’m going to turn my attention to the other Atmos motherboard and see if I’ll have a better luck with that.

Yeah, baby! Yeah!

Turns out it was the multiplexer that was soldered onto the motherboard all along. The original was a bitch to desolder and I managed to lift one pad. Luckily the trace was on the component side, so I managed to get enough solder onto the leg of the socket.
Zap, ping, explode and shoot all work.

I need to give the case a good scrub. I also need to find screws for the case and the motherboard as most of them are missing.

Thank you so much for all the help and advice!

Now, it’s time to turn my attention to the other motherboard.

Well done