Oric Atmos not booting - vertical lines

[john]

I've got a oric atmos that's been sitting for years.

Trying to troubleshoot why its not booting (New to oric's)
Found a 7905 that was giving a 4.2V which I've replaced and its now a stable 5V.
Also found a badly soldered resistor between the clock and ULA.

So now its at least doing something. Was giving a black screen before.
I've connected it to a TV via RF and on the speaker I'm getting constant white noise and vertical lines on the screen.

None of the ram chips are getting hot. Only the CPU is getting slightly warm.

Does the vertical lines give any clues/

[Dbug]

Looks like there are 40 vertical bars, which matches the number of columns displayed by the ORIC, but normally there would be black borders all around the screen and only 240x224 pixels visible in the center area, so not sure if the ULA is, or is not working

[john]

Is there anyway to confirm if its the ULA other than to replace it?

I've run through some troubleshooting guides and checked:
- 5V is stable
- Reset is good
- Checked various clock pins - all good.
- Installed test rom and not seeing any difference to the screen. Looks the same ever with the rom removed.

[Symoon]

"on the speaker I'm getting constant white noise" this seems unusual to me, and if I'm not mistaken could mean a problem with the VIA or the sound chip (I'll let real harware experts confirm or not )
Not sure if these prevent from booting though.

[john]

I've tried the reset trick by pull reset to ground after power on and it fixes the white noise issue. No sound at all after a reset so clearly some reset issue in power up.

However this has still not fixed the issue, though the screen does look a little different now but still with vertical lines.

[Dbug]

Looking at the ROM disassembly, assuming the CPU is working, we would have the RESET handler call the F88F routine, which starts by setting up keyboard repeating parameters, then immediately checks the RAM in FA14 before calling F8B8 to setup the rest of the system.

So if FA14 is executed, you'll see the typical 010101 bit patterns appearing all over the memory when the values $AA and $55 (AA>>1) are written to each byte.

Since this is not what we see on the screen, the only alternatives I see are that:
- The CPU is not actually running
- The memory is not being written to
- What the ULA is showing has no relation to what actually is in memory

[john]

The plot thickens.

Removed the 6502 and replaced with a known good. No difference.

When I notice that D7 on the 6502 was shorted to ground. Turns out a fault with IC12 RAM. Replaced...still no difference. Noooo

Think I'm going to replace/test all the RAM. Failing that will order a spare ULA.

[john]

Some progress - removed all the RAM and tested. Found 2 faulty and replaced but still the same screen issue.

However if I remove all the RAM I get clear vertical bars with no screen 'noise'

Adding the RAM back and the screen goes back as shown in the first pic.

One other test I tried was I found another ROM here which just plays a beep on the speaker which works! (Screen is still garbage)

https://forum.defence-force.org/viewtop ... 706#p20706

According to the post this proves CPU, VIA, AY are ok.

Does this mean its looking like the ULA is faulty?

[Chema]

I'd say the ULA is not working. As Dbug said there are not borders, and it does not resemble what's on the memory. A problem with the voltage regulator suggest something nasty happened.

If you have an oscilloscope you may check data or address lines for activity.

[john]

I've now hooked it up to a RGBtoHDMI board. Thought it might be an issue with the RF side but seeing the same thing. (So I guess that at least rules out the RF section)

I tried flashing the diag rom to a 27C128 and don't see any difference on startup.

[Silicebit.]

Hello John!

Please, check the multiplexers (74LS257).

[john]

Thanks for all the assistance you guys. It's alive!!!! But now more issues

Slicebit - you were spot on with the '257s

Both faulty - each one had a gate inputs shorted to ground.

So now it boots up and screen looks 100%. I thought finally...it's fixed......

I connect the keyboard back and pretty much none of the keys work properly.

Pressing 1, ESC, Z, left arrow, del, A, return I only get a D displayed
Pressing x, q, 2, \, down arrow, ], s I get only a 3 displayed.

So this corresponds to COL 5 and 6 doing something.

Tested the 4051 and it's working. So it must either be the 6522 or the 8912?

The speak beeps when I press these keys so I'm thinking its the 6522?

[Dbug]

Check that the connector is not offset by one pin, some machines have the pins that don't match on each side

[john]

You are a legend - that was the exact problem. All working now.

[Dbug]

Well, let's more say that it's "experience", been there, done that