Oric Atmos problems

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RetroUser
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Oric Atmos problems

Post by RetroUser »

Hi everyone,

Thank you for having me on the forums. I guess it's the usual story of buying a computer off eBay and having problems with it.

The story begins with buying an Oric Atmos in an untested state which was okay, could be lucky or not as the case maybe. On opening the computer (I hadn't turned the computer on as of yet), found the ROM was inserted upside down. Removed the ROM to find a leg was missing. Soldered a leg on so I could read the ROM contents. The contents didn't make sense as couldn't see any ASCII text such as Oric. However, on close inspection noticed some bytes were the same and some weren't when compared to the ROM image available online. Turned out the reason some bytes were different was because bit 4 was 1 when it should have been 0 in comparison to the ROM image online. Those bytes that were the same as the ROM image had 1 in bit 4 anyway so assume the ROM was returning a 1 for bit 4 for every byte.

Burnt an EEPROM with the ROM image online and inserted into ROM socket. Turned on and was greeted by black screen with white thin vertical lines must like the ones shown https://forum.defence-force.org/viewtopic.php?t=1820 but with more space between the lines. Also noticed the 6522 got really hot to touch. Have replaced the 257s and RAM, as suggested in the post, but still getting the white lines.

Have done some probing and found there's low resistance on D4 from RAM (PIN 14, IC15) to 6522 to 6502 and ROM socket to 5v. The resistance is around 25 ohms. The resistance is lowest from pin 18 of the expansion port to 5v at around 2 ohms. Looks like a short between D4 and 5v? Have removed the expansion port connector as had a similar problem on another computer (not Oric) which was fixed by reflowing a connector but doing this didn't work with the Oric. Hit a brickwall as can't see anything obvious linking 5v with D4.

My next suspect is the ROM socket but does this problem sound familiar to anyone and should I be looking somewhere else?

Almost forgot, I've burnt an EEPROM with the diag ROM image but that does the same of displaying thin white vertical lines when the computer is turned on.
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iss
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Re: Oric Atmos problems

Post by iss »

Welcome here! :)

If the lines are exactly as in the old post and the bottom right black square is present - this means bad 6522.
But in your case the first suspects are the DRAM chips.
As alternative you can try my Diag ROM too (attached below, can't find it in old posts) - it doesn't use RAM at all and if there is a "beep-beep" from speaker it means 6502+6522+AY-3-8910 = OK.
Attachments
diag-4k.rom.zip
(356 Bytes) Downloaded 13 times
RetroUser
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Re: Oric Atmos problems

Post by RetroUser »

Hi,

Many thanks for the reply. I've tried the ROM and there was a change but the computer didn't start up. I've attached a couple of images to show what I'm getting. The first image is what I've been getting with the 4K diag rom from your reply. The screen isn't stable and the green dots shimmer. There's no sound from the the speaker. The second image is what I've been getting using another diag ROM or when the Atmos OS ROM is inserted. I've removed the old 6522 and tried another 6522 and get the same.

Edit: Just to add, I've been measuring resistance between various pins and Vout on the 7905. Checked the the 7905 and gave an output of -4.92volts.

Edit2: Had a thought and measured the voltage between pin 34 (ground) and 18 (D4) of the expansion port. There's a steady -4.86v between the pins which I guess wouldn't do any of the ICs any good. Does this mean there's a short between Vout of the LM7905 and D4? Is that possible?
Attachments
With 4K diag rom.png
With old diagnostic ROM.png
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iss
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Re: Oric Atmos problems

Post by iss »

Hm, this smells like bad 6502 (which doesn't exclude bad RAM too).
Do you have more serious equipment oscilloscope?
If so you can check for activity on the address and data busses.
RetroUser
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Re: Oric Atmos problems

Post by RetroUser »

Okay, I'll try an oscilloscope. The edits in my reply can be ignored as got the pins wrong on the expansion port as didn't take into account board was upside (doh! :oops: ). Re-did the measurements and there's low resistance (25 ohms) between D4 and ground (short?). Have removed the 6522 and still get the white vertical lines shown in my previous reply and there's low resistance on D4. Don't think it's the 6522 or not solely due to the 6522. Will remove the 6502 and ULA as those are the only other ICs I can think of where D4 goes through (already tried with the ROM out and resistance was the same, unless there's a problem with the ROM socket as the ROM was the only socketed chip).

Edit: might try lifting D4 leg on 6502 and see what happens.
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Dbug
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Re: Oric Atmos problems

Post by Dbug »

I wonder if it would be possible to build a minimalistic device to plug on the expansion bus, which basically would just have enough components to power and control a single LED, and then have a minimalistic diagnostic cartridge which does only the bare minimum tests that are needed when you are not sure if your VIA, RAM or Display are working, basically using blinking sequences to indicate how far it has gone:
- No blinking -> The 6502 did not even manage to run or access the LED at all
- Fast blinking -> There is a problem with the RAM
- Slow blinking -> There is a problem with the VIA
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iss
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Re: Oric Atmos problems

Post by iss »

Dbug wrote: Mon Jun 27, 2022 10:59 am I wonder if it would be possible to build a minimalistic device to plug on the expansion bus...
Very nice idea! Meditate on this, I will :mrgreen: <- this is Yoda
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Re: Oric Atmos problems

Post by RetroUser »

Lifted D4 leg and resistance between D4 on IC16 (pin 14) and ground is the same as pin 14 on the other RAM ICs, so safe to assume 6502 is bad?

The cartridge is a great idea as would do away desoldering randomly!
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Re: Oric Atmos problems

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RetroUser wrote: Mon Jun 27, 2022 2:13 pm... so safe to assume 6502 is bad?
You already desoldered 6522, so another 40-pin chip will be no problem and you probably have one which is known good. Use socket. ;)
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Re: Oric Atmos problems

Post by RetroUser »

Have some turned pin sockets and thought I had a few 6502's spare but can't find them anywhere :? Will have to keep looking. Many thanks for all of the help, I wouldn't have thought it was the 6502 as they tend to be pretty reliable. Wonder if the problem was due to the ROM because the D4 on the ROM is stuck high? Did someone insert the ROM the wrong way around damaging the CPU and ROM?
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Re: Oric Atmos problems

Post by iss »

RetroUser wrote: Mon Jun 27, 2022 4:12 pmDid someone insert the ROM the wrong way around damaging the CPU and ROM?
Probably not but who knows - our humble 30+ years old Oric can tell lot of survival stories.
Honestly I don't exclude bad RAM chip(s) too. So far the ULA seams to be OK which is good :).
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Re: Oric Atmos problems

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Found the 6502s and inserted one. Still not starting correctly but there is a change. If I put in the Atmos ROM (not the original but one I've burnt to EEPROM), then get a black screen (ULA problems?). If I put in the MJB Diagnostic 1.08j ROM, then I get a white screen with black borders all around most of the time. Occasionally, the screen looks like the first image (the one with a vertical line of white rectangles and the rest of the screen is small green rectangles that jitter) I attached in a previous post except all of the small rectangles are white and jitter. If I put in the 4K diag ROM uploaded to this thread, then I get the same image as shown in a previous reply, the one with a vertical line of small white rectangles and the rest are green. (edit: forgot to add there's no sound from the speaker with this ROM inserted).

I'm suspecting everything connected to D4 (ULA, CPU, VIA, EEPROMs and IC16, which is new) are damaged, possibly even the new devices because I've been testing with the old CPU?
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Re: Oric Atmos problems

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I desoldered the ULA and put a socket in. Think there may have been a bad joint(?) because put the ULA back in and turned the computer on and there's a change for the MJB Diagnostic ROM. Instead of going to a white screen the computer displays an image like the first one with white and green rectangles. When I press the reset button the screen clears to solid white with black borders but it's stuck on this screen as pressing the reset button again has no effect. Atmos ROM is the same and results in a black screen, and other diag ROM produces the same image of white and green rectangles and there's no sound from speaker. Two steps forwards and ...
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Re: Oric Atmos problems

Post by Dbug »

What do you mean by "desoldered the ULA" Oo
I never had an Oric on which the ULA or ROM were not on a socket!

Maybe you should attach a good quality picture of your motherboard, top and bottom, so we can see what kind of changes had been made to it :shock:
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Re: Oric Atmos problems

Post by RetroUser »

Okay, I'll get some pictures later. Only the ROM was in a socket. ULA was soldered to the board.
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