Hello Everyone. I need your help please 🙏

[ManicMiner1977]

Hi everyone,

Thank you for letting me join your forum and in advance of your help.

I have a faulty Oric Atmos I recently bought from Ebay. It refuses to boot to BASIC and instead sits on a red/black barred screen. Reset button doesn't do anything.

Original PSU putting out 12v DC @ 3.5A.

Replaced all socketed RAM and checked original in Commodore Plus 4. Multiple diagnostic and use suggests OK.

Replaced ROM with 1.22 UK/ Diagnostic.

Tried a BBC Micro Model B 6502 CPU. No difference. CPU good in BBC afterwards. Replaced with original 6502. Tried bridging pin 1 and 40 as recommended reset to overcome c21 issue.

Found an apparently faulty Capacitor @ c23. Not sure of purpose of this but blown top and powdered on legs. Removed in anticipation of replacing.

Tried without this capacitor and get a different screen with more colours which changes on every subsequent power up.

Please see attached pictures. Last is original from Ebay. First is after removing c23 Capacitor.

Please can someone point me in the right direction before I start with a multimeter tracing around the board?

Best regards

Gavin.

[iss]

...

Welcome here!
Check: both multiplexers '257 (IC8,IC20), tracks with continuity tester.
Measure the resistance between 5V and GND rails (anywhere but after the 7905) - it should to be ~250 Ohms.

[Dbug]

Original PSU putting out 12v DC @ 3.5A.

That seems quite high, already with the standard 9V PSU the Oric voltage regulator gets easily burning hot, so it's definitely not an original PSU, or it is not working as expected.

Regarding the direction, Is it center positive?

[Chema]

Maybe it's stupid, but all that red instead of white while using RGB output... doesn't it point to either a bad cable, bad ic22 or something around that, or a bad ULA? (I'd check in that order) it is usually black and white, with possible atributes rasing colors all around.

Dbug is right. If the PSU outputs 12 volt (not measured with a multimeter, but nominally) that is not correct and could have damaged things.

80% of times it is due to ram problems, including the ram multiplexors, so I would also check those.

[ManicMiner1977]

Thanks everyone.

The PSU is original, centre +ive with a multimeter. Open not connected to the Atmos. 12v/3.5A. I thought it quite high for a 9v rated PSU.

I'll try the multiplexers too.

Does anyone know what that c23 cap us for. It is sited just above the AY?

I'll make the checks.

Thank you.

[ManicMiner1977]

Also the voltage regulator has been upgraded to TRACO Power TSN 1-2450A. This was done prior to ne buying it.

[iss]

...

The 12V PSU is OK for short tests only else the 7905 will get very very HOT and when the case is closed then things can go ugly.
The C23 - 47nF is just decoupling one change it but Oric should boot without it.
About the TRACO Power TSN 1-2450A: you have to increase the C21 (the reset cap.) form 1uF to 4.7uF...10uF..22uf - this allows TRACO startup and stable 5V.

[ManicMiner1977]

The Cap @ C21 is 25v 10uF. It looks it has been replaced due to the solder.

I'll take some measurements.

[ManicMiner1977]

...

Welcome here!
Check: both multiplexers '257 (IC8,IC20), tracks with continuity tester.
Measure the resistance between 5V and GND rails (anywhere but after the 7905) - it should to be ~250 Ohms.

Hi There,

Checked for continuity across the multiplexer main track and they seem OK.

5v is reaching every chip on the board from what I can see.

The resistance between 5v and ground on all caps, chips, etc seems to be 177 Ohms.

Checked all RAM, CPU, ULA, 8337, AY

[iss]

The resistance between 5v and ground on all caps, chips, etc seems to be 177 Ohms.

177 Ohms is bit low but acceptable (maybe TRACO lowers it).
Try with changing the '257.
Is any chip extremely hot?

[ManicMiner1977]

Hi There,

Are you saying to replace the multiplexers first?

Under a thermal camera, most obvious chips are around the 40-50 C. I do notice that the ULA is almost cold @ 26 C and doesn't seem to heat up at all. Is this anything to consider? I see ULA go for around £20+ so don't want to replace unless necessary.

I thought because I have a stable image the ULA is fine. However I am not an Oric Expert.

Cheers mate

[iss]

Are you saying to replace the multiplexers first?

Indeed! It's easy to change the multiplexers.
And yes, ULA was ~5$ but it's already 10$+shipping At least It's proven they are good.

[ManicMiner1977]

Cheers mate

[Vyper68]

Don't forget the Oric uses the 2Mhz version 6502A and 6522A not the bog standard 6502 or 6522 used in other computers. You can use a 65c02 with a bodge wire and pin one disconnected.

Can you burn a test EPROM?