Atom scrambled screen

discussion of games, software, hardware & emulators relating to the Acorn Atom
djj
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Atom scrambled screen

Postby djj » Fri Nov 24, 2017 7:12 pm

Hi all from a new member,
Have recently been given an Atom by a friend, on trying it out, the screen is full of scrambled garbage, see pic.
Can anyone tell me if this is a common, hopefully easily fixed issue ?

He also gave me a BBC Master, which appears to work OK, but is missing the 1,2,3,0 and . keys, any suggestions as to where would be the best place to get replacements ?

Thanks for any help
Dave
Atom screen.jpg

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hoglet
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Re: Atom scrambled screen

Postby hoglet » Fri Nov 24, 2017 7:29 pm

Have you tried pressing BREAK?

If that makes no difference, most likely something is stopping the 6502 from running properly. You will get the same symptoms if the 6502 is removed completely.

Usually it's a RAM fault, as the 2114L RAM chips are notoriously unreliable. Probably IC51/IC52.

Can you post a photo of this insides?

Dave

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roland
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Re: Atom scrambled screen

Postby roland » Fri Nov 24, 2017 8:37 pm

Welcome another Dave :lol:

There are plenty of reasons what can cause this issue. But if the mainboard is in good condition then we can fix it. There are many threads on this board about this behaviour so you can start doing some research on the ram chips, but also check the clock signal, the 8255 etc. The best way to start is to remove as much chips as possible and start with a real minimal Atom.

Welcome to this forum, enjoy your visits here and we're glad to assist you.

Greetings from the south of the Netherlands,
Roland
256K + 6502 Inside
MAN WOMAN :shock:

djj
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Re: Atom scrambled screen

Postby djj » Fri Nov 24, 2017 8:59 pm

Thanks for the speedy replies guys.
Mboard pic attached
Atom mboard.jpg

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roland
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Re: Atom scrambled screen

Postby roland » Fri Nov 24, 2017 10:46 pm

That Atom is almost a minimum Atom. You can start to swap the 2114's like Hoglet suggested. Make sure that the pins are blank and have no oxidation. One of our forum members had an Atom with bad contacts due to oxidation.

Btw ... What test equipment do you have, eg volt meter, oscilloscope...?.

If you have a volt meter check if evey ic has at least 4.8V. A bad power supply can also cause the atom not to start up.
256K + 6502 Inside
MAN WOMAN :shock:

djj
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Re: Atom scrambled screen

Postby djj » Sat Nov 25, 2017 12:35 am

Thanks Roland,
Don't really have any electronics knowledge, I have a multimeter.
Do you mean to test each IC's voltage individually ? If yes, do they all have a standard set of pins for I/O voltage ?
Hope you don't mind the really basic questions.
Dave

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roland
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Re: Atom scrambled screen

Postby roland » Sat Nov 25, 2017 8:46 am

If you look at the IC you will see (in most cases) a small notch at one side. This is the top of the IC. Left to this notch you will find pin 1. This pin often has an additional marker like a small circle pressed into the IC housing.

In this position the +5V, often indicated as Vcc, is at the top right side of the IC, the opposite pin of pin 1. The 0V, GND, is at the left lower position of the IC. For example, a 74LS04 has GND at pin 7 and Vcc at pin 14. The Atom ROM IC's have GND at pin 12 and Vcc at pin 24. This goes for almost every IC with 24 pins or less in the Atom. The large IC's like the 6502, 6522, 6847 and 8255 have a different pin layout. I will leave this as an exercise to you to do some research on Google (hint: try datasheet and 6502) to find out what their power pins are.

As long as you don't create a short circuit you can do no harm to the Atom when you're measuring the voltages. Just make sure the multimeter is in the voltage DC position, the first step > 5V.

We don't mind if you're asking basic questions. In fact, it's sometimes nice to help people without doing thorough research :)
256K + 6502 Inside
MAN WOMAN :shock:

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oss003
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Re: Atom scrambled screen

Postby oss003 » Sat Nov 25, 2017 9:10 am

Hello Dave,

welcome to the forum, maybe this link will help you: viewtopic.php?f=44&t=6318&p=60505&hilit=minimal+configuration#p60505

Greetings
Kees

crj
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Re: Atom scrambled screen

Postby crj » Tue Nov 28, 2017 3:17 am

roland wrote:I will leave this as an exercise to you to do some research on Google (hint: try datasheet and 6502) to find out what their power pins are.

Actually, searches like "6502 pinout" often get you what you need more quickly.

(And I can recommend this handy diagram of EPROM pinouts to anyone who's not yet found it.)


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