New user - fixing an Atmos

If you want to ask questions about how the machine works, peculiar details, the differences between models, here it is !
How to program the oric hardware (VIA, FDC, ...) is also welcome.
virtvic
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New user - fixing an Atmos

Post by virtvic »

Hi there, Virtvic here!

I received an Atmos from a friend of mine recently - this is a computer I've been wanting for a long time. I'm sure I'm not the only one thinking these have one of the best industrial designs and best keyboards of ALL the 8 bit micros.

The Atmos came with no RAM, but has had sockets fitted already.

I plugged it in with some 4164 I had in my stash and noticed the +5 voltage was down. I fitted a new 7809 voltage reg and am still only getting +4.7v, which I know isn't enough. I've already replaced the electrolytic caps and am just about to replace the disc caps too (I know they probably aren't a problem, but look pretty gunky, so prob good to do them and it only takes a few minutes)

If I can get the thing to boot, the first thing I'll be getting is a Traco switching voltage reg and then an Erebus (it's sat in by ebay cart as we speak...)

I have also read about replacing one of the caps with a higher value to help the reset circuit boot the computer - but I need a decent +5v first.

I even burned a diagnostic eprom and I can see some sort of text, but the image is garbled.

The image jumps about a bit, changes and has static like chatter from the speaker.
atmos.jpg
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Chema
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Re: New user - fixing an Atmos

Post by Chema »

While our hardware experts come here to help, let me just pop in to welcome you wish you luck with this repair.

Check the memory multiplexers, it seems they are also prone to failure and remember the Oric needs fast memories (100 ns).

A voltage drop may indicate a damaged chip and considering someone peeked inside and soldered sockets, something nasty may have happened.
Last edited by Chema on Tue Apr 26, 2022 9:20 pm, edited 1 time in total.
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Dbug
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Re: New user - fixing an Atmos

Post by Dbug »

We can clearly see that you have 40x27 characters visible, so looks like the ULA is just fine, but since the image us jumpy and stretched, I will assume that some of the garbage has enabled the 60hz video mode.
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kenneth
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Re: New user - fixing an Atmos

Post by kenneth »

Welcome!
virtvic wrote: Tue Apr 26, 2022 7:42 pm ...I fitted a new 7809 voltage reg and am still only getting +4.7v..
7905 Only. :mrgreen:
Maybe your 9v psu is no more strong enough?
virtvic
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Re: New user - fixing an Atmos

Post by virtvic »

kenneth wrote: Tue Apr 26, 2022 9:01 pm Welcome!
virtvic wrote: Tue Apr 26, 2022 7:42 pm ...I fitted a new 7809 voltage reg and am still only getting +4.7v..
7905 Only. :mrgreen:
Maybe your 9v psu is no more strong enough?
Ooops, beg your pardon - I did replace it with a 7905 - I read about the weird way the Oric & Atmos reverse the polarity (???)

I have 2x 74LS257 on order.

I have 4164B -10 RAM - is that fast enough? (not sure which way the speed goes?)
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Chema
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Re: New user - fixing an Atmos

Post by Chema »

virtvic wrote: Tue Apr 26, 2022 10:38 pm I have 4164B -10 RAM - is that fast enough? (not sure which way the speed goes?)
Yep, I think that's the one.

If your PSU is new and gives enough power, you may have a drop due a faulty chip. Memory and multiplexors are the usual suspects, and you already replaced the ROM... what about removing the ULA and any other socketed chip and see if the voltage is still low? Can you measure the current? It should be below 500mA (which is the maximum of the original PSU, I bet it should be less... around 300mA).

You may already know there is a repair guide here:

from which I extracted this (section 2):
1. Remove ULA and Oric ROM from sockets and store them.
2. Check 5v-0v rail resistance against known good board (approx 225R). If lower than 50R skip
powering up and go straight to step 2.4
3. Power up from a current limited PSU (9V DC/Max 700mA, DC Jack centre positive). If 5v rail is
4.95v to 5.05v, power supply is good, return ULA to socket, and go to section 3.
4. See 01_0vRail_7905.jpg. Check C8 next to the DC input jack is not short circuit (replace if
needed). Unsolder and lift the “OUT” leg of 7905 from board or cut trace as shown. If +5v is not
present between separated (0v) “OUT” and (+5v) “COM” leg, unbolt and replace 7905. Go back to
step 2.2 and start again
virtvic
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Re: New user - fixing an Atmos

Post by virtvic »

How would I check the multiplexers?

I would like to rule these out before the ardous task of desoldering the ULA (and perhaps the 6502 as well)
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Chema
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Re: New user - fixing an Atmos

Post by Chema »

virtvic wrote: Wed Apr 27, 2022 7:54 pm How would I check the multiplexers?

I would like to rule these out before the ardous task of desoldering the ULA (and perhaps the 6502 as well)
I am not sure, to be honest. Apart from substituting them, that is. Maybe check their activity with an oscilloscope? Maybe desoldering them and check the voltages to see if it still drops below 4.9 or so, or measuring current draw.

I think the ULA is socketed, so that should be easy. Why not check the voltages on the main chips? Starting near the regulator to see if there is something obvious.

Please, let other hardware experts tell, but I think that, if your PSU is in working condition, you should have a good voltage to start with. Removing all chips that are socketed and check it in different points to see if you can find an evident culprit could be a good idea.
virtvic
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Re: New user - fixing an Atmos

Post by virtvic »

The ULA, 6502 and 6522 are all soldered :(

A previous mod has all the RAM socketed and the BIOS.

I don't have a scope, or the knowledge on how to use one...

I'll remove the 2 multiplexers and see if that makes any difference to the voltage. I will have to socket them anyway, for the new ones that are on their way.
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iss
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Re: New user - fixing an Atmos

Post by iss »

Check this old post how to measure the resistance between +5V and GND tracks. What are your readings?
iss wrote: Wed May 19, 2021 10:23 pm For a working board the readings should be roughly ~250 Ohms.
If the difference is more than +/-50 Ohms (i.e. <200 or >300 Ohms) there should be something broken.
virtvic
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Re: New user - fixing an Atmos

Post by virtvic »

The readings are all around 150 ohms, indicating something is broken. I’m hoping it’s one of the multiplexers.
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iss
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Re: New user - fixing an Atmos

Post by iss »

Indeed, check the '257 and '04, Unfortunately, without scope there is no other way than desoldering.
You can control the resistance just after removing the suspected IC and if it goes to something close to normal 250 Ohms - this will be good sign ;).
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Re: New user - fixing an Atmos

Post by HigashiJun »

BTW, nice and funny Trinitron.

:)
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virtvic
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Re: New user - fixing an Atmos

Post by virtvic »

Just removed the old 257's and replaced with socketed new ones - no change.

The voltage is still +4.7v

I guess the next thing is to remove the 6502 and check the voltage again, then the ULA, then the 6522?
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Re: New user - fixing an Atmos

Post by Silicebit. »

Hi virtvic!
virtvic wrote: Tue Apr 26, 2022 7:42 pm ....

I even burned a diagnostic eprom and I can see some sort of text, but the image is garbled.

....
This suggest that CPU and ULA are working. Have you checked that RAM is really ok?
Oric user since 1984. YouTube
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