Oric Atmos problems

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.
RetroUser
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Re: Oric Atmos problems

Post by RetroUser »

Here are pictures of the board as it currently is. I've had to change capacitors C23 (it broke) and C1.
Atmos bottom.jpg
Atmos top.png
Gone backwards as don't get a stable picture anymore. Turn on the computer and the vertical lines are back and scroll up the screen and jitter. Touching/tapping the board results in interference so think something is loose. I'll go back over the solder joints just in case.
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iss
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Re: Oric Atmos problems

Post by iss »

It looks good.
If you insert only ULA, 6502, 6522 (no! RAM) in this state and "beep-beep" ROM Diag will be there sound?
Else, the next step is long and boring - checking all tracks with continuity tester using schematics :(.
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Dbug
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Re: Oric Atmos problems

Post by Dbug »

With the missing expansion connector, and the fact you said there was no ULA socket, to me it almost look like if the machine had originally be used as as a "part doner" to repair other machines.

Or is that you who removed the IDC connector on the back?
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Re: Oric Atmos problems

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I removed the IDC connector as found a direct short between pin 18 and 5v. Had a similar problem with a BBC micro where one of the pins was shorted and reflowing the connector (BBC micro) fixed the problem but didn't with the Oric so removed the connector to see if there was anything obvious under it.

The computer had all of the components, RAM was Inmos 2600, ULA, CPU and VIA soldered onboard. Only the ROM was damaged and the data is corrupt on reading. I've bought a RAM tester off eBay but it's late arriving. I'll be able to test the RAM to make sure it's okay.

I'll try the diag ROM with RAM removed as soon as I can and let you know.
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Re: Oric Atmos problems

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Just tried the diag ROM with all of the RAM removed and still no sound. However, not tested the speaker and just assuming it works. The image on the screen has changed where instead of small green rectangles that jitter they have turned to purple spots that jitter.
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iss
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Re: Oric Atmos problems

Post by iss »

Can you measure the resistance between +5V and GND with no chips installed?
If it's < 200 Ohms then the only remaining soldered AY-3-8192 is faulty too :(.
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Re: Oric Atmos problems

Post by RetroUser »

I've measured resistance between 5v and GND and it's around 150 ohms. I'm guessing this isn't good.
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Re: Oric Atmos problems

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2 steps forward ...

Have an update where I think the Oric is partially working but still has problems. I have removed and socketed the AY-3-8912, and replaced the ROM socket because it looked a bit poor. I measured the resistance with the AY-3-8912 removed from the board and got a resistance of around 230 ohms with what I thought was all of the major ICs removed, turned out I had left the ULA in the board. 230 ohms seems better than 150 ohms. I guess a new AY-3-8912 is necessary.

Have been testing after socketing the AY-3-8912 and think there's some activity from the board because I can hear a faint clicking/popping sound from the speaker when iss' test ROM is inserted. The sound is very faint but there's a definite clicking/popping which repeats while power is on and where there's a pause between pops of around 1/2 second (could be a bit longer). Below is what is shown on the screen with iss' ROM inserted.
What is shown on screen when iss diag rom is installed.png
I've tried another diag ROM and can hear a sound also coming from the speaker. Sounds like a longer beep and again not very loud. CPU gets hot too, noticeably hotter than any other chip on the board.

The test108 rom doesn't do anything different where below of the screens shown when the test108 rom is inserted. The image below is what is shown when power is applied.
test 108 rom before pressing reset switch.png
The image below is shown when the reset button is pressed.
test 108 rom after pressing reset switch.png
I think there may be a problem with the speaker as noticed a couple puncture marks in the speaker. Can see them in the image of the board uploaded in the previous replace. The speaker damage could be contributing to the low volume from the speaker?
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Dbug
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Re: Oric Atmos problems

Post by Dbug »

I don't think the holes in the speaker influence the volume, that would be more like some unwanted vibrations adding some noise to the sound.
I would personally suspect the LM386 on the left of the ULA, that's the amplifier sound.
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Re: Oric Atmos problems

Post by RetroUser »

Fitted a new LM386 but it didn't make a difference to the loudness of the audio. Gives more substance to the problem being the AY-3-8912. I've got a replacement AY-3-8912 on the way.
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Re: Oric Atmos problems

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Just a small update. Got a new AY-3-8912 and fitted it. Checked board resistance and get 217 Ohms with all chips but RAM inserted. With the old AY-3-8912 I could hear a very quiet clicking/popping sound coming from the speaker when iss' diag ROM inserted. With the new AY-3-8912 inserted there's no sound from the speaker which is odd. Would hope to get the same quiet clicking/popping or the correct sound at the correct volume. CPU gets hot but not too hot to touch. VIA is warm and AY-3-8912 is warm. Screen behaviour is still the same so assume that the ROM is sending audio to the speaker but there must be another problem :( :? Question is what next? Will begin probing with the oscilloscope.
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Re: Oric Atmos problems

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Made progress and found the problem. The problem was one of the 74LS04s was bad (IC21). I replaced the 74LS04 and powered up the Oric with iss' ROM but didn't hear any sound. However, screen pattern was different. Reset the computer and saw a familiar pattern of mostly black screen with white horizontal block lines on the right of the screen. I've seen that pattern in a post on this forum. Put the Oric ROM (on an EPROM not original because the original ROM is broken) in and turned the computer on and got the following.

Oric booted.png

It works! Many thanks for all of the replies, they have been really helpful. Did go a bit over the top and socket all of the ICs when only needed to replace a 74LS04 (although the CPU, VIA and sound chip were bad). I guess the computer is futureproofed as can change any of the ICs easily.

However, the computer isn't 100% fixed because I didn't hear anything when iss' ROM was inserted so I still think there is a problem with sound, either the sound chip (which is nos) or LM386 or something in between. Could be the speaker as it has some damage.
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Re: Oric Atmos problems

Post by iss »

Cool and congrats! :)
Fixing the sound should be easy when AY replacement comes.
Check the C2 is looks like broken on the photo but maybe it's only the shadow of C1.
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Re: Oric Atmos problems

Post by Dbug »

Yeah, pressing the keys should generate key clicks, and CTRL-G should play "PING", that's easy way to test the sound.
If you are suspicious of the speaker, the sound output is also available on the TAPE DIN connector.

Generally speaking, sound on the Oric is:
CPU 6502 -> VIA 6522 -> AY-3-8912 -> LM386 -> Speaker

Since the machine boots and display the text, we know the two first are fine.

The AY is used for the sound, but also for the Keyboard and Printer port, so if you can type characters the AY is probably fine, but I guess it's possible that the I/O ports are not damaged and only the sound generator, but that would be rare :)
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Re: Oric Atmos problems

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Just tried the computer with the keyboard plugged in and the letter K is displayed repeatedly when the computer is turned on. Sometimes a carriage return must happen because the cursor moves down a line randomly and not wrap. I've tried the mjb test108 ROM and it gets to the VIA test page and displays the message "VIA test fail" so could be the VIA chip is bad or something related to the VIA. I've tried another VIA, with and without the sound chip inserted and unplugged the keyboard, and get the same behaviour and VIA test fail message. I've got a few more VIAs to try. They should be okay as bought the VIAs a couple of years ago and tried them in a BBC micro (another broken computer story) in the System VIA slot and the computer turned on okay from recollection. Wouldn't have thought VIAs would have gone bad in antistatic foam in a box?
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