Oric-1 Repair

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Vyper68
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Oric-1 Repair

Post by Vyper68 » Tue Apr 09, 2019 10:00 am

Hi
I have been repairing an Oric-1 for a friend this week and have made some progress. I had to fit a new 7905 and once I did I was getting a reading of 3.5V so something was pulling it low. I went for the RAM at first but rather than cut the traces on the 0V Rail I desoldered them one at a time ( lesson learned for the future - Cut the 0V rail and save some time :D ) I was still getting a reading of 3.5V until so I was a bit perplexed until I touched some of the Logic chips on the modulator/clock circuit.
Long and short of it is that a LS74 and LS123 were toast so I have pulled them and the Board is getting a 5V reading and is steady. I have a few LS74's on order as I had run out building some AY Boards for the Spectrum so it will be a few days before I can test it again. The only things I am worried about is what would make those two logic IC's fail and that the little PROM TSP24S10N is also getting very hot and I don't know how hot these things get from memory and I am not aware of a modern day replacement nor do I have the code to program a new one. If this is also faulty are there any replacements available or is it one part that needs to be taken from another Oric.
It also has no factory modifications on this one so I will need to do those before I'm finished provided I get it working.
Bazinga!

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Re: Oric-1 Repair

Post by Chema » Tue Apr 09, 2019 1:42 pm

I don't know if th the PROM chip (246x4 bits) is still available (http://pdf.datasheetcatalog.com/datashe ... 5205_1.pdf) and I seem to recall somebody asking for the contents of the chip.

So, basically, I think it should be taken from another Oric, that is the bad news. The good news is that it is not necessary at all if you use the RGB output. It is only used (IIRC) for generation of the color of the video signal when using the TV modulator.

Found: Here are the contents of the PROM: viewtopic.php?p=10940#p10940

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Re: Oric-1 Repair

Post by mikeb » Tue Apr 09, 2019 5:49 pm

Contents of the PROM chip are also in the Leycester Whewell "Advanced User Guide" book.

+1 on Chema's point: This PROM and related LS ICs are only needed if you want a working colour UHF output.

What would make those chips fail? Who know what was done to the Oric before you got to it? :) Could be just "bad luck", could be any number of "I was just looking inside the case and slipped with a probe/screwdriver" situations that you'll never get the truth of. That's how repairs work. Often very important information is either omitted, or actively covered up, so you have to trust nothing.

IC 26 and (27 or 24) don't look to be accessible from outside Oric, i.e. they don't connect to external ports or anything. They just create the waveforms (counts) to drive the PROM. So, unless it's bad luck, someone may have been trying to do something around that area and had an accident!

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Re: Oric-1 Repair

Post by Vyper68 » Tue Apr 09, 2019 6:04 pm

Thanks for the replies guys, helpful advice.
Yeah, I have no info on the Oric's history other than it was a non-runner and bought or donated to my friend as such. So what happened I do not know. It was never opened since it was sold as the Seal was intact and untampered until I lifted it off.
I was sure it would be the RAM but I guess you live and learn, Good news about the PROM only affecting the UHF Modulator, so it could still be used even if the UHF was not working and most people today use the RGB anyway. :D
Bazinga!

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Re: Oric-1 Repair

Post by mikeb » Wed Apr 10, 2019 5:30 pm

Vyper68 wrote:
Tue Apr 09, 2019 6:04 pm
It was never opened since it was sold as the Seal was intact and untampered until I lifted it off.
Useful info -- sounds like the bad luck fairy visited then.

To be honest, it's that long since any of the Oric/Atmoses I've got had a warranty label on, I'd forgotten they came with them! :roll:

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Re: Oric-1 Repair

Post by Vyper68 » Sat Apr 13, 2019 7:31 am

I had some more duff logic chips on the UHF modulator circuit the other LS74 and LS04. I have not got it working yet sadly as there now seems to be no Clock signal from the 12Mhz crystal or IC21 is stuffed as well. I am starting to think that someone plugged a PSU socket in the modulator or something equally as odd. This Oric had no modifications at all, I may try a new AY Chip but I thought that was just Keyboard and Sound?
I will keep plugging away though, it's a learning experience as my " repair thing " is Atari 8-Bits normally. I used my Atmos daily back in the day and still use my Atmos now but aside from recaps to improve the reset circuit my experience is limited to reading your advice and the service manual.
Bazinga!

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Re: Oric-1 Repair

Post by mikeb » Sat Apr 13, 2019 6:06 pm

Vyper68 wrote:
Sat Apr 13, 2019 7:31 am
I had some more duff logic chips .... I have not got it working yet sadly as there now seems to be no Clock signal from the 12Mhz crystal or IC21 is stuffed as well. I am starting to think that someone plugged a PSU socket in the modulator or something equally as odd.
OK, it's quite unlikely that anyone managed to power it through the UHF socket (you'd have to really try to be that clumsy), but even then, I'm not sure you'd back-feed into the main Oric that way. What I will point out is that trying to plug in the DC jack while sat at the FRONT of Oric, by feel, usually leads to the DC jack going in the expansion port. That is usually pretty bad. That's far more likely, sadly. 9v into TTL inputs is bad ...
Vyper68 wrote:
Sat Apr 13, 2019 7:31 am
I may try a new AY Chip but I thought that was just Keyboard and Sound?
It is keyboard and sound. But as usual, you have to start from the fundamentals. Do you now have a 5v rail that is 5.00v +/- 0.2v? If not, weird stuff will happen. Then, without a 12MHz clock into the ULA, you can't even begin to work out what is and isn't working (unless it's obvious that smoke is coming out somewhere :). So 12MHz into the ULA is required, and then hopefully 1MHz coming out of the ULA to the CPU, and onward to the 6522 VIA chip.

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Re: Oric-1 Repair

Post by Steve M » Mon Apr 15, 2019 12:15 pm

I might well have a 24s10. I'll have a look. If I don't have a new one I have a lot of old boards that you might be able to pull one from.

BTW I used to carefully peel off the warranty label and restick it. You could do it 2 or 3 times before the glue failed.

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Re: Oric-1 Repair

Post by Vyper68 » Sun Apr 21, 2019 8:19 am

mikeb wrote:
Sat Apr 13, 2019 6:06 pm
Vyper68 wrote:
Sat Apr 13, 2019 7:31 am
I had some more duff logic chips .... I have not got it working yet sadly as there now seems to be no Clock signal from the 12Mhz crystal or IC21 is stuffed as well. I am starting to think that someone plugged a PSU socket in the modulator or something equally as odd.
OK, it's quite unlikely that anyone managed to power it through the UHF socket (you'd have to really try to be that clumsy), but even then, I'm not sure you'd back-feed into the main Oric that way. What I will point out is that trying to plug in the DC jack while sat at the FRONT of Oric, by feel, usually leads to the DC jack going in the expansion port. That is usually pretty bad. That's far more likely, sadly. 9v into TTL inputs is bad ...
Vyper68 wrote:
Sat Apr 13, 2019 7:31 am
I may try a new AY Chip but I thought that was just Keyboard and Sound?
It is keyboard and sound. But as usual, you have to start from the fundamentals. Do you now have a 5v rail that is 5.00v +/- 0.2v? If not, weird stuff will happen. Then, without a 12MHz clock into the ULA, you can't even begin to work out what is and isn't working (unless it's obvious that smoke is coming out somewhere :). So 12MHz into the ULA is required, and then hopefully 1MHz coming out of the ULA to the CPU, and onward to the 6522 VIA chip.
Well the crystal was not working for a start, so I have replaced it with a new one and the problem remains. Could you maybe post a picture of how the waveform should look from IC21 and the ULA. I can pick up a waveform on my little Ocilloscope from pin 2 and pin 15 of the ULA but it doesn't look very clean.
Bazinga!

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Re: Oric-1 Repair

Post by mikeb » Sun Apr 21, 2019 5:35 pm

Somewhere in here ...

http://oric.signal11.org.uk/files/pub/u ... Schems.pdf

You will find a bunch of actual waveforms (scoped from a live Oric and composited) for the 12MHz/1MHz clocks, RAS, CAS and other stuff ...

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Re: Oric-1 Repair

Post by Vyper68 » Mon Apr 22, 2019 9:36 am

Thanks for the help mikeb you are a star! That will help greatly. It does seem like the clock is there now but I have a horrible feeling the ULA and main IC's have been damaged by whatever event occured.
Still it won't put me off, I have a AY chip, 6502, 6522 and RAM, I can burn a new ROM and I will get this working again eventually.
Bazinga!

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Re: Oric-1 Repair

Post by Vyper68 » Tue Apr 23, 2019 12:07 pm

I think I will have to write this one off as a bad job, the ULA is toast and so is the ROM so I am not going to spend more time on this one.
Bazinga!

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Re: Oric-1 Repair

Post by Chema » Tue Apr 23, 2019 1:04 pm

Ah, bad luck then... It is possible to find a new ULA (there were some at sale on ebay, iirc), but if several chips are damaged it might not be worth the effort.

Sorry to hear :(

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Re: Oric-1 Repair

Post by mikeb » Tue Apr 23, 2019 5:08 pm

If the damage is really that widespread, then it would be a very long job repairing it. Given that it was still warranty sealed, I do think someone's done one of two things:

1) Powered it with a totally inappropriate adapter (AC output, wrong polarity, 12V or even higher etc).

2) Powered it with the right adapter, but into the expansion port.

At most you might save the case, speaker, and keyboard membrane as a donor for another Oric, or frame the motherboard and hang it on the wall as retro-art ... :(

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Re: Oric-1 Repair

Post by Vyper68 » Sun Apr 28, 2019 1:34 pm

Yes it is frustrating, given how many logic chips have been damaged and the ULA and ROM don't work having tried them in my Atmos seperately I don't hold out much hope for the other IC's.
That said, I picked up an old Oceanic 118n 5.25" Floppy drive to pull the 6502 and 6522's out and I have a working AY-8912 chip from my Melodik ZX Spectrum interface. I only need a ULA as I can burn a ROM to get it up and running.

Just need to keep an eye out for that and have another go when one ( ULA ) turns up. The case and PCB is as you say in good condition and would be worth hanging onto.
Bazinga!

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