Replace XT1 and XT2 crystals

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Equinoxe
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Replace XT1 and XT2 crystals

Post by Equinoxe » Sun Mar 10, 2019 8:07 pm

First I have to say thanks to Dbug and the Oric community for creating such an amazing Oric resource - I never would have imagined my first computer from 1984 would still be so enthusiastically supported in 2019!

On that topic I imported a couple of dead Orics recently to Canada both of which were dead (I'll create a separate thread on these shortly) and I noticed one of them (which has no video output at all) has crystal XT1 missing and XT2 appears to have either corrosion or was replaced (also there appears to have been some repair around the RF modulator making we wonder if this Oric was killed by a PSU short that I've read about elsewhere).

My question is would any 12MHz 8.867MHz work or are these special in any way? Also would my assumption be correct that a missing XT1 would definitely cause a no video/dead behaviour? Also do I need to replace the 8.867MHz (XT2) if I'm only using Scart video?

Thanks in advance!

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mikeb
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Re: Replace XT1 and XT2 crystals

Post by mikeb » Sun Mar 10, 2019 8:44 pm

Oh yes.

XT1 (12 MHZ) is the main oscillator, from which all other clocks derive. No CPU, no ULA, nothing of any use.

XT2 is only needed IF you want a PAL colour output on the UHF modulator. It does have to be exactly 8.8672375 MHz or the colour carrier frequency will be wrong. Edit to add: It is twice the 4.43MHz which is used in the UK for PAL colour. I note you are in Canada, so I guess this is irrelevant to you :)

Your RGB (DIN) output will work once the ULA is clocked at 12MHz, it doesn't depend on XT2.

Get XT1 replaced before going much further, the only thing is to get the right physical size. There are different types, cut for series resonant, parallel resonant, fundamental or third overtone etc. but I have no idea which Oric uses :(

Mike.

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Equinoxe
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Re: Replace XT1 and XT2 crystals

Post by Equinoxe » Mon Mar 11, 2019 1:59 am

Thanks very much for the quick reply - You mention different types but the service manual simply states 'XTAL 12MHz' so I'm guessing it would be the most common option (most places I'm looking for these don't seem to state the type - simply '12Mhz Crystal 'x' package and 'x' tolerance') - Do you think there would be any great harm done if I simply try the default, most common type that I can find?

Thanks again!

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iss
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Re: Replace XT1 and XT2 crystals

Post by iss » Mon Mar 11, 2019 8:33 am

Any 12 MHz will work. For repair I prefer to use crystals with small footprint:
12mhz.jpg
12mhz.jpg (10.06 KiB) Viewed 335 times
BTW, many Oric's have a transistor soldered next to modulator (i.e. not only a capacitor) - this is not a repair but 'standard' feature :).

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mikeb
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Re: Replace XT1 and XT2 crystals

Post by mikeb » Mon Mar 11, 2019 6:48 pm

Equinoxe wrote:
Mon Mar 11, 2019 1:59 am
You mention different types but the service manual simply states 'XTAL 12MHz' so I'm guessing it would be the most common option
I did look in the service manual to see if there was a hint or a part number. But no :(

As @iss says, go with an easily available small part and try it. Once you've got the 12MHz clock, and 1 MHz clock to the CPU, and some working power rails (5v), then things should look more hopeful.

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Equinoxe
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Re: Replace XT1 and XT2 crystals

Post by Equinoxe » Tue Mar 12, 2019 7:06 pm

Many thanks I've ordered the crystal and will post updates on progress as it's made. In the meantime I've attached pictures of the mobo in question and I'm curious if anyone has any thoughts on why the underside of XTAL2 looks in bad shape and also what the two extraneous solder additions might have been for immediately to the right of XTAL2 on the component side?
Attachments
20190309_154624.jpg
20190309_154603.jpg

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mikeb
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Re: Replace XT1 and XT2 crystals

Post by mikeb » Wed Mar 13, 2019 6:38 pm

The underside of XT2 looks like someone has desoldered/replaced this XTAL using an inappropriately large soldering iron, and failed to clean the flux up. It's messy as anything, lots of scraping about :)

The extra solder blobs/pads you mention -- I'm guessing, but if you mean the two silver pads at the bottom of the long fat track, they are just vias (through connections) in the 0V rail bonding both sides of the board together. They're a bit full of solder, may have been used as an attachment point for a 0v wire?

If that's not what you mean, add a big circle around the thing you did mean.
00_0vRail_FullView.jpg

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Equinoxe
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Re: Replace XT1 and XT2 crystals

Post by Equinoxe » Thu Mar 14, 2019 5:10 am

That sounds about right. This machine had a repair sticker from a radio shop so I'm wondering if the XTAL was harvested for another project (im hoping that's the most significant fault) incidentally do you have a similar diagram showing the 5v rail by any chance so that I can check continuity while I wait for the components. And on a side note what do the letters mean in your diagram?

Thanks again for your help with this mystery machine :)

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Re: Replace XT1 and XT2 crystals

Post by mikeb » Thu Mar 14, 2019 6:07 pm

Repair shop with really big soldering irons :)

I don't have a highlighted 5v rail, and that's connected to the letters -- it was a picture borrowed from my Oric Repair notes. It indicates good places to cut the 0v rail to isolate faulty parts that are overloading the power rail. It's a hand-annotated high res scan of a part stripped board.

http://oric.signal11.org.uk/html/repairguide.htm

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