Microdisc Service Manual

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.
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Vyper68
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Microdisc Service Manual

Post by Vyper68 »

Hi all,
Does anyone know of or have a service manual for the Oric Microdisc. I have one for the Oric-1/Atmos but not the disk drive.
Thanks.
Bazinga!

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mikeb
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Re: Microdisc Service Manual

Post by mikeb »

Not sure if there's much similarity, but I have a schematic and bare-board scans of the Cumana interface here :-

http://oric.signal11.org.uk/files/pub/cumana/

No service manual though.

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Re: Microdisc Service Manual

Post by iss »

Vyper68 wrote:
Wed Aug 19, 2020 4:42 pm
... service manual for the Oric Microdisc
It's in the DF-Library, see Manuel du Microdisque - Oric Microdic Manual.

BTW, did you noticed the typo (Microdic)?

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Re: Microdisc Service Manual

Post by Vyper68 »

Thanks Mike & iss, I have the Cumana scans & schematics for both of them. I was hoping for some more technical information like the service manual for the computers.
Is there any documentation on the optimal setting for RV1 available?
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Re: Microdisc Service Manual

Post by iss »

I don't think there is good and real Service Manual for Microdisc.
Attached is a part of CEO-MAG article (in French) with some details about improvements for Microdisc controller and RV1 adjustment.

In common, RV1 is used to adjust the falling edge of the MAP signal relative to F2's rising edge.
So the best rule is (as per Oric Service Manual p.15):
MAP timing is important, MAP is a 250ns pulse, negative going with its leading edge occuring 80 to 100ns before the rising
edge of phase 2 (output from pin 39 of IC5).
But for me this is bit 'theoretical' because everyone knows the (in)famous saw-tooth-like shape of the F2 signal.
Practically, by trial-and-error you should find such position of RV1 by which DOS works stable without freezes. That's it! ;)
Attachments
microdisc-150-11-14.pdf
(212.61 KiB) Downloaded 49 times

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Re: Microdisc Service Manual

Post by Vyper68 »

Thanks iss that’s very interesting :D
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Re: Microdisc Service Manual

Post by Steve M »

I've been trying to fix my Opel (Cumana) system.
I took the board from a working Cumana interface and put it in the Opel case. I took the Opel board and put it in the Cumana case.

The Opel still doesn't boot. The Cumana with the Opel board boots.

I think I have a power issue. I'm not sure if it's the power supply for the board or the one for the drives - but power seems important !!!

When I find time I'll be getting a different power supply for the drives and see if the Opel boots then. If not I guess it's the main power supply, but it seems to be giving the right output.

I also seem to have got the drive cable the wrong way round. I'm not sure how that happened but it doesn't help!!! I seem to have killed my 3" drive. It's got a cable connector that can go on either way. So that might have been on wrong at some point. Argh!

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Re: Microdisc Service Manual

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Steve M wrote:
Fri Sep 04, 2020 1:27 am
... but power seems important !!!
I'm fairly sure you do need power for it to work :D

Is it variation in the grounding arrangements between the two boards (that whole thing on the Cumana schematic about cutting the ground track so that things AREN'T connected) -- maybe this is biting you in some way? Mixing and matching boards/power supplies might fail for that reason.

All traceable back to the use of a 7905 and the weird grounding arrangements when you end up with more than one "ground" at different potentials.

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Re: Microdisc Service Manual

Post by Steve M »

mikeb wrote:
Fri Sep 04, 2020 5:47 pm
I'm fairly sure you do need power for it to work :D
Yes, but maybe Vyper hasn't got enough. I'm not sure my power supply is putting out enough amps, but it seems to have the right voltage.
mikeb wrote:
Fri Sep 04, 2020 5:47 pm
Is it variation in the grounding arrangements between the two boards (that whole thing on the Cumana schematic about cutting the ground track so that things AREN'T connected) -- maybe this is biting you in some way? Mixing and matching boards/power supplies might fail for that reason.
I'm not sure, but I don't think so. I'm thinking it may be the drive power supply. I posted you the voltages I was getting in a PM but I think you may have missed it. It was just under 12V which I thought might be near enough - but maybe it is a bit low?
mikeb wrote:
Fri Sep 04, 2020 5:47 pm
All traceable back to the use of a 7905 and the weird grounding arrangements when you end up with more than one "ground" at different potentials.
Maybe, but I don't think anything has changed in that respect.
Attached is the 10V power supply.(bottom) How many amps is that? It's the same in both Cumana and Opel.
The other ones are the drive power supply.
Attachments
P1240166r.JPG
P1240164r.JPG
cumanaPSU2.JPG

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Re: Microdisc Service Manual

Post by Vyper68 »

mikeb wrote:
Fri Sep 04, 2020 5:47 pm
Steve M wrote:
Fri Sep 04, 2020 1:27 am
... but power seems important !!!
I'm fairly sure you do need power for it to work :D

Is it variation in the grounding arrangements between the two boards (that whole thing on the Cumana schematic about cutting the ground track so that things AREN'T connected) -- maybe this is biting you in some way? Mixing and matching boards/power supplies might fail for that reason.

All traceable back to the use of a 7905 and the weird grounding arrangements when you end up with more than one "ground" at different potentials.
Can you elaborate on the issue of the grounds and what I would need to do to rectify it as my DIY Cumana board hangs at “Insert System Disk” and doesn’t do anything apart from after a period of time random characters appear on screen which I put down to VR1 which seemed to go away when it’s set to 1.5K.
I am about to send it away to get looked at as I’m stumped and it’s beyond my diagnostics kit at present. I have the power going in to the Cumana and then a pigtail from there to the Oric. I haven’t cut any tracks on the ground.
Bazinga!

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Re: Microdisc Service Manual

Post by Steve M »

I'll need to check the measurement on mine but it was different.
I think if you're getting the 'Insert System Disk', that is a good sign.
Is the drive an A drive?

How did you piggy back the Oric supply ?

Pic shows the power connections. Red and Black are the power supply in. The black and white cables are marked PWR. These are the output which powers the Oric. (If you have all the components.)
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P1010011.JPG

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Re: Microdisc Service Manual

Post by Steve M »

If you measure between the middle leg and the one nearest to RV1 on the board I'm getting .925.
Make sure you have this little wire in place.
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Re: Microdisc Service Manual

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Steve M wrote:
Wed Sep 09, 2020 12:13 pm
I posted you the voltages I was getting in a PM but I think you may have missed it. It was just under 12V which I thought might be near enough - but maybe it is a bit low?
No PM alert showing, so unless you mean a while ago? But the 12V rail is usually not critical (5V is!) and often sags when motors kick in.
Steve M wrote:
Wed Sep 09, 2020 12:13 pm
Attached is the 10V power supply.(bottom) How many amps is that? It's the same in both Cumana and Opel.
The other ones are the drive power supply.
Can't tell from the transformer, but the LM340T regulators are specced as 1A each. Note, that's only when heatsinked properly, sitting like that, they will just burn up and shut down!
Vyper68 wrote:
Wed Sep 09, 2020 3:22 pm
Can you elaborate on the issue of the grounds and what I would need to do to rectify it
Not entirely, because my brain hurts when I think about it. Yours probably will too now ...

Original Oric with its wall wart: The wall wart is entirely floating. There is no "ground", just two conductors that are about 9v DC apart. They enter Oric. The more positive one passes straight through and becomes known as the 5V rail, the more negative one passes through a 7905 and becomes 5v less than the other one. At which point, Oric internally decides to call it GROUND, and connects it to things like 0v rail, can of modulator/UHF socket, cassette port, printer port, expansion port.

If you plug in a peripheral of some kind, it can take 5v and Ground from the expansion port, and everything is fine. No issues.

But the disc interface: First, you throw away the wallwart. Then, the disc interface creates its own 5v (and 12v too), it uses it internally. It also provides power back to Oric. The fun comes when feeding that power into the DC input of Oric, and also maintaining a common ground at the expansion port, which you have to do so that the circuit makes sense between the disc interface, and Oric.

A simple answer would be to create +5v/ground and shove it into the expansion port of Oric, ignoring the DC input totally. This won't work because the 7905 in Oric will be unhappy that is being force fed +5v in its output, with nothing on the input. So they didn't do that.

Instead, a power feed (pigtail?) sticks out of the Cumana (and other?) disc interfaces, to power Oric.

Remember: The "ground" on the DC jack coming in is NOT the same as the "ground" on the expansion port. There's about a 4v difference between (that 7905 is scrubbing off the excess as heat).

If you inadvertantly connect these "grounds" together you bypass the 7905 (short input to output). On the Cumana circuit (or at least some versions) there is an annotation warning NOT to common the grounds as drawn because it would bypass the regulator in this way.

It's always a problem where people use an arbitrary Power Supply to run Oric (bench power supplies, PC power supplies etc.) because there is a chance that the "ground" coming out of the power supply is actually connected back to mains earth, and NOT floating. This will power Oric fine on its own, but the moment you start connecting a monitor, TV, cassette, printer, expansion devices, you run the risk of THAT device's ground ALSO being tied back to mains earth. At which point the two grounds (4v apart) meet and Oric dies.

Your problem with your disk interface and setting of VR1/screen corruption is separate to all this fortunately!

Steve: I don't know what the wire is you're advocating, but the annotation specifically shows Pin 34 of the ORIC expansion connector must NOT be connected to Cumana local ground. "+5V common link on PCB, NO GND LINK". It looks like someone has "added the missing wire" from ground to P34 (is that the Oric port, or the peripheral port)? If so, that trace wasn't missing, it was intentionally not there (it's wrong on the schematic, hence the note).

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Re: Microdisc Service Manual

Post by Steve M »

mikeb wrote:
Wed Sep 09, 2020 6:21 pm
Steve: I don't know what the wire is you're advocating, but the annotation specifically shows Pin 34 of the ORIC expansion connector must NOT be connected to Cumana local ground. "+5V common link on PCB, NO GND LINK". It looks like someone has "added the missing wire" from ground to P34 (is that the Oric port, or the peripheral port)? If so, that trace wasn't missing, it was intentionally not there (it's wrong on the schematic, hence the note).
Well, I've got two that are both like this.

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Re: Microdisc Service Manual

Post by Steve M »

mikeb wrote:
Wed Sep 09, 2020 6:21 pm
Can't tell from the transformer, but the LM340T regulators are specced as 1A each. Note, that's only when heatsinked properly, sitting like that, they will just burn up and shut down!
They are normally bolted to the case with some thermal compound.

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