Circuit diagram errata...

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Dave
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Circuit diagram errata...

Postby Dave » Tue Feb 26, 2019 7:16 pm

I spotted a little mistake in the QL schematic from the service manual. It's not an easy mistake to make, but if you follow the schematic, Very Bad Things could happen.

Screen Shot 2019-02-25 at 10.32.29 AM.png


IC35 is the 5V regulator. IC36 is -12V (7912) and IC37 is +12V (7812). Note that the two ground pins of the 7812 and 7912 meet, and logically, this is ground, with zero potential. 0 volts.

If you follow that 0 volt line to the right and down, notice it's connected to the DC input voltage, which is 9V. It should not connect there, but continue down to GND.


IC21 is listed as a 74LC257, but is 74LS245

On the Issue 6 schematic, IC23, the 8302, is shown connecting to DB[0..7], but it connects to D[0..7] - this changed for Issue 6. (This is quite clever.)

What other errors do people see?

I'll collect them and do a fresh drawing in the coming weeks, while I have some extra free time. When it's done I'll post it here for checking, then send it to Dilwyn for publication.


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mk79
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Re: Circuit diagram errata...

Postby mk79 » Tue Feb 26, 2019 10:45 pm

Dave wrote:What other errors do people see?

I'll collect them and do a fresh drawing in the coming weeks, while I have some extra free time. When it's done I'll post it here for checking, then send it to Dilwyn for publication.
Or you could just use the updated schematics that is already available from Dilwyn http://www.dilwyn.me.uk/docs/hardware/index.html (QLissue5.pdf and QLissue6.pdf)...


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Dave
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Re: Circuit diagram errata...

Postby Dave » Wed Feb 27, 2019 3:04 am

This *is* helpful, because it lists all the errors and corrects them in the drawing.

It's a PDF. I will need to convert it to a schematic.

Thank you for the pointer, though. I downloaded the flawed schematics from right above those links and didn't see them!


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Re: Circuit diagram errata...

Postby tcat » Tue Apr 23, 2019 4:01 pm



Hi,

I have just spotted the update, many thanks for that.

I wish to ask, do we see R107/108 flying on top of ZX8302 ULA for Iss#6 boards, what is the use of `0' Ohm resistors R102/103.

What is the use of TC1 540pF trimmer, does it have to be adjusted over the years on Iss#5 boards?

Many thanks
Tomas


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Dave
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Re: Circuit diagram errata...

Postby Dave » Tue Apr 23, 2019 5:54 pm

The trimmer is to fine tune the 32,768 Hz crystal to improve the accuracy of the clock built into the 8302. The clock was never wildly inaccurate, because it was not battery backed so it never had time between resets to accrue wild errors, but would instead acquire the ultimate error ;)

I can't speak for the resistors/links you mention.


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Re: Circuit diagram errata...

Postby NormanDunbar » Tue Apr 23, 2019 8:32 pm

Would the zero ohm resistors just be there to assist the pick and place machines that they had back in them days? Easier to PandP a resistor than a wire link? I think they still use zero ohm resistors for that purpose nowadays.

Can I still call a zero ohm resistor a resistor? ;)

Cheers,
Norm.


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Re: Circuit diagram errata...

Postby tcat » Wed Apr 24, 2019 7:34 am

Ah, haven't thought of that, production costs during board population and assembly, keeping pipelines filled.

How about bona fide resistors R107/8 on top of ZX8302 ULA on Iss#6 boards, this bit I am not understanding, yet. I know they are part of documented mod (R102/3) for Iss#5, but do we see them on Iss#6, they seem pulling down MD RAW I/O to -12V rail?

My only Iss#6 has them actually fitted.

Many thanks.
Tomas



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