Transformer/ power supply

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Godzil
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Re: Transformer/ power supply

Post by Godzil » Sat Jun 17, 2017 2:31 pm

Then look again at the track. You will need a bridge rectifier for BOTH regulator, not just one, a 78xx need a DC input not an AC.
And here the 7805 is directly fed by one side of the transformer which is AC voltage, that is absolutely not the way to do thing.

So here is the schematic of the thing:
Capture d’écran 2017-06-17 à 14.26.09.png
Using a direct tap from the transformer to feed the 7805 is really wrong, and it is really surprising that this thing is working. Because Ground is rectified, but the voltage in the input of the 7805 is still oscillating, so the 7805 see an AC voltage and not a DC, it is not meant to be input with an AC voltage, at all.

And for the transformer characteristics we have no proof that it is symmetrical, but that is not the problem here, as long as the input voltage for the 7805 and the one for the 7812 are both DC and higher than the output voltage for each.

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kenneth
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Re: Transformer/ power supply

Post by kenneth » Sat Jun 17, 2017 5:14 pm

I am not agree with you Godzil, indeed, we can use a rectifier for each regulator, that will make a schematic respectful of electronic theory, but sometimes, when the manufacturers want to make a low cost device, they use the middle point of the transformer, who is not oscillating despite the appearance, but I understand that the way they connect their componants is really weird... :wink:

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Steve M
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Re: Transformer/ power supply

Post by Steve M » Sat Jun 17, 2017 10:04 pm

Godzil wrote:
Sat Jun 17, 2017 10:22 am

I will try to make a schematic from that power supply, but can't clearly see the track on the "bottom" side of your picture, but it looks really weird to have a tap of the transformer to feed directly the 7805, it should have a bridge rectifier before. And if the 4700uF capacitor has failed, the 7805 is not going to work well.
If you look at the top picture you can see the bridge rectifier - a RS402L. Havent' been able to look at it today. Had to dig the garden in hot sun.

It is the four connections below (in the underside picture) the ones where the output wires connect.

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coco.oric
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Re: Transformer/ power supply

Post by coco.oric » Sun Jun 18, 2017 7:42 am

Linked to the high level electronic competencies i haven't, i can't help. But Steve, can you put on the forum (for the mag) a global pic of your device.
Thanks a lot, Didier
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Steve M
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Re: Transformer/ power supply

Post by Steve M » Sun Jun 18, 2017 5:28 pm

I'll try and do some pics. It's basically a black metal box. Not very exciting.
The original configuration was with a single 3" drive right in the middle. There's a metal blanking plate which hides where a second drive can sit to the right.
So it's a black box with a slot in the middle - or a black box with two slots. :?

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Steve M
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Re: Transformer/ power supply

Post by Steve M » Mon Jun 19, 2017 1:14 pm

Wasn't able to get a sensible resistance reading.

The other capacitors are 104, (as used on the Cumana interface).

Need to reassemble it before trying to get any other readings. Took some pictures for Didier though :)

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Re: Transformer/ power supply

Post by Godzil » Mon Jun 19, 2017 8:54 pm

Kenneth: you are right, I think it is that heat that kill my brain ;-)

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Steve M
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Re: Transformer/ power supply

Post by Steve M » Mon Jun 19, 2017 10:25 pm

I got the big capacitors and swapped those and got over 5V so tested the system. Drive lights went on but it wouldn't boot. So I guess the interface is faulty too.
I tried swapping drives just to see if the drive was duff but now the drives won't light up either. So maybe the power supply has gone again or maybe the interface has got worse.

It's a bit beyond me now. :(

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coco.oric
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Re: Transformer/ power supply

Post by coco.oric » Tue Jun 20, 2017 5:36 am

Took some pictures for Didier though
Thanks a lot Steve, i've got the pics and discovered this big oric extension.
A future file on the mag.
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