Transformer/ power supply

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Steve M
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Transformer/ power supply

Post by Steve M » Tue Jun 13, 2017 10:11 pm

I have a Opelco (Opel) system. This is basically a Cumana interface in a larger case with an extra power supply to power the floppy drives.

Mine has a dual power transformer for the drives i.e. it has two outputs. This isn't working.

So I'm looking for a replacement - is there any chance? Where can you get transformers with dual output ???

Failing that I have seen smaller single output transformers chained. Is there anywhere I can get a pair of power supplies that would power my disk drives.

(The original Opel had a single 3" drive. But I really need to power two.)

I could use a PC power supply but it won't fit inside the case.

Any ideas ???

In the meantime I've ordered some of the components found on my power supply and maybe if I replace these it might fire up again. There's a couple of voltage regulators and a couple of axial capacitors. There's also a black plastic thing. I don't know what it is so I hope it isn't that that has gone.

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Re: Transformer/ power supply

Post by kenneth » Wed Jun 14, 2017 5:10 pm

It will be intresting to know where the voltage is down, an electric shock may destroy the varistance, a fuse-componant, or the input coil, you can test the last one with an ohmmeter with the power off, switch on, and test the output coil with a voltmeter, and finally the output regulator voltage. I think that if the transformer is ok, it'll be better to replace the failed componants. My answers will be sharper if you can show pictures of the device. :wink:

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Re: Transformer/ power supply

Post by Steve M » Wed Jun 14, 2017 8:03 pm

Here's a picture
Two voltage regulators 7805 and 7812. I'm guessing 5V and 12 V
Axial capacitors 16V 4700uf and 25V 2200uf

There's a bridge rectifier RS402L Can't get one of these very easily. Could I substitute a similar one ?

Havent' poked my probes at it. I'm a bit scared of 240V. But the interface powers up but the drives don't.
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Re: Transformer/ power supply

Post by Steve M » Wed Jun 14, 2017 8:06 pm

Here's a shot showing it is a Cumana power supply.
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Re: Transformer/ power supply

Post by Godzil » Wed Jun 14, 2017 10:51 pm

If the interface is up, that probably mean it is the 12V which is dead (and the 5V is working fine and dandy)

So that only leave the 7812 so unless the 25V 2200uf have miserably failed and make a shortcut (which is possible) it is the likely culprit.

I would do a single shortcut check with a multimeter accross the two lead of the capacitor with everything plugs in the device (but not main obviously)

78xx are quite hard to kill, but they dislike heat. I see some white thing aroung the ring, are they normally attached to the chassis (in metal?)

Do you have a photo of the bottom side of the PCB?

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Re: Transformer/ power supply

Post by Silicebit. » Thu Jun 15, 2017 5:15 pm

Steve M wrote:
Wed Jun 14, 2017 8:03 pm
Here's a picture
Two voltage regulators 7805 and 7812. I'm guessing 5V and 12 V
Yes. :-)

Steve M wrote:
Wed Jun 14, 2017 8:03 pm
There's a bridge rectifier RS402L Can't get one of these very easily. Could I substitute a similar one ?
Search for GBL01.

Steve M wrote:
Wed Jun 14, 2017 8:03 pm
Havent' poked my probes at it. I'm a bit scared of 240V. But the interface powers up but the drives don't.
If the interface powers up, then the power supply must put out 12V and 5V, regulators are in serie, first 7812 and after 7805 to lower the voltage from 12V up to 5V. As Godzil said, a picture of solder side could help.
Oric user since 1984. YouTube

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Re: Transformer/ power supply

Post by kenneth » Thu Jun 15, 2017 9:27 pm

You can disconnect the 2200uf condenser and check that the 12V regulator is switched on again, we can verify if the chemical condenser is too old.
Very important: when the load is connected to the regulators, the cooler is required, (the metal plate screwed behind the regulator with the sticky thermic paste) :wink:
If the bridge rectifier has the same shape its characteristics are enought (the regulators need 1a max) and bridges like this can provide 2A and more.
Last edited by kenneth on Thu Jun 15, 2017 9:36 pm, edited 1 time in total.

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Re: Transformer/ power supply

Post by Steve M » Thu Jun 15, 2017 9:27 pm

Hmm. I'm getting just under 12V and around 3.8V

Is guess 3.8 is low - but is it enough to power a floppy drive?

Drives won't fire up. I tested the drive in my other system and it works.

Maybe the interface is faulty?

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Re: Transformer/ power supply

Post by kenneth » Thu Jun 15, 2017 9:43 pm

3,8v in place of 5v it is too low. At least we need around 4.8v.

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Re: Transformer/ power supply

Post by Steve M » Thu Jun 15, 2017 10:03 pm

So I'll change the voltage regulator. Should that sort it?

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Re: Transformer/ power supply

Post by kenneth » Fri Jun 16, 2017 6:05 pm

Maybe a component of the interface that spends too much energy ?
You can check it by touching the heating surface of the component.

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Re: Transformer/ power supply

Post by Godzil » Fri Jun 16, 2017 8:43 pm

With your multimeter in Ohm mode, with the system unpluged from main (important) what is the resistance on the +5V rail?

For a 7805 to not regulate correctly to 5V and drop that low that mean that either something draw a lot of current on the 5V rail (like a failed component)

the other reason is that the input voltage is too low and the 7805 can't regulate at it's nominal value.

Can you take a picture of the back of the power supply? (the track side)

Does the transformer really have two output? (aka the input for the 7812 is not corrected with the one for the 7805)

And what it the input voltage for bot regulator?
Also I'm not sure if you have mesure the voltage with or without the load (aka the floppies and electronic board)

If you get 3.8V on the 5V rail WITHOUT the controller connected, either the capacitor is really faulty (that can happen) or it is the 7805, but I've never see a broken 7805, they really are hard to kill.

Also if the input voltage for the 7805 with and without the load it lower than ~7-8 volts, the transformer is faulty and in this case replacing it will be complicated..

Oh one thing you can do too, is on the input side of the 7805, do the measure with your multimeter in the AC mode (and not the normal DC) if you don't get 0V*, we can suspect that the bridge rectifier is faulty

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Re: Transformer/ power supply

Post by Steve M » Sat Jun 17, 2017 12:45 am

I measured without a load, so I think both voltages are probably down. (The 12v was 11.85v or something.) I expected it to be more without a load - not sure if that's right, but when I measured a different variable voltage power supply it was putting out more than the voltage it was set to.

I've got one capacitor but not the other. (It is on order.)

I'll have another look with later to test resistance. Not sure what it should be ?

It looks like the two outputs are connected to the the same connections.

How can I tell if the transformer is OK?
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Re: Transformer/ power supply

Post by Godzil » Sat Jun 17, 2017 10:22 am

Ok I don't have the thing in my hands so my intepretation maybe incorrect, it seems the transformer have two output, but what the heck are they feeding the 7805 with a non DC voltage?

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.

I will try this afternoon to look more at thatm

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Re: Transformer/ power supply

Post by kenneth » Sat Jun 17, 2017 12:39 pm

As I can see on the picture the transformer is a classic one with two coils connected in serial mode and the output is in the same configuration, the middle output connection is used for a symetric rectifier, then I suppose that the transformer gives the same voltage in the output (for ex 2x 9v)....
Godzil wrote:
Sat Jun 17, 2017 10:22 am
Ok I don't have the thing in my hands so my intepretation maybe incorrect, it seems the transformer have two output, but what the heck are they feeding the 7805 with a non DC voltage?
Because this connection is the "middle point" of the voltage made by the rectifier (if the coil gives 9v), the 5v regulator is powered with 9v input, and the 12v regulator is powered with the two coils, so 18v. :wink:
Last edited by kenneth on Sat Jun 17, 2017 1:19 pm, edited 2 times in total.

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