Please suggest a working power supply (12 Volt) for the internal CPC 3" drive.

[TFM]

Hi! 

Can somebody please suggest a PSU which works for the 3" drive of the CPC6128. I bought two from Amazon, neither one of them works. Tested another one I had at home, doesn't work either. BTW: The power supply from the CTM does work.[nb]But I don't want to use my broken CTM just to supply 12 Volt.[/nb]

[Velktron]

"Doesn't work" is a too vague statement. Does the supply not work? It doesn't output 12V DC as advertised? Is it even of the correct polarity? Or does the drive not budge despite everything else being OK? Have you taken measurements with a multimeter?


The CPC isn't too demanding of its 12V PSU, actually: it's rated at 0.4 A max, and even many generic transformers can cover that. Can you post a link to the PSUs you already tried?

[TFM]

- Does not work means the drive is not spinning
- Output is about 12 V (multimeter)
- Polarity is correct (Plus at inside pin)

Here they are:
Amazon.com: BINZET 12 Volt 2 Amp Power Adapter, AC to DC, 2.1mm X 5.5mm Plug,
Amazon.com: EPtech AC DC Charger Power Supply Cord PSU Adapter 5.5mm x 2.1mm 12V

To connect it to the CPC6128's 12 V cable I try to use this one:
Amazon.com: JacobsParts DC Coupler Female to Female F/F 5.5mm x 2.1mm Barrel

Got no idea what I did wrong. 

[Bryce]

Hate to be the one to give you the bad news, but the 12V supply is the other way around - Plus on the outside, ground on the inside.

Bryce.

[Prodatron]

Wasn't that a typical "Amstrad rule" AFAIK that they put plus at the outside?

[Bryce]

Amstrads: 5V has Ground on the outside, 12V has ground on the inside.

Bryce.

[Prodatron]

Ok, I only remembered, that the Amstrad NCxxx have Plus at the outside as well.

[Bryce]

True. Several other classics used the inside ground too. I usually add stickers under my machines to remind me of the voltage and polarity when I haven't used the machine in a while. Saves me a lot of repair work

Bryce.

[chinnyhill10]

Wasn't that a typical "Amstrad rule" AFAIK that they put plus at the outside?

That was Sinclair mainly. Amstrad carried on the negative tip for the grey +2 as due to time constraints the machine is essentially just a Speccy 128k. The black +2 has a total redesign of the power circuits and uses a DIN connector.

[chinnyhill10]

Hi! 

Can somebody please suggest a PSU which works for the 3" drive of the CPC6128. I bought two from Amazon, neither one of them works. Tested another one I had at home, doesn't work either. BTW: The power supply from the CTM does work.[nb]But I don't want to use my broken CTM just to supply 12 Volt.[/nb]

if it came from Amazon and cost a few quid then it a) might not provide enough current and b) is probably a death trap.


Anyone who follows me on Twitter will have seen last week I took apart a cheap PSU. Terrifying. Tiny wires from the mains pins and nothing shielding the mains pints from hitting the main board if they moved. Let alone the lack of filtering caps compared to a decent PSU. Awful. And that wasn't even the cheapest I've seen!

[Prodatron]

Just a stupid question, as I am not an electr(on)ic expert: Did it have a reason to put plus outside? As what I learned is always to put ground outside for security reasons

[Bryce]

"Best practice" would be to have the ground on the outside.

@Chinnyhill: Is there a link to that teardown that I can view without being a twitter member?

Bryce.

Edit: Putting the positive on the outside on the 12V is actually a really bad idea. It makes the potential short circuit much worse if you try to plug the 5V plug in the 12V socket.

[Velktron]

Ouch. Very ouch. Positive on the inside means trouble, and you might hace fried something.

But since the CPC actually has its own male round connector for 12V that is supposed to plug into a female socket, may I ask how you connected it to the PSU? Did you use a DIY gender changer?

[remax]

But since the CPC actually has its own male round connector for 12V that is supposed to plug into a female socket, may I ask how you connected it to the PSU? Did you use a DIY gender changer?

Yeah, last link in his post.

[Bryce]

The 12V supply is only used to power the motors. The motors will have run backwards a bit, but I don't think anything would be damaged.

Bryce.

[chinnyhill10]

The 12V supply is only used to power the motors. The motors will have run backwards a bit, but I don't think anything would be damaged.

Bryce.

Which is why when you plug in a CPC with no power to the drive you can still see faint light in the LED.

[chinnyhill10]

"Best practice" would be to have the ground on the outside.

@Chinnyhill: Is there a link to that teardown that I can view without being a twitter member?

It was only a photo of two PSU's I took side by side, opened up. Others have done far better jobs and I've seen far worse PSU's.

[Bryce]

The electronics in the drive is powered by the 5V rail and this is what controls the LED.

Bryce.

[Velktron]

The 12V supply is only used to power the motors. The motors will have run backwards a bit, but I don't think anything would be damaged.

If the switch engaging/disengaging the main drive motor was mechanical, e,g. a relay, then sure. But the CPC actually uses a Darlington transistor for the switching, and that might not be rated for passing reverse voltages/currents.

Even if it is, the main DC motor which just rotates the disk might indeed have no trouble other than a bit of backwards running (it's unusual to use such a motor in a floppy, BTW. Most use direct drive spindles with brushless stepper motors). But the motor moving the heads might, as that one should be a stepper motor controlled by electronics (unless Amstrad managed to cheapen out even on that, or it's using the 5V line instead).

[TFM]

Amstrads: 5V has Ground on the outside, 12V has ground on the inside.

Bryce.

Oh crap! That's the problem?!? It really has Plus on the outside! Damn, wouldn't have expected that (expecially since 5V has Plus inside).


Ok, so I need to solder an inside/outside changer, and the problem is solved, right?

[remax]

@Chinnyhill: Is there a link to that teardown that I can view without being a twitter member?

[Velktron]

Ok, so I need to solder an inside/outside changer, and the problem is solved, right?

Yes, hoping that nothing was fried the first time around.

Wasn't that a typical "Amstrad rule" AFAIK that they put plus at the outside?

Sony also did it on a lot of their portable audio gear, and the Japanese Famicom and the various Famiclones also used an outer positive arrangement.

[chinnyhill10]

Thanks for digging that out! The one on the left is a better quality supply and the one on the right is a cheaper one. The wire came straight off of the mains pin when I opened it up and look how thin it is. Additionally if you look at the one on the left it has a fairly thick plastic insulator between the pin and the board. Nothing on the right!


And yet I've seen far far worse than the PSU on the right. PSU's with half as many components in and mains pins that will fall out under the slightest force.

[pelrun]

Edit: Putting the positive on the outside on the 12V is actually a really bad idea. It makes the potential short circuit much worse if you try to plug the 5V plug in the 12V socket.

It means that if you do that then both supplies get shorted to ground, which just triggers whatever overcurrent protection the supplies have; as opposed to joining the 5v and 12v rails together and having significant reverse current flow back into the 5v rail, probably damaging it.


The real abomination was when Amstrad pointlessly reversed the polarity on the single input socket on the NC100, essentially guaranteeing you'd blow the internal fuse if you tried a replacement PSU.

[Bryce]

If the switch engaging/disengaging the main drive motor was mechanical, e,g. a relay, then sure. But the CPC actually uses a Darlington transistor for the switching, and that might not be rated for passing reverse voltages/currents.

Even if it is, the main DC motor which just rotates the disk might indeed have no trouble other than a bit of backwards running (it's unusual to use such a motor in a floppy, BTW. Most use direct drive spindles with brushless stepper motors). But the motor moving the heads might, as that one should be a stepper motor controlled by electronics (unless Amstrad managed to cheapen out even on that, or it's using the 5V line instead).

A Darlington pair should survive having 12V put across it the wrong way around.

Bryce.