Spectrum Computing

Is it possible to modify a board to use an ATX power supply?
I remember that I saw in the past a schema to use it (soldering wires in specific points on board to supply the voltages) but I never seen it again.
All the original power on the board is removed (coil, etc, etc)...

Thanks.

Can I ask why you'd want to do that? seems like overkill (and aren't the voltages wrong anyway).

If the ATX PSU generates -5V, yes.
If the ATX PSU does not generate-5V, but does generate-12V, also yes, but different wiring and depending on the board issue of your ZX Spectrum, some modifications.

In all cases, I strongly recommend using polyfuses (best) and quick blow / fast rated fuses to provide protection the the ZX Spectrum in case of short circuit fault.

If you let me know which issue board you want to modify, is it a rubber key or other model, if your ATX PSU generates a -5V supply, and depending on the above, if you want to use a DIN socket and plug for the power connection, or wire to an edge connector.

Note that the basic scheme does not support any expansions that make use of the +9V rail available on the edge connector.

Mark

beanz wrote: ↑Tue Dec 04, 2018 3:55 pm Can I ask why you'd want to do that? seems like overkill (and aren't the voltages wrong anyway).

Because the board is damaged where is the part of power supply.

1024MAK wrote: ↑Tue Dec 04, 2018 4:28 pm If the ATX PSU generates -5V, yes.
If the ATX PSU does not generate-5V, but does generate-12V, also yes, but different wiring and depending on the board issue of your ZX Spectrum, some modifications.

In all cases, I strongly recommend using polyfuses (best) and quick blow / fast rated fuses to provide protection the the ZX Spectrum in case of short circuit fault.

If you let me know which issue board you want to modify, is it a rubber key or other model, if your ATX PSU generates a -5V supply, and depending on the above, if you want to use a DIN socket and plug for the power connection, or wire to an edge connector.

Note that the basic scheme does not support any expansions that make use of the +9V rail available on the edge connector.

Mark

It´s an Issue 3 and my power supply has all required voltages.
I can wire to edge connector if it´s possible. The easiest way.

PS: in Brazil, our clone TK90X has a way to do that. But the board is a bit different from ZX.
http://www.tk90x.com.br/Projetos_FontePC_2.html

I see you have already removed some of the components from your board. For the benefit of others, I will still list the components that need removing.

ZX Spectrum, Issue 3 board, suppling from an external regulated PSU

Remove the following components from the ZX Spectrum PCB:
REG - 7805 voltage regulator
COIL - The coil
TR4 - ZTX650 or ZTX651 or ZTX653 or TIPP31

The external supply must produce all three supply voltages simultaneously when operating.
Feed in via the following edge connector contacts:
Pin 3 underside +5V
Pins 6 and 7 underside 0V/GND (use both connections)
Pin 20 component / top side -5V
Pin 22 component / top side +12V

Please ensure you get the correct connections.

The wires for the +5V and the +12V supplies should be rated at a minimum of 1.4A. The wire for the 0V/GND should be rated at a minimum of 3A. Or use two wires in parallel each rated at a minimum of 1.4A. Keep the wiring as short as possible.

Fit fuses as detailed below. The first fuse from the PSU should be the conventional type of fuse. Then wire this to the resettable Polyswitch fuse. That is wire the conventional and the resettable fuse in series. Then wire to the ZX Spectrum.

Fuse the +5V supply at 1.5A or 1.6A using a fast / quick blow / quick acting type such as this Siba 1.6A Fast Ceramic Fuse 250V. Siba part number 179 021.1.6. Then a resettable fuse such as a Bourns 0.75A Multifuse. Bourns part number MF-R075-0-99.

Fuse the +12V supply at 1A using a fast / quick blow / quick acting type such as this Siba 1A Fast Ceramic Fuse 250V. Siba part number 70-007-33/1A. Then a resettable fuse such as a Littlefuse 400mA Polyswitch Fuse. Littlefuse/Raychem part number RXEF040.

Fuse the -5V supply at 250mA using a fast / quick blow / quick acting type such as this Siba 20mm 250mA Fast Ceramic Fuse 250V. Siba part number 70-007-33/0.25A. Then a resettable fuse such as a Littlefuse 100mA Polyswitch Fuse. Littlefuse/Raychem part number RXEF010.

If you are unable to get or use resettable / Polyswitch fuses, the values of the conventional fuses should be reduced as follows:

Fuse the +5V supply at 1A using a F1A fuse (fast / quick blow quick acting type).

Fuse the +12V supply at 500mA using a F500mA fuse (fast / quick blow quick acting type).

Fuse the -5V supply at 100mA using a F100mA fuse (fast / quick blow quick acting type).

I strongly recommend ceramic 20mm fuses for the +5V and +12V supplies. The fuse for the -5V supply can be a glass fuse.

Mark

Thank you!

Fuses are meant to protect the source, which is unnecessary in the case of PC PSU.

Dan Antohi wrote: ↑Tue Dec 25, 2018 9:53 am Fuses are meant to protect the source, which is unnecessary in the case of PC PSU.

No, fuses are to used to protect any part of the system that may be damaged by a short circuit or heavy overload. In the case of the ZX Spectrum, I recommend fuses to protect the PCB tracks and any wiring.

The original PSU can only deliver a limited current. Whereas a PC PSU can deliver a much greater current, many magnitudes greater. So in the event of a short circuit, the PC PSU could deliver so much current, that it could burn out the PCB tracks or wiring. Overheating wiring will cause insulation to melt, this can be a fire risk.

Mark

True, yet nobody ever considered installing fuses in a PC (for example). I wonder why.