original power supplies

[David G]

The ZX81 was originally kitted out with different power supplies/mains adapters/wall warts/PSU/AC adapter

Basic specs are:

• DC output: 9.75 volts

• Amperage: 600-700ma to 1.5A

• connector: 3.5mm plug, positive tip (center is +)

0.600A is just enough current to power the ZX81 itself and 16K RAM PACK. If a printer or other add-in is used, it needs a higher amperage power unit

These are unregulated units. Final regulation occurs inside the computer at the 5V regulator. Unregulated AC to DC converters are used to convert alternating current (AC) into direct current (DC) without stabilizing the output voltage

Initial tests:
* Unloaded voltage. Plug the power supply into the wall. The maximum voltage output will be up to 16 volts DC
* Verify the polarity: center of the plug is positive, outside sleeve is negative
If the voltage is not higher, that's a good sign. Plug it into the machine
* Loaded voltage: 9-12V

Units include:

• UK Sinclair UK 700 a.k.a. UK700

• UK Sinclair UK 1200 a.k.a. UK1200

• EURO 700

• USA DVE 91A

• USA Sino-American 9-10

• USA Sinclair (Dynamic 05D0003)

FUSES: UK units have an internal thermal fuse

UK and EURO power adapters match the upswept kick of the ZX81 case design

photo from valoroso.it

[David G]

I'll start with this one as it is the most common power supply in USA where I am located

The ZX81 version "Timex/Sinclair 1000" shipped with a 0.650A output supply, by either Sino-American or DVE

• MODEL NO: DV-91A

• OUTPUT: 9.75V DC 650ma

• shape: cube

DVE
PLUG-IN POWER SUPPLY
MODEL NO: DV-91A
INPUT: 120V AC 60Hz 16W
OUTPUT: 9.75V DC 650ma
LISTED UL 883K E59477 (-)—C—(+) CSA LR 38470
R-1
FOR USE WITH COMPUTER
CAUTION: INDOOR USE ONLY
MADE IN TAIWAN [date code, e.g. '1082' (October 1982)]

[1024MAK]

The UK PSUs are:

UK700 - 240V AC mains input. Output rated at a nominal 9V DC, maximum current 700mA (0.7A). When not connected to a load (the computer), depending on the local mains voltage, the output can be as high as 16V DC.

UK1200 - 240V AC mains input. Output rated at a nominal 9V DC, maximum current 1200mA (1.2A). When not connected to a load (the computer), depending on the local mains voltage, the output can be as high as 16V DC. Although rated at 1200mA (1.2A), because this type was intended for use with a ZX Printer, please consider this the maximum figure for intermittent use. For continuous use I suggest 700mA (0.7A).

A ZX Spectrum UK1400 PSU (nominal 9V DC, maximum current 1.4A) can be used to supply a ZX80 or ZX81 (or T/S 1000 or T/S 1500). But you will need a 2.1/5.5mm "barrel" to 3.5mm plug adaptor.

All of these UK PSUs have an internal thermal fuse.

In Europe, some European versions of these PSUs were available, the UK part of the type number being replaced with "EU" IIRC. They had a rated mains AC voltage of 220V I think.

Mark

[David G]

Alternative supplier for Timex/Sinclair

• MODEL: 9-10

• OUTPUT: 9V DC 1.0A

• shape: sloping cube

SINO-AMERICAN
MODEL: 9-10
PLUG-IN POWER SUPPLY
FOR USE WITH COMPUTER
LISTED UL 771K CSA LR 34266 [date code, e.g. '1282' (December 1982)]
INPUT: 120V 60Hz 15W
OUTPUT: 9V DC 1.0A
(-)—C—(+) MADE IN TAIWAN

CAUTION
INDOOR USE ONLY
PRECAUTION
DOMESTIQUE USAGE SEULEMENT
SA-48

[edit: add photo of back side]

[David G]

Thanks Mark. I have updated post one with this information

The UK PSUs are...

[David G]

USA ZX81s shipped with a Dynamic power supply

• MODEL: 05D0003

• OUTPUT: 9.75V DC 650ma

• shape: rectangle back with cube front

FOR USE FOR sinclair

CAUTION USE INDOORS ONLY
DYNAMIC INSTRUMENT CORP
MADE IN U.S.A. 3 82 [date code]
CAT NO. 05D0003
120V 60HZ 12 W
LISTED UL PWR SUPPLY 581
9.75 VDC 650ma

some are unbranded, such as this later revision marked:
CAUTION: USE INDOORS ONLY
7 82 D
LISTED UL PWR SUPPLY 581 B

[EDIT: add unbranded photo]

[David G]

The ZX81 UK version shipped with a 700ma or 1200ma (1.2 amp) output supply. The quality differs as there were several suppliers for it

• Model: UK 1200

• OUTPUT: 9v DC AT 1-2A

• shape: rectangle with upswept top to match the ZX81 case

• UK power supplies have an internal thermal fuse

sinclair
ZX POWER SUPPLY
IN: 240v AC ~ 50Hz.
OUT: 9v DC AT 1-2A.
DISCONNECT FROM MAINS WHEN NOT IN USE.
FOR INDOOR USE ONLY.
UK 1200

This one has been fitted with modern MK safety plug

[David G]

Mark, is the "internal thermal fuse" replaceable? Is it something to look at if the PSU has no output?

All of these UK PSUs have an internal thermal fuse.

[1024MAK]

Depends on the PSU manufacturer.

Many have the thermal fuse soldered on the circuit board separately. It’s a bit tricky to change as it’s under the transformer which is also soldered to the board. Normally they are in an insulated sleeve. And yes, spares are still available. I forget now what temperature they are rated to blow at. Keep in mind that it’s recommended to use a heat shunt on each of the legs when soldering and be quick! Otherwise you may blow it from the heat of soldering...

The other position is embedded in the primary winding of the transformer. I’ve never tried replacing any of these. On a transformer from something else (not Sinclair related) the fuse was on top of the primary winding under a layer of insulation tape.

If there is no output voltage, disconnect the PSU from the mains. Now measure the resistance of the primary winding by testing across the AC input either across the transformer primary winding pins or across the mains plug pins.

If you get a result that is open circuit (infinity / over-range) it’s likely the thermal fuse has blown.

Mark

[1024MAK]

Post above edited.