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Timex Sinclair 1000 / ZX81 - power supplies

billdeg

billdeg

Technician
Joined
Nov 18, 2003
Messages
3,885
Location
Landenberg, PA USA
I noticed today that all of my Sinc 1000 / zx81 power supplies were reading above 12V rather than the 9v they're supposed to be sending. If you find one of these systems make sure you test the power supply before you use it. You could fry your system.

I tested 8 or 9 power supplies, all bad. I was able to hook up a 9 volt battery to do some quick testing, but they don't last long.

Anyone here - show of hands - have a working 1000/zx81 that they can reliably use?

bd
 
Bill

I just checked out mine (ZX-81) and it was reading 13.6V when not plugged into the unit? I haven't measured the voltage on the ZX81 board though? Are you sure the computer doesn't regulate it down to 9V?

I had my ZX-81 running for about 5 hours straight last week without any ill effects?

Tez
 
From what I know of them, the power supplies will read upwards of 12v when not under a load, also they are unregulated. There's a regulator on the computer itself that is supposed to correct this but I like to either replace the stock ps's with better regulated ones or add a regulator if I can if there is enough room in the case. Or given that, I have even recased some of them into larger bricks to accommodate the larger bulk.

So billdeg they're not really bad, just bad quality from the factory. See if you can get a cheap-o laptop power supply and re-cable it for the 1000/ZX; that seems to be my best thing to do, and they last forever and rarely get hot (unless they are also defective, happens sometimes but not often). I do this for SNES consoles and Sega stuff, their power supplies are really bad quality.

Nathan
 
The system regulates it down but for the long term this isn't probably good for the system. If a brick says 9.75v that's what should be coming out, right? Practicaly none of my old bricks is sill accurate to the voltage on the label. As a computer preservationist I say use new and good power.
 
Dwight Elvey said:
Hi
Put it on your ZX81. You'll find that under load, it is close
to the 9.75V.
Dwight
Click to expand...

I am confused. When I check the output of a regulated supply it's 9Volts (or I can adjust it to be). Isn't what's printed on the power brick/wall box supposed to match the output. For example if it says 9.75 600mA, does that mean 9.75v only when there's a load equivalent to the 600mA? Why wouldn't I expect the volts with no load to be what the unit says it will be?

In otherwords, how does one check a power supply *before* it's plugged into the computer?

Bill
 
Hi Bill,

You're suffering from a common misperception. Power supplies have to be tested while running under load.

Think about the difference your car engine makes when driving down the highway at 2000 RPM vs. running at 2000 RPM while sitting in neutral. Those are two entirely different measurements.

Those little wall wart power supplies are amazingly reliable. Don't write them off until you can test them properly.
 
billdeg said:
I am confused. When I check the output of a regulated supply it's 9Volts (or I can adjust it to be). Isn't what's printed on the power brick/wall box supposed to match the output. For example if it says 9.75 600mA, does that mean 9.75v only when there's a load equivalent to the 600mA? Why wouldn't I expect the volts with no load to be what the unit says it will be?

In otherwords, how does one check a power supply *before* it's plugged into the computer?

Bill
Click to expand...

Hi
You put a load on it. Easiest one is a light bulb.
Lamps are close to constant current. This means
6 watt 12v bulb should be about right.
Dwight
 
Hi
First, most wall warts are unregulated. Many are not even filtered.
The voltage can vary quite a bit under load.
Most linear supplies do well with no load and often can
be set with no load.
Many switchers need a minimum load or they don't work to
well.
As was mentioned, powersupplies should be tested under
load. You don't always need to max them out but some
load is recommended.
Dwight
 
I have learned something here. I assumed that the voltage was constant no matter the load and it was the mA rating that told a person how well the power supply varied/controlled the voltage to keep it constant. I am not an EE. thanks!

bd
 
Check out the unregulated S-100 voltages on a big system like my Z-2D! The +/-16 V lines are around +/-20 V with no load, and the +8 V line is usually at +10V. As soon as you plug in the ZPU and a RAM board, it drops down to something a little more reasonable. Since the only regulation you have is a set of very large capacitors, they charge up to the peak voltage of the rectified AC coming off the transformer. With a load, they settle down toward the RMS voltage. Too much load and you start to get a lot of ripple on the supplies as the transformer can't charge the filter capacitors fast enough. This is also why you'll see 16 or 25 V rated capacitors on +5 V supplies.
 
I've got several working T/S 1000s, and several presently nonworking due to keyboard flexstrip breaks.
All of my power supplies are working fine.

As mentioned, your no-load voltage is going to be a lot higher than the voltage under load. I have a dedicated load box for testing power supplies, but a resistor of the right value will work for a DC supply.
 
Perhaps Bill is more used to Commodore power supplies. I recently tested some spare ones, and my multimeter read 5.05 - 5.15V (!) on the 5V DC line and about 10V on the 9V AC line while unloaded. I didn't measure how much those voltages drop once plugged into the computer, but I would think not too much from the specs: 5V DC 1.5 - 1.7A, 9V AC 1.0A.
 
saundby said:
I've got several working T/S 1000s, and several presently nonworking due to keyboard flexstrip breaks.
All of my power supplies are working fine.

As mentioned, your no-load voltage is going to be a lot higher than the voltage under load. I have a dedicated load box for testing power supplies, but a resistor of the right value will work for a DC supply.
Click to expand...

Hi
Using resistors can be done but remember, they may need to be quite high in
wattage for many supplies. I recommend having at least a 25% rated load when
checking for voltage setting and at least a 75% when checking for ripple.
A 200W supply would need a large resistor in wattage.
This is why I like using lamps when I can. They come in a nice
range of wattages and as I said, as long as you're within a couple volts
of the rated value, are close to constant current.
Things like headlamps are usually around 100W some place. A 6V headlamp
makes a nice load for may large 5v supplies. Things like brake lights
and dash lights work better for small suppies like on the ZX81.
Dwight
 
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