Testing ZX81 Power Supply

Mousey

Forgive the total noob question, but I have a ZX81 UK 700 PSU (and no ZX81) - how would I go about testing it is giving the correct output? I have a multimeter, but am not really sure what I'm doing with it (I know - I shouldn't be allowed near electronic stuff). I don't want to sell it on if there is a chance it'll blow up someone's ZX81!

1024MAK

Tomorrow I will post some pictures showing you how to do a simple voltage test.

Meanwhile, do you have any 12V lamp bulbs rated at between 1W and 5W? E.g. Car lamps?

Mark

Mousey

Thanks Mark, I look forward to your post.

I just had a rummage in the back of the car and have found 2 spare bulbs that are both 12V & 5W. I am intrigued as what comes next... :)

1024MAK

Photos on Flickr
I used test leads and a proper lamp holder. But if you don't have these, use stranded wire with the ends stripped off. Wrap the bare ends around the electrical connections (make sure there are no short circuits) and then use tape to hold the wires in place. Wrap the tape as tight as you can.

Your voltages may be slight different, as the Sinclair PSU is an unregulated type.

Do you want any extra words?

Mark

Mousey

Thanks Mark, I really appreciate your help here. I only have a set of red & black leads going from the multimeter to probe type ends, so I need to get some of those crocodile clip extensions or wrap bare wires around each end, as you say.

I don't have a bulb holder, and looking at the base of the spare bulb it doesn't seem to have 2 contact points that I could tape wires to. I'll take a piccy tomorrow when I have more time. It's frustrating that I am so ill equipped to do this, but I don't mind spending a few quid on some bits as they may came in useful another time.

RobeeeJay

If you don't mind spending a few quid, and really that's all you'd need to spend, some high watt resistors would be the easiest thing imho.

Mousey

RobeeeJay wrote: »

If you don't mind spending a few quid, and really that's all you'd need to spend, some high watt resistors would be the easiest thing imho.

Sorry to be totally thick, but I've looked up high watt resistors (they are cheap, as you say), but I'm not sure how I would use them to test the PSU?

lunarul

Just do the same as in the pictures, but with a resistor instead of the light bulb (light bulbs are essentially resistors)

1024MAK

But keep in mind, the resistor will get very hot. You need a 22 ohm resistor rated at 8W or more. If the resistor has mounting lugs on it, that means to operate at it's rated power, it needs to be mounted on a heatsink. But if you keep the test to only 20 to 30 seconds, and keep clear of the resistor, it should be fine.

Or instead use two 10 ohm 5W resistors wired in series.

Alternatively if you have a suitable soldering iron and some solder, you could just solder wires directly to the lamp cap. But it does depend on the type of cap.

Mark

Mousey

1024MAK wrote: »

Alternatively if you have a suitable soldering iron and some solder, you could just solder wires directly to the lamp cap. But it does depend on the type of cap.

I'll probably do this instead of sourcing a bulb holder or using a resistor. In the meantime I found a pack of 10 crocodile clip leads on eBay for just £2.80 delivered! Now all I need is a suitable bulb, as the ones I found have the wrong kind of cap.

Thanks to all for their help with this, especially Mark - are you the guy producing "Mark Fixes Stuff" on YouTube?

[EDIT] I've saved the web page with photos locally, so if you want to remove them from Flickr then that's fine.

1024MAK

Nope, I'm not the "Mark Fixes Stuff" on YouTube.

Oh, don't worry. I have two Flickr accounts. I tend to leave pictures up for years...

Mark

RobeeeJay

"Mark Fixes Stuff" should be renamed, "Mark Opens Boxes" these days. :(

Thanks for filling in the resistor blanks Mark, I didn't have the time to calculate them when I posted.

Mousey, they aren't hard to get:
http://www.ebay.co.uk/itm/Ceramic-Wirewound-Crossover-Resistor-10-Ohm-20-Watt-5-/230791149873?hash=item35bc397931

That's what you are looking for, but you can get them cheaper if you get them from China/Hong Kong and want to wait, or shop around a bit more. You won't need a heat sink, they won't burn your hands off, just don't leave them connected for very long. But if you have the bulb bits to hand may as well use that I guess.

(Basically with both methods all you are doing is putting a load on the unregulated power supply that draws an appropriate amount of current to test it)