Sinclair ZX80 / ZX81 Forums

[Malcohol]

Hi all.

I bought a ZX81, modified for composite display, off eBay recently. It came with no power adapter, but I presumed that a universal one would be fine. Unfortunately, I've tried about 4 different ones and none of them work.

I read that the ZX81 requires 9V at 600/700mA. I haven't been able to find a power supply at that specification, but do have ones above the required current spec. I always thought that devices wouldn't draw more power than they need. Is this wrong?

I've tried ones at 300mA, 500mA, 1200mA and 1700mA. The one at 500mA shows a blury version of the correct display. The higher ones show the machine booting with a correct display, but then the screen goes blank. Is this behaviour consistent normal operation, or is it likely that the machine is faulty?

I'm have *very* basic electronics knowledge, so perhaps someone on this forum could help me with the following:
* If I connect an ammeter into the circuit when the ZX81 is connected to the 1200mA supply, will I be able to measure the actual current being drawn?
* Would it be possible to make a circuit that absorbs the surplus by placing some sort of resistor in parallel with the ZX81?

Thanks for any help,

Malcolm

[Mike3]

The higher current shouldn't make any difference.. it will only draw what it needs,, try replacing the voltage regulator...I believe its a 7805....

Mike

[XavSnap]

Hi,

Yes, It's a 7805...
But, an inverted polarity on the +9 socket can destroy the 16k rampack... and the ULA ship!

Use the ZX81 power adaptator to avoid it.