PSU Voltages.

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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flaviosr
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PSU Voltages.

Post by flaviosr » Fri Aug 10, 2018 7:31 pm

Dear All,

I am putting order in my small Sinclair Collection (1 × ZX80 in kit, 1 × ZX81, 2 × Spectrum, 1 × QL), testing everything.
I was using one of two Spectrum when it simply turned off... I was a bit scared when I decided to measure the voltage from the PSU and it was 0.9V...I breathed a sigh of relief.
After this I decided to measure the voltage of all my 3 PSUs and I found what follows:
° ZX81: 19V
° Spectrum 1: 12.5V
° Spectrum 2: 14.5V (after a while the "dead" PSU decided to get back to life).
Anyway the above voltages are much above the 9V declared...
I started looking over internet and I have found in one site that Spectrum PSUs are normally 15V without load (if this is right, the 12.5V is too weak).
Nothing about the ZX81.

Can you help me understand what is happening?
3 dead PSUs?
Someway to repair them?

Thank you
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chequered flag
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Joined: Sat Mar 10, 2018 7:22 am

Re: PSU Voltages.

Post by chequered flag » Sat Aug 11, 2018 6:07 am

The power supplies are unregulated. They should give 9 volts at maximum current. Less than maximum current will give a higher voltage.
The PSU that sometimes gives 0.9 V may have a short in the cable when you bend it in certain angels.
The PSU that gives 14.5 V is close to the maximum voltage the Spectrum can handle. If a PSU fails and give too high voltage then the Spectrum may fry.

https://www.electronics-tutorials.ws/bl ... upply.html

To test the PSU you must apply a load that draws the maximum current. Then measure the voltage. It should be at least 9V.

The voltage is regulated inside the Spectrum and the excess voltage is transform to heat.
To reduce heat and prolong life you can replace the PSU to a modern regulated that always gives 9 V up to maximal current.
Be careful with polarity (plus, minus). If you get it wrong then the Spectrum will fry.

https://www.apgsensors.com/about-us/blo ... r-supplies
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1024MAK
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Re: PSU Voltages.

Post by 1024MAK » Sat Aug 11, 2018 4:31 pm

See also this thread...

Mark
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flaviosr
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Re: PSU Voltages.

Post by flaviosr » Sun Aug 12, 2018 8:59 am

1024MAK wrote:
Sat Aug 11, 2018 4:31 pm
See also this thread...

Mark
It's mine! :lol:
I registered in ZX81, Spectrum and QL forums! One for each. I notice now that the ZX81 and Spectrum are... very equal! :D
And thank you for your answer for the video quality! :D I am looking now for your suggestions...
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