The Sinclair QL Forum

Hi again guys, I just introduced myself in the Wellcome section. As I said I'm a new user of a Sinclair QL, I'm waiting for a new keyboard membrane from sellmyretro which will arrive in days. Meanwhile a neighbor of mine gave a homemade PSU. Connecting it to my QL I see the menu screen correctly, both using an RGB cable and using the antenna one. Out of curiosity I went to check the output voltages from the plug: on the blue I have 18.5 V alternating current and on the red 7.6 V direct current.
I read around on the internet that direct current should oscillate between 10v and 12v while alternating current should be 15.6v.

Can I use this PSU? Or do I risk doing damage?

Unfortunately I don't have any micro drive cassette to be able to test to see if it can handle the load.

Robbizz wrote: ↑Mon Jun 05, 2023 6:19 amoutput voltages from the plug: on the blue I have 18.5 V alternating current and on the red 7.6 V direct current.

When I was fixing my PSU these correct values were quoted.

Chr$ wrote: ↑Wed Dec 25, 2019 9:15 pm Around 16v AC sounds about right for one of the required voltages. There should be about 16v AC and 9v DC and on the plug the middle pin is GND.

Did you put a load across the pins when taking a measurement? The 7.5V means you will waste less heat on the 5V regulator if it works. The 18.5V is a bit high so damage might occur eventually due to continuous over stressing components. My no load measures Red 11V DC & Blue 16.6V AC

Ruptor wrote: ↑Mon Jun 05, 2023 10:27 am Did you put a load across the pins when taking a measurement? The 7.5V means you will waste less heat on the 5V regulator if it works. The 18.5V is a bit high so damage might occur eventually due to continuous over stressing components. My no load measures Red 11V DC & Blue 16.6V AC

This afternoon I will do some measurements with the computer turned on, in this way I can actually understand what the voltages are. Thank you

Robbizz wrote: ↑Mon Jun 05, 2023 10:46 am This afternoon I will do some measurements with the computer turned on, in this way I can actually understand what the voltages are. Thank you

Maybe use a light bulb or something to put a load on the power supply. I'd avoid trying the QL because if the power is indeed too high, you'll damage something.

bwinkel67 wrote: ↑Mon Jun 05, 2023 11:23 am Maybe use a light bulb or something to put a load on the power supply. I'd avoid trying the QL because if the power is indeed too high, you'll damage something.

Actually the guy who sold me the transformer already has two QLs both powered by the same transformers. He reassured me that he built these transformers based on his own experience. I was just wondering (that's why I wrote here) what other users of these voltages think. I just now measured the voltage when the QL is powered and turned on: I get 7V on the DC section and 14.6v on the AC. I have not used any bulbs or other loads with these measurements, I simply measured from the inside with a tester. Incidentally, the guy who gave me the transformer told me that it can be used for hours without the internal heat sink getting hot.

7V on the DC end might in fact be a little tight (7V is the minimum the 7805 needs to create the internal 5V). But that's rather the "might not work under heavy loads, but won't break anything" end. The AC end seems fine to me.

tofro wrote: ↑Mon Jun 05, 2023 3:55 pm 7V on the DC end might in fact be a little tight (7V is the minimum the 7805 needs to create the internal 5V). But that's rather the "might not work under heavy loads, but won't break anything" end. The AC end seems fine to me.

Thank you. This can give me peace of mind until I find an original power supply. By the way, I'm aware that the original spectrum power supplies are not very reliable; I don't know if due to the fact that they were built on the cheap or simply the capacitors need to be replaced (perhaps both). If I take an original power supply and replace the capacitors, can I use it safely, or should I continue using the one I already have?

Robbizz wrote: ↑Tue Jun 06, 2023 9:42 am

tofro wrote: ↑Mon Jun 05, 2023 3:55 pm 7V on the DC end might in fact be a little tight (7V is the minimum the 7805 needs to create the internal 5V). But that's rather the "might not work under heavy loads, but won't break anything" end. The AC end seems fine to me.

Thank you. This can give me peace of mind until I find an original power supply. By the way, I'm aware that the original spectrum power supplies are not very reliable; I don't know if due to the fact that they were built on the cheap or simply the capacitors need to be replaced (perhaps both). If I take an original power supply and replace the capacitors, can I use it safely, or should I continue using the one I already have?

I've had QLs since the very beginning and never had a PSU die on me. And mine are just original like they left the factory and their innards have never been touched. They're pretty reliable. Simplicity also has some advantages

Robbizz wrote: ↑Tue Jun 06, 2023 9:42 amThank you. This can give me peace of mind until I find an original power supply. By the way, I'm aware that the original spectrum power supplies are not very reliable; I don't know if due to the fact that they were built on the cheap or simply the capacitors need to be replaced (perhaps both). If I take an original power supply and replace the capacitors, can I use it safely, or should I continue using the one I already have?

Originals are as rare as hen's teeth & expensive so I wouldn't bother. The main advantage of getting an original PSU is it plugs in without modifying the computer but if you have a QL plug any PSU with suitable voltages is fine.

Ruptor wrote: ↑Tue Jun 06, 2023 12:45 pm

Robbizz wrote: ↑Tue Jun 06, 2023 9:42 amThank you. This can give me peace of mind until I find an original power supply. By the way, I'm aware that the original spectrum power supplies are not very reliable; I don't know if due to the fact that they were built on the cheap or simply the capacitors need to be replaced (perhaps both). If I take an original power supply and replace the capacitors, can I use it safely, or should I continue using the one I already have?

Originals are as rare as hen's teeth & expensive so I wouldn't bother. The main advantage of getting an original PSU is it plugs in without modifying the computer but if you have a QL plug any PSU with suitable voltages is fine.

The 3D printed power connectors work well, no need to alter computer!