The Sinclair QL Forum

Greetings everyone. I just purchased my first QL. I acquired a power supply from Keelog and am connecting to a modern TV via composite video (RGB cable is on the way). The computer powers on just fine with the expected startup screen but after 2-3 minutes it shuts down. I disconnected both microdrives and this time extends to over 11 minutes, but it still inevitably shuts down.

I let the computer run without the regulator while measuring the voltage. It was a steady 8.97V and never dropped, so I think that the power supply is OK. With the regulator attached, it will start at 8.38V, slowly decrease to about 8.0V then rapidly decrease to 7.8 and then the computer powers off. The regulator and heatsink only get warm, never hot.

As far as I can tell, all of the capacitors are original. Does this seem like a capacitor issue?

Any advice is greatly appreciated!

Hi

If the power supply is okay, try measuring the 7805 Voltage regulator ouput voltage, or try a new voltage regulator, that is faulty.

The QL is not prone to capacitor ptoblems, but changing them would hurt.

I looked at the Keelog power supplies, they look good, maybe a little on the expensive side.

Hi Derek. This is my third Keelog PSU (others are for Commodore 64 and Amstrad CPC 464) and they have been great.

I am just wondering if I have a faulty PSU. Is it normal for a voltage regulator to have a slowly decreasing supply voltage? The 9V supply to the voltage regulator seems to be directly connected to the PSU's 9V pin, which would exclude a capacitor being the issue. The output voltage holds steady at 4.85V until the input voltage drops to 7.8V.

On the other hand, the 7805 has a minimum input voltage of 7V which I don't think is being reached in my case. At 7.8V, it stops supplying the output voltage, so the 7805 could be faulty. I do not have any spares but am going to test it with a different PSU, if I can rig something together.

I tried a different 7805 with more or less the same results. It stayed on 30 seconds longer and the supply voltage was able to go lower than 7.8V before output voltage was lost.

As mentioned earlier, the supply voltage starts at 8.39V and slowly decreases. As it gets closer to 8.1V that accelerates. As it hits 8.0V it will plunge rapidly.

I connected the 7805 directly to the Keelog PSU. The input voltage holds steady at 8.98V. The output holds steady at 4.91V. The load is low but everything is normal. The issue only occurs when under load. With microdrives disconnected, the computer will stay on for 11 minutes. With microdrives connected, this is reduced to about 3 minutes. Is there anything to help conclusively prove that the issue is inside the computer, or if it's the power supply?

Many apologies for all of the posts. I was able to rig up a separate 9V power supply and everything is stable. You can't see it in the image but the 9V pin on the regulator is bent up / is not connected to the QL plug. I have already reached out to Keelog and hopefully they won't give me too much trouble with a warranty replacement. Thanks for reading, and thanks to Derek for his reply.

HI,

I can sell you an original QL Power Supply.

Thanks Derek but I live in the US. Otherwise, I'd take you up on that.