The Sinclair QL Forum

Hi,

I have an original untouched QL UK power brick which seems to be reliable at the moment and provides a clean 9v DC according to my scope. I'm wary of old PSUs especially from some of my other machines like the C64 where they're known to fail destructively and send 9v AC where it shouldn't be sent. Should I trust my QL supply, or have others done what I did to my C64 PSU and replace the DC regulation circuit with something modern like an LM259 regulator https://www.ebay.co.uk/itm/395716971624?

Best

Martin

Hi,

I placed a small buck converter inside the PSU, the 9-10V as input and the SMPS set to deliver 6.5 or 7 v. This to limit the heat from dissipated by the 7805 inside the QL.

/Chris

martins wrote: Sat Nov 09, 2024 1:07 pm Hi,

I have an original untouched QL UK power brick which seems to be reliable at the moment and provides a clean 9v DC according to my scope. I'm wary of old PSUs especially from some of my other machines like the C64 where they're known to fail destructively and send 9v AC where it shouldn't be sent. Should I trust my QL supply, or have others done what I did to my C64 PSU and replace the DC regulation circuit with something modern like an LM259 regulator https://www.ebay.co.uk/itm/395716971624?

Best

Martin

I would classify the QL power supply as "mostly harmless".

In true Sinclair fashion, there's not much in there that could go bonkers, and in many years of QLing, I have never seen one rampage. They either work or don't.

Measurig the output voltage before connecting it to the QL won't hurt, though - As you already did.

OK great thanks very much!

Chris_68 wrote: Sat Nov 09, 2024 1:41 pm I placed a small buck converter inside the PSU, the 9-10V as input and the SMPS set to deliver 6.5 or 7 v. This to limit the heat from dissipated by the 7805 inside the QL.

Should be treated with caution. 7V is the very minimum if the QL still has its original 7805 regulator.
And regulation gets significantly worse below 8V input voltage.

Hello

I don’t know if this is of any help, but fey months ago I started a QL, which was dormant for 36 years with it’s original PSU. Of course, before that I checked the voltages and everything was normal (yes, I heard a lot of horror stories about old Commodore C64 power units), and is working fine now…


Good luck...

Jože

Hi, I think replacing the regulator circuit with modern technology like LM259 regulator might be a wise choice. This helps to improve the stability and durability of the PSU, avoiding the risks associated with old PSU.

Hi, there is no LM259 as such - probably a step-down switching regulator of the LM 259x family is meant, e.g. the LM2596.
Using a switching regulator reduces heat inside the QL case a lot and therefore improves the durability of the QL itself.

I don't think it will improve durability of the PSU though. Without additional measures, this type of regulator requires large amounts of input current when starting up. More than the QL's original linear regulator. Adding extra stress to the PSU on every power-up.

I have found the original QL power supplies very robust, unless they were abused.