The Sinclair QL Forum

Hi,

I recently plugged my old original QL PSU to my QLs (I have a modern replacement for everyday use) and got unexpected results. This PSU has always worked great.

But now when I plug it to QL #1 (goldcard) the QL doesn't boot and instead produced a very scary buzzing sound. When plugged to QL#2 (512k memory expansion) the QL does start but the bottom half of the display cannot sync correctly. Looks like a defective PSU with capacitor in need of replacement...

But first, I decided to check the voltage levels on the scope. With no load, i.e. with the scope probes directly attached to the PSU output, I get the following:

9V DC rail: 11.3V with a +/-32mV variation
15.6V AC rail: 17.4V rms (50.4V p2p)

These are higher than they should, but maybe normal because there is no load? What do the expert say ?

For information, I have a Teroid voltage regulator in both QL instead of the old 7805.

Francois

P.S. No QL were hurt during these tests they both work fine with replacement PSU

Probably more useful to monitor the voltage levels on the main 9v output whilst the QL is running. The requirements for the 15Vac supply are a lot less demanding - and even the higher voltages are within the specification of the small regulator IC's that reduce this to +/- 12V DC

My guess is that the 9V DC input is collapsing under load and that's probably your issue.

Pr0f wrote: Sat Nov 04, 2023 4:26 pm Probably more useful to monitor the voltage levels on the main 9v output whilst the QL is running. The requirements for the 15Vac supply are a lot less demanding - and even the higher voltages are within the specification of the small regulator IC's that reduce this to +/- 12V DC

My guess is that the 9V DC input is collapsing under load and that's probably your issue.

You are probably right. I may check the PSU behaviour under load. That should not be too difficult.

Are the original PSU known to fail after 40 years ? Do people re-cap / repair them ? Or should I buy another modern replacement ?

François

Hi,

I replaced mine but not because it was necessary, more to be on the safe side. Measure under load as suggested, is the DC signal noisy?

/Chris

There is a capacitor in the power supply - but the circuit is a little unusual as well - I seem to have lost the circuit for it - but my memory tells me there is a thyristor in the circuit acting as a regulator...

Hi,

I don't remember the source, most likely from the forum but here are the schematics.

/Chris

For info my PSU is an american 120V type, but I guess it changes only the transformer part of it.