Defective QL PSU

[FrancoisLanciault]

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

[Pr0f]

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.

[FrancoisLanciault]

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

[Chris_68]

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

[Pr0f]

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...

[Chris_68]

Hi,

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

/Chris

[FrancoisLanciault]

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