Q.L. PSU Repair

[Lurch666]

Swapped a microdrive from my spares/repairs Q.L. to my working one and got some microdrive tapes to load.

Then I lost the T.V. signal.After a bit of checking it seems I'm lost the 15v ac output from my PSU.

Since they are hard to come by I will try and figure out the circuits of my PSU while it's apart and post a log of how (if) I fix it-I'm hoping it's just a simple component fail as if I've got 9v dc output the 15v ac must still be coming from the transformer.

[1024MAK]

As far as I know, there are two different designs of Sinclair QL PSU. They both have the same case. But inside the circuitry is different.

It's a tight squeeze with the components being densely packed, so although I have had one apart, I have not reverse engineered it.

On the web, I have come across a schematic. If/when I find the link, I will post it up (but don't hold your breath).

Failure of the 15V AC output, but with the 9V DC output okay is a bit strange. The 15V AC is likely to just be fed from a secondary winding on the transformer. Transformers are one of the most reliable components (unless underrated). If I were you, I would check the low voltage cable. Test with a multimeter on ohms or continuity to see if there is a break, or intermittent break in the cable.

Mark

[Lurch666]

I couldn't find ANY schematics of the Q.L. PSU when I was looking to make one-that's why I'm going to try and sort out the circuit from mine.
It doesn't look too complex and I should be able to make a rough diagram of it's circuitry.






I have removed the transformer to check the voltages and it seems to me that the transformer pin (4) that connects to the 15v ac wire of the plug that goes to the Q.L. is only giving 2-3v ac.The unused pin next to it (5) is giving 15v ac so I might try connecting that pin instead of the faulty one.



It seems confusing that the 9v dc is complex because it's getting fed from most of the remaining pins of the transformer-I would have thought it would just be 9v ac put through a rectifying bridge and then smoothed.
Anyway when I got a day off I'll map this into a proper circuit diagram.

EDIT:1st two pictures got cut off.Right click and view image to see them properly.

[1024MAK]

It's complex because some of the windings and diodes are switched in or out of circuit depending on the output voltage, which in turn depends on the output current...

One design uses a SCR (Silicon-Controlled Rectifier also known as Thyristors).

Mark

[Lurch666]

O.K. scrappy circuit diagram warning.
I've never done this before (can you tell...)
I haven't marked the pins or values for most of the components but if anyone out there needs more info I can put more detail in.
Just hoping someone who understands electronics better than me can make sense of this.



(It's supposed to be 9v ac at the top but my handwriting was never any good).

I have now put the PSU back together with pin 4 of the transformer disconnected and pin 5 supplying the 15v ac and it's now powering the Q.L. properly again.

[Silvester]

I couldn't find ANY schematics of the Q.L. PSU when I was looking to make one-that's why I'm going to try and sort out the circuit from mine..

See http://www.dilwyn.me.uk/docs/hardware/QLperip.zip

[Lurch666]

Wow thanks dude.Much clearer than mine.
This should be put somewhere it can be found easier.

[1024MAK]

Ahh, I knew I had seen two different versions. I have downloaded it before, but had forgotten it was in a zip file

Explains why I failed to find it when I searched my own hard drive and searched my bookmark links...

Mark

[tcat]

Hi,

Just looking at the PSU schematics in PDF, it is an interesting circuitry.

I'd love to ask about elyt capacitors and their function.
Are these for wave forming, you mentioned the PSU is switched, or are they just for smoothing purpose?
Is it a good idea replacing them with new ones and what values you would recommend?

Old capaciros I have seen originally fitted on QL boards are blue, while nowadays production is in black, so perhpas replacing all elyts inside QL might also be a good idea. Could anyone advise?

Many thanks
Tom

[1024MAK]

Yes, the large electrolytic capacitors are to smooth the rectified AC signal so that the resulting DC no longer drops to zero every half cycle. If there is excessive ripple ( test using an oscilloscope or a good multimeter) on the output while under load (connected to the computer), they should be renewed.

Mark