Leaky Electolytics in Philips 'scope

Thanks for that. I've now got it out and it's evident it's been repaired before. I'll try to post some pictures later on. Actually all the axial electrolytics have tested fine, surprisingly. I have spotted one small, old-style polyester cap that's cracked but aside from that, nothing obvious apart from 'historical charring' which someone has cleaned up and replaced a couple of diodes. At least now I can get at it, power it up and make some voltage checks! Does anyone know if it needs to see a load to produce an output? Being of a certain age, I'm more of a linear psu person...

If it's a SMPS (with presumably universal input voltage), what's the switch for?

And so I repeat (to Cursitor): when you are bypassing the input filter be sure your mains voltage is being applied to the proper input to the SMPS.

Dave

Thanks, Arfa. Well I snipped the filter completely out of circuit and checked it and it clocked at 0.9ohms so for sure something fused inside. It's rated at 250V/1A and I just happen to have a new spare one which is unusually fortunate. Now the psu board is out of circuit I feel a bit happier about zapping it with full volts. I'll give it a whirl in the morning as it's getting late here now. cheers, cd.

That makes a lot more sense now, Dave, thanks.

Are these available on-line?

If the PS can accept 115 or 230 and if there's only 2 wires for mains input to the PS, then it *is* universal input.

If there's a switch, then I can't imagine only 2 wires input to the PS (the filter/switch can't drop half the mains without a transformer)...?

Admittedly confused, Dave

There are *two* mains input sockets on the board - in addition to the rear panel switch. I plan to post a pic of the board shortly, then all will become clear hopefully.

... Phil

Changed the subject to reflect the progress made over the last day or two. I've now uploaded pics of the PSU board. You can see where it's been repaired before and evidence of a previous burn-out. It looks a little tired in places overall. Check it out:

Ah, so the Philips' ps has a voltage doubler at its front end controlled by the "mains switch". The input to the ps *proper* is the same--240--fed by either the 240 mains directly or the 120 mains, doubled.

Thanks, Dave

Its also a pretty good way to fry a variac - they're not as robust as regular fixed mains transformers.

An old service engineer's trick is to bridge the fuse with a 60W light bulb, then you can check voltages into the mains front end and get some idea what's holding things up.

Its worth remembering the lightbulb filament has a PTC characteristic, so there will be some inrush surge - calculating the value for an equivalent power resistor would avoid that.

Input to the ps proper is either rectified (240vac) mains or rectified (120vac) mains *doubled*, depending on switch position.

Dave

The usual scheme being the main electrolytic is split into a series pair across the plus and minus lugs of the bridge rectifier.

For 230V operation the switch is left open - for 120V, the switch is closed to connect the junction of the 2 electrolytics to one AC lug on the bridge, that converts the bridge into a voltage doubling rectifier.

In text books, you often see a voltage doubler with the same layout as a bridge rectifier, but 2 of the rectifiers are replaced by capacitors - the original diodes of a full complement bridge don't do any harm in a doubler even though they're not needed, but they are needed for the non-doubling configuration and leaving them there makes for simpler switching.

Some clarification of this statement would be nice. I have an old Variac and don't want to see it get damaged. It has an inline fuse within it so why isn't that in itself sufficient protection?

And those big electrolytic storage caps are only rated at 350V. I'd have thought 400 min would be more suitable!

That's exactly what Philips have done in this particular PSU.

I need to remind people, that the wiper of the Variac (Autotransformer) needs to be fused. The windings are rated for current, so if it's 10 A at 120 V. It;s also 10 A at 10 V. A primary fuse won;t protect it.

I am. But the problem at present is that I need to establish if the scope actually works before I spend money on it. That means contriving some temporary power supply (or *supplies* more likely) capable of generating the wide selection of voltages that the scope itself requires. One of the rails needs 60V and my largest PSU tops out at only 40. It's a PITA. :(