Switching PSU Theory of Operation / Repair Tips & Techniques

From: Douglas Quebbeman <dhquebbeman_at_theestopinalgroup.com>
Date: Tue Jul 25 14:54:08 2000

[..snippage..]

> > No, if I measure across the positive and negative, I'm seeing nearly
600vdc.
>
> Eeek!. That's double what I'd have expected. Does this thing run off the
> normal 115V mains, or 220V? How does it (effectively) quadruple the mains
> input?

[..more snippage..]

> > : switcher, which runs the 280V DC through the torroidal transformers to

> > : produce the other voltages. The Big Bridge is to convert the output of
the
>
> That's more what I'd have expected. About 300V supply to the chopper
> stage, not 600V.
>
> Can I ask a silly question? Are you sure you're reading the voltmeter
> correctly. You've not got a 'range doubler' turned on or something (I've
> been caught by this one...)

You may not have yet seen the correction I posted, but I was seeing that
only when I measured the + terminal of one of the blue electrolytics, and
the - terminal of the other. Why would I do that? I'd just gotten out of
bed and hadn't had coffee yet. YES, A BAD TIME TO MESS WITH HT! After I
saw that I realized my time would be better spent letting the dog out
and preparing to leave for the office rather than risking my life while
I was still half asleep. No more messing with it except when alert, I
promise!

> > > ****PAGING TONY DUELL****
>
> Did you honestly think I'd not reply to a question like this :-).

Be sure to check the quote level to see that this wasn't me who said it.
 
> > I can probably sketch the HT portion out, but this PSU includes two
rather
> > complex looking boards (although they're only 2-layer) with custom PALs
and
> > the like.
>
> PALs? Are you sure? It's not at all common to have significant amount of
> digital electronics in an SMPSU. Most of the time it's either simple
> analogue circuitry either as discrete components, or one of the well-know
> SMPUS control chips.

These look like digital boards to me. I'll pull 'em and either scan them
or take a digital photo, as well as record a few part numbers.

Keep in mind that being overly proprietary was one of the things that
killed Prime.

> > One symptom I hope leads me to finding the problem is that the PSU has
> > four LEDs onit, 3 green, 1 yellow, the green are status indicators for
> > +5, +12, & -12; the yellow LED is an indicator for the AC. Although the
> > AC feed to the PSU is fine, the yellow LED glows dimly, not brightly, as
> > it once did. The green LEDs are unlit.
>
> That's a starting point. Trace back from that 'AC' LED to find what it's
> really monitoring. Maybe that supply line is low.

> > > That suggests that the mains rectifier is still functional, they don't
> > > 'chop' the ac mains.
> >
> > Uh, being an analog idiot, forgive me if this is a stupid remark, but
> > I thought "chopping" the AC with a rectifier yielded DC; when I measured
>
> No. 'Rectifying' is turning AC into DC. 'Chopping' is turning that DC
> back into a square wave (at least in voltage, as I mentioned last night,
> the current waveform is complex).

Ok. I thought chopping referred to the way the waveform looks before vs.
after. Before it's a sine wave, crests about and below the axis. After
being rectified, there are only crests above the axis. That looks like
chopping off the bottom part of the wave to me. Sorry.
 
> > If it matters, the room in which I keep the Prime was hotter than
> > I usually let it get when I have the Prime running (I don't have
> > central A/C, so I have a window unit in an adjacent bedroom and
>
> In which case look for dried-up (high ESR) electrolytic capacitors. This
> is a very common heat-related problem.

Would I not have to open them to make that determination? I though they
contained PCBs or somesuch toxic chemical.

regards,
-doug q
Received on Tue Jul 25 2000 - 14:54:08 BST