25 Jul
2000
25 Jul
'00
10:54 a.m.
[..snippage..]
Be sure to check the quote level to see that this wasn't me who said it.
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.
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.
Would I not have to open them to make that determination? I though they
contained PCBs or somesuch toxic chemical.
regards,
-doug q
> 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 :-). > 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. 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 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. 
25 Jul
25 Jul
1:47 p.m.
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4:11 p.m.
New subject: OT: PCB Toxicity (was: Switching PSU...)
Rumor has it that Tony Duell may have mentioned these words:
and someone else said...
> Would I not have to open them to make that
determination? I though they
> contained PCBs or somesuch toxic chemical.
[snip]
Secondly, the electrolyte in these capacitors is not
PCB-based (PCBs are
not electrolytes anyway...). The electrolyte is not particularly harmful.
Actually, according to somebody who works with _large_ transformers and
capacitors, yes, PCBs are dangerous _in quantity_, but unless you're
really silly, the amount in one capacitor, or even all the capacitors in
an HV PSU, is unlikely to do you any harm. I can't really comment on this
-- it's not my field.
Did a study back in High School (near 15 years ago, so bear with me...) on
PBB (PolyBrominatedBiphenyls) and PCB (PolyChlorinatedBiphenyls) - excuse
the spelling if it ain't perfect - but IIRC PCB's aren't too harmful to you
- until injested. They're fire retardant chemicals, and they got mixed up
with batches of Dairy Feed Supplements at an elevator here in Michigan, USA
back in '75 (again, IIRC).
Moderate to high concentrations *in the body* do bad, bad things,
especially to small children & unborn fetuses (or would that be fetii? ;-)
and it does travel thru cows milk, women's breast milk & the placenta. It
can cause birth defects, miscarriages & serious mental problems, as (once
again, IIRC) it can affect your nervous system in a bad way.
So, if you're not feeding 50 pound bags to your dairy cattle, or serving up
"Capacitor Stew for Two" for you and your pregnant wife, I wouldn't worry
about the toxicity level of any PCB's (should they even exist) in caps.
At least that's what the voices tell me... ;^>
Take care,
Roger "Merch" Merchberger
--
Roger "Merch" Merchberger --- sysadmin, Iceberg Computers
Recycling is good, right??? Ok, so I'll recycle an old .sig.
If at first you don't succeed, nuclear warhead
disarmament should *not* be your first career choice.
26 Jul
26 Jul
8:44 a.m.
New subject: OT: PCB Toxicity (was: Switching PSU...)
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25 Jul
25 Jul
4:32 p.m.
----- Original Message -----
From: "Tony Duell" <ard(a)p850ug1.demon.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Wednesday, July 26, 2000 8:17 AM
Subject: Re: Switching PSU Theory of Operation / Repair Tips &
Techniques
> > > 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?
This is starting to sound like it has either a separate + and - supplies
with a common ground, or
it's some kind of voltage doubler. Since I can't imagine why they'd
need to do a doubling act
for the primary side of a SMPS I'm inclined to suspect the former.
> > > : 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.
Yes, sounds like a classic description of the mains rectifier/filter
side of an average SMPS.
> 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
Yes, but aren't the 2 capactiors in series, something like :
------------+-------------------- +ve
|
=====
-----
|
------------+
|
=====
-----
|
------------+--------------------- -ve
Well they could be if they were across a potential higher than the
invidual voltage rating of the caps.
That gives you an effective voltage rating equal to the sum of the
ratings of the caps. The penalty is
that it divides the effective capacitance by the number of caps in the
series circuit (assuming all are the
same value - it's a bit more complex if they have different values)
Have you got a + where there should be -?
The above diagram looks wrong to me. If this was a +/- supply, then the
lower cap should have it's anode connected
to the common (centre) ground point, or it's effectively reverse
polarised.
It would be more common to find caps in parallel to increase the
capacitance (opposite of series - the total
c of the circuit is equal to the sum of the values of all the caps in
it.)
And you're seeing 600V between the 2 points
I've marked '+ve' and
'-ve',
right? That's still twice what I'd have
expected -- I'd expect peak
mains
voltage (i.e. 115*sqrt(2)) across each capacitor, or
about 320V
between
+ve and -ve in that diagram.
The Prime techie mentioned a bridge, so I assume it's not a doubler
circuit,
that leaves some kind of +/- supply but I'm blowed if I know how they
achieve that
out of a bridge. I'd definitely like to see a circuit for this one.
I am still puzzled as to how it gets 600V without
either other large
capacitors, or a mains-frequency transformer.
Ditto.
I will just ask again -- this thing does run off 115V,
right?
I keep wondering the same thing, if it was multiphase it might make
sense, hmmm,
I *wonder* if this was originally 3 phase and been converted to single?
2 mains rectifiers
in series? With a common rail? Sounds ugly but possible I suppose.
"When you have eliminated the impossible, whatever remains, however
unlikely, must be the truth."
Sherlock Holmes
(Sir Arthur Conan Doyle)
Hmmm.. Well, whatever you do, don't work on it
_after_ a hard day's
work...
> > > > ****PAGING TONY DUELL****
> >
> > Did you honestly think I'd not reply to a question like this :-).
No, not really....;^)
> > 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.
How old is this box of tricks anyway? I must agree, if there is a lot
of logic there,
from the description of all the led monitoring etc, it
might be more to
do with that than
part of the supply proper. Possibly control logic to ensure the system
won't power
up if the supplies are out of spec, or shutdown the whole thing if one
fails etc...
> > In which case look for dried-up (high ESR)
electrolytic
capacitors. This
> > is a very common heat-related problem.
Indeed. Probably the commonest fault in PSU's, which tend to get
pretty hot
anyway. The startup resistor is still a potential culprit here too.
Cheers
Geoff Roberts
Computer Systems Manager
Saint Mark's College
Port Pirie,
South Australia
geoffrob(a)stmarks.pp.catholic.edu.au
netcafe(a)tell.net.au
ICQ: 1970476
26 Jul
26 Jul
12:36 p.m.
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8763
days inactive
8764
days old
5 comments
4 participants
participants (4)
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ard＠p850ug1.demon.co.uk -
dhquebbeman＠theestopinalgroup.com -
geoffrob＠stmarks.pp.catholic.edu.au -
zmerch＠30below.com