On Sun, 20 Aug 2006 08:11:33 GMT, mike wrote:

John, N9JG wrote:
Your advice is elegant in its simplicity.

And OPTIMISTIC in it's generality.
While one might imagine situations where this technique is
useful, it would be better to define a strategy that takes
into consideration the exact configuration of the PS in
your unspecified amplifier.

If your caps don't need reforming, any technique will work.
If they do, you're gonna have some current flow. It's not
hard to imagine a scenario whereby you have 25W or more
dissipated in your caps. I don't call this safe. A 100W
light bulb will keep you from blowing up the rectifiers while
you're blowing up the caps.

I had this happen with a 3 week old amp. One of the low voltage caps
blew. It sounded like a shotgun being fired in my basement. The
"can" hit the top of the 8877 amp and flattened out like a 38
wadcutter hitting something solid.

It doesn't have to be a boat anchor to have a cap blow.


Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Thanks for the (mostly helpful) replies, guys.

Applying each one of the two ends of the 220V coil of the transformer
to the 220V legs of the amplifier plug made sense. I guess I was
mostly wrestling with how to handle the gounded / grounding lug of the
amplifier plug.

Gary said to just leave it floating, but wouldn't grounding it be safer?

wrote:
Thanks for the (mostly helpful) replies, guys.

Applying each one of the two ends of the 220V coil of the transformer
to the 220V legs of the amplifier plug made sense. I guess I was
mostly wrestling with how to handle the gounded / grounding lug of the
amplifier plug.

Gary said to just leave it floating, but wouldn't grounding it be safer?

Since you will be running the variac through a 110/220v isolation
transformer, you will lose reference to the utility ground, so there is
no reason to connect up the ground on the amp. If by some chance the
110v windings in the amp are on different cores, or are actually 2
transformers, then you would need a common (ground) reference for the
two. This is probably not your case, be nice if you had a schematic.

Gary N4AST

j

All he was asking was how to hook up a variac. Seemed simple enough.

Gary N4AST

True. But why lose the chance for an argument? - my issue was with the
over complication of a simple task....and if someone needs to ask
questions on how to set up a variac and a transformer, then the keep it
simple approach (ie, the light bulb) has much to recomend it.

Andrew VK3BFA.

Andrew VK3BFA wrote:
tirade 1 deleted

Rubbish. Where do you get this 25w figure from,
tirade 2 deleted

Well, you could do the math...it's easy.
I was too lazy to do the nonlinear math for the lamp.
But an experienced (good) engineer knows how to
quickly make approximations to weed out BAD ideas.
That skill is uncommon. You'd think it's obvious.
And the GOOD engineers do it instinctively.
But the other 99% just don't get it. They'll head
down the dead-end road 'till they reach the end
then bang their head on the barricade. A (good)
engineering manager shines a light on those dead-end
barricades. See, there are situations where a light
bulb can fix a problem.

When the problem is largely unspecified, a (good)
consultant requests specificity. It's easy to
ASSUME that it doesn't have tube rectifiers, but we
don't really know.
When the consequences can be BAD, and the problem
is unspecified, a (good)
consultant recommends a conservative approach.
And when the original poster merely requested clarification
on a conservative approach, it's low risk. Proposing
a higher risk option is ill advised.

A (good) teacher tries to understand the implications of
the question and keep the student out of harm's way.

I'm gonna pull some numbers outa my ass, 'cause that's
where I keep my common sense. That way, I get a new
supply every day.

The series light bulb likely works 99.99% of the time.
But doing nothing probably works 99.98% of the time.
If your estimates are different, state them. How many times
have you turned on your amp? How many times has it blowed up
because you didn't reform the caps?

At the risk of repeating myself...IF your caps NEED reforming,
it's wise to bring them up slowly with a variac. Adding the
series light bulb will make it even safer, but only if appropriately
sized and you're paying attention...there's that pesky specificity again.

If you're really interested in reforming caps, take them out
and use a variable power supply with a BIG resistor separately for each
one. Do it very slowly...but nobody really asked that...
Ya see, if you've got series caps...more pesky specificity...
a leaky one can cause the good one to fail. But nobody aked that
either. And maybe they've got equalizing bleeders that can mitigate
the problem...assuming one's not open and making the situation
worse...but nobody asked that either.

An interesting question might be, "how do you refurbish an amp that
hasn't been turned on in a while?" But nobody asked that...

Shooting from the hip, firing off the one true, onesizefitsall, solution
when you don't know the particulars and are not accountable for the
result is...
well...it's the "internet way"!

mike

mike wrote:
Andrew VK3BFA wrote:
tirade 1 deleted

Rubbish. Where do you get this 25w figure from,
tirade 2 deleted

Well, you could do the math...it's easy.

Shooting from the hip, firing off the one true, onesizefitsall, solution
when you don't know the particulars and are not accountable for the
result is...
well...it's the "internet way"!

mike

RANT
ON................................................ ..................................
I am NOT disputing your theory - its correct as far as it goes, youve
left out lots of other factors that, ideally, should be considered in
this case, But its irrelevant to the original question. And in real
life I would ignore it as being too anal retentive.

If the original poster needed to ask how to hook up two transformers,
then your quasi mathematical analysis would be totally innapropriate to
the job specifications. Did you think of this at all? It is covered in
most 1st year engineering course...(RTFM)

My method, with the light bulb, might be crude, (it is crude, so?) but
it will work. And have proved that it works, more times than I can
remember. Linear and switching power supplies.

A simple question - using the light bulb method, whats the WORSE that
can happen? If the amp is horribly faulty (and lets not even bother
about doing insulation tests on the (old) transformer, that would make
it even more complicated) then the light bulb will light to full
brilliance.

Even an engineer could see that theres something wrong, and then fault
find it. (After doing the math, of course)

The variac method - again, the same simple question - whats the WORSE
thing that could go wrong. And I will answer that, not from theory, but
from practice. The voltage is slowly ramped up until something fails
abruptly and catastrophically. With no current limiting beyond a mains
fuse in the variac (You did check its rating, didnt you?) the electros
will disintigrate like shotgun cartridges generating lots of additional
work. (no harm was done to the person, being a trained engineer they
had safety goggles on...didnt they?)

(The above assumes its a linear supply - if its SMPS, then the
switching devices sacrifice themselves in a vain attempot to save the
fuse)

I have spent my entire working life in this field - working as a
technician fixing up the design problems of engineers. Stuff that any
competent technician would know NOT to do, from practical experience.
In my working life, I have met approx 5 competent engineers. The rest
were theoretical bull**** artists who soon moved into management purely
on the strength of having a degree - they were as hopeless at that as
they were at engineering, but there they had less chance to do actual
damage.

Are you a "working" engineer - do you own a soldering iron? - lots of
theory, plenty of if/but/maybe/ because/planning, but no real
appreciation of what the problem is - firing up an old amplifier. Thats
all - its not a design exercise, theres no cahce of a Nobel Prize in
this one, the project doesnt need to be managed, no consultants needed
to give a "proffessional " opinon, no SPICE models needed - just how
to power up an amplifier with minimal damage.



RANT
OFF............................................... ................................................

woo........

TIRADE
ON................................................ ..............................................

And I am heartily sick of "armchair experts" making life bloody
complicated when someone asks a legitimate question that is best
answered without a theoretical textbook analysis - I have seen far too
many people at radio clubs made to feel inadequate because some
smartarse engineer has given a long winded incomprehensible, irrelevant
answer to a simple question. These people leave, and dont bother to
come to any more meetings, or ask any more questions.

One of the best teachers I ever had was an engineer - a very rare
person, a brilliant teacher and a competent engineer. He woked out
where the question was coming from, ie the level the student was
capable of understanding - he then started from basic principles, with
great patience, and took the student forward until they could
understand the rocket science behind the question.

And if you want to see a good textbook using this method, grab a copy
of 'The Art of Engineering" by Horowitz(sp) and Hill. read it - you
might learn something from it about communication skill.

If you cant help, dont hinder.


Andrew VK3BFA.

I

And if you want to see a good textbook using this method, grab a copy
of 'The Art of Engineering" by Horowitz(sp) and Hill. read it - you
might learn something from it about communication skill.

If you cant help, dont hinder.


Andrew VK3BFA.

Hi Andrew, don't you mean "grab a copy of 'The Art of Electronics' by
Paul Horowitz and Winfield Hill"? As far as I can tell H & H never
wrote 'The Art of Engineering'.

You made some halfway good points in the age old "me Technician you
Engineer" argument, but you lost credibility when you recommended a
book that doesn't exsist. Attention to detail, Engineers do,
Technicians don't :-).

If you can't help, don't hinder.

This had nothing to do with the orginal question-sorry.

Gary N4AST