Ian Field wrote:

"Michael A. Terrell" wrote in message
...

Arfa Daily wrote:

"petrus bitbyter" wrote in message
.nl...

"Arfa Daily" schreef in bericht
...




Didn't you walk right into the problem? It seems that by a yet
unknown
cause the circuit sometimes runs into some state that makes it
oscillate
or in some other way fries the resistors. This kind of intermitted
faults are the most difficult to solve as most of the times there
seems
to be no problem. Even your observations with the scope may mislead
you
as connecting the probe may change the circuit enough to change its
behavior.

petrus bitbyter


Yes, there may indeed be some kind of intermittent problem. I will put
them all back up again on Monday, and see how they perform this time.
As
to my 'scope muddying the waters, it's generally pretty well behaved
in
that respect. It's a high quality 100 MHz job, which is always used
with
a x 10 low capacitance probe. If that is having much of an effect on
the
circuit, then it must for sure be a pretty poor design. To be honest,
I
still think that the problem lies with me somehow. Something that I
did
differently between the first and second times that I tried them, but
I'm
not sure what ...

Arfa

Off course you may have made a mistake. Humans make mistakes, even I
sometimes do
But if so, what on earth fried those resistors the first time?

petrus bitbyter



Every example that I've seen so far, has the resistors badly discoloured
and
the print and substrate scorched. On some, one of the resistors has been
open, so the network has not been doing the job it was put there for, at
all. They are 3 watt resistors, and even when the supply *appears* to be
running correctly, they get hot enough for you to say that they are
probably
dissipating a good 3 watts, and maybe a bit more, so I would guess that
you
would have to say that from that angle, it's a badly designed bit of
circuitry. I am fairly convinced that the purpose of the network is to
attenuate the big spike that occurs on the leading edge of the switching
waveform. This would tie in with why they have used about the biggest
film
resistors they could get, rather than using a higher power wirewound
type,
which would have a fair bit of inductance. I would also surmise that they
have used two x 150 ohm resistors rather than a single 330 ohm, to try to
spread the dissipation a bit.

When you replace the resistors, they still run hot, with no visible signs
on
the 'scope of any 'RF' on the waveform, so you'd have to say that it *is*
working correctly. What led to this thread in the first place was that
when
I was initially evaluating these supplies for the company that wants them
repairing, after I replaced the resistors, they ran very hot when the
supply
was loaded, but seemed to just run 'acceptably' hot when it was idling.
Likewise, when loaded, the switching FET got very hot, but was almost
cold
at idle.

However, when I next tried them - same conditions for i/p voltage and
load,
as far as I was aware - they now seemed to be working much better in that
the resistors were just acceptably hot for all conditions, loaded or not,
and the FET remained cool also. So this has now led me to believe that it
must have been something I was doing differently - and wrongly - when it
was
running very hot. So, a mistake ? Yes, probably. As you say, we all make
them, and this has got to be one of the easiest trades for doing it in.

I've just heard from the company that they are sending a bunch more up
this
week, so if there's some more of this type amongst them, I'll have some
more
'untouched' ones to see what happens this time.


Are you using metal or carbon film resistors? You could have a
resonance with a cap & metal film.

There's always low inductance thick film resistors, but they have to be
generously rated to minimise heating - they were a constant hassle in the
video O/P stage in one of the TCE CTVs, running too hot and repeated thermal
cycling took its toll on the pins attachment pads.


Carbon film is less inductive. Carbon comp would be ideal, but most
EEs these days don't know they exist. Low power SMD metal film
resistors do a lot better at UHF and Microwave frequencies. We used
them at 10 GHz, with no problems. Larger, high power metal film on a
round core are a spiral of metal.


--
You can't have a sense of humor, if you have no sense.