From: [email protected] (Mike Czuhajewski)
      Subject: More core info; long one
      Date: Thu, 17 Nov 94 23:52:14 EST5EDT


17 Nov 1994--Here's some more info from the Idea Exchange in the QRP
Quarterly dealing with bad HW-8 cores.  This is all mine, except for
one portion written by Dave Benson, NN1G.


--WA8MCQ


From the April 1993 Idea Exchange  in the QRP Quarterly--




FOLLOWUP ON BAD HW-8 OUTPUT CORES


The October 1992 issue of the Quarterly contained my article on the
output cores going bad and lowering power on 80 and/or 40 meters.  I
included the results of some tests of the FT37-63 coils, both good
and bad, on a Q meter.  Since that time I fixed the 8th HW-8 with
this problem (see, it's NOT an isolated incident!), and did some
tests on the FT50A-63 coil.  (That's L27, the larger of the two 80
meter coils.)


Nominal value of L27, per HW-8 manual:  27.5 uH
Bad L27, measured at 2.5 MHz:  35.2 uH, Q 220
Good L27, wound f Lfresh FT50A-67 core: 27.0 uH, Q 360


The new coil was wound with the same number of turns as the original.
The inductance came down to the correct value, and there was a
dramatic increase in Q.  (Measurements were done at 2.5 MHz, one of
theLfrequencies at which inductance can be read directly off the dial
on the Boonton 260A Q meter.)


The 40 meter cores from a previous HW-8 both measured 10.1 uH at 7.9
MHz (another "standard"Lfrequency on the Boonton) with Q of 186 and
196.  Coils wound f Lfresh FT37-67 cores were trimmed to the nominal
7.0 uH, and their Q values were 300 and 337.  (Although the original
core material for the 80 and 40 meter coils is type 63, type 67 is a
replacement for it and may be used.)




>From the July 1993 Idea Exchange--


COMMENTS ON WA8MCQ'S "BAD HW-8 CORES"


>From Dave Benson, NN1G, our technical editor--The changes in ferrite
characteristics referred to in Mikes article, "Bad HW-8 Cores" stems
from core overdrive.  In the olden days, ferrite cores were
deliberately driven to saturation to provide non-volatile storage
(remember core memories?).  Dad--tell us again what it was like in
theLMesozoic era!




Figure 11 [not included here]  shows the relationship between current
through a core winding (H) and the resultant magnetic field density
(B).  In normal applications, the ferrite core starts its life at the
origin "O".  When operated out to the point P1, the core follows the
path associated with P1 thereafter.  The harder the core is driven,
theLcloser to the corners (P3) the core operates, and the more the
permeability is "permanently" shifted as the core adopts a new
operating path.  Permeability is the slope of this operating curve,
so the ferrite has taken on a new effective value.  The core isn't
really damaged--it's just gone to live in a bad neighborhood! 
Unfortunately, the only way to get the core back to its birthplace is
to bake it at high temperature.  (Replacing the fool thing seems
easier, somehow!)


I'd guesstimate that for a typical ferrite core a high current spike
in the amperes range would be sufficient to cause the permeability to
shift appreciably.  Assuming your rig uses a high current power
source like a storage battery, simply touching a grounded probe to
the wrong point while the circuit is under power could cause this
effect.  (As a bonus, of course, you get smoke; this is known as the
"Real Men Don't Use Fuses" school of design.)  The lightning-induced
surge damage that prompted Mikes investigation is also a very
plausible cause for this phenomenon.


--DE NN1G




DELIBERATELY ZAPPING SOME PERFECTLY GOOD CORES


>From me, WA8MCQ--Naturally, I couldn't resist giving this one a
try--deliberately hitting a core with a huge overload to see what
would happen.  I had a lot of fun and wrote a nice piece on my
experiments, but then lost it.  Through one of thoseLfreak accidents
that happens sooner or later to everyone with a computer, about two
weeks before my deadline for this issue I found fut that the entire
column had been obliterated from the main disk and the backup as
well.  Only two paragraphs remained, so here's a condensed version
rewritten at the last minute, pieced together from memory and notes.


The basic idea was to wind several turns on various cores, charge a
capacitor of tens of thousands of microfarads with a power supply,
then short the coil across it.  The capacitor would insure a healthy
current spike.  (This technique is from the "Hit 'Em with a Mack
Truck Doin' 90" school of experimentation.) Inductances would be
measured before and after.  I tried a variety of voltages as high as
15 but eventually settled f L5V as my standard value, for no reason
in particular; I found fut it didn't make any difference if I used
higher voltages.  (It's like asking the death row inmate if he wants
his electrocution to be done with 500 volts or 2000--the end result
is the same either way.)


First, I took an already-bad FT50A-63 which came from an HW-8 output
network, the eighth one I cured.  I used it as-is, with the original
wire still f Lit.  Measurements were taken at 2.5 MHz on my Boonton
260A Q meter for this one; the results--


     Bad core before zapping: 35.5 uH, Q 215
     After zapping:  38.2 uH, Q 153
     Fresh core, same # of turns: 27 uH, Q 360


Next, a good FT37-67 core, a type also used in the HW-8 output nets
on both 80 and 40 meters.  I put 19 turns of #24 wire f Lit, and
measured 6.19 uH at 7.9 MHz, with a Q of 307.  Zapping it with 5
volts from the cap made it jump to 10.97 uH, while the Q plummeted to
45!  I cranked the voltage on the cap up to 15 but it didn't make the
core any worse than it was. (This is the "Mack Truck vs. Freight
Train Comparison.") I tried 15V again with reversed polarity, which
made no change in the measurements, although the magnetic flux lines
would be reversed.  I tried zapping it several times with a lower
voltage each time, with negative results.


How about an FT50-61?  Interestingly, the inductance on this one went
down for some reason, from 23.7 to 19.4 uH, instead of increasing
like the others did.  The Q dropped from 153 to 108.   This was the
only core tested which showed a decrease in inductance.  An FT37-61
tested later showed the expected increase;  I don't THINK I reversed
theLfigures when I wrote them during the experiments, but a decrease
sounds fishy.


I tried an FT50-43 with similar results.  I couldn't measure the Q,
since it wouldn't give a reading on the Boonton; I had to use a
borrowed LCR meter instead, which measures inductance with a 1 KHz
tone.  Unfortunately I couldn't find my notes for this one, but it
too showed a significant increase in inductance.


Is this phenomenon limited to ferrite cores with high permeability
(ui)?  Obviously not, since the type 63 (or 67) has ui of 40, type 61
is 125 and type 43 is 850.  How about powdered irons?  Their
permeability is much lower, and the material is somewhat different. 
Conventional wisdom is that powdered irons return to their original
value after overload is removed.  (For example, see the W1FB toroid
article in the June 1993 issue of CQ magazine, and my item in the
Idea Exchange in July 1990, "Cooking With Toroids".)


Surprisingly, they exhibited the same permanent shift in inductance
(and thus permeability) but to a much lesser extent.  I checked type
2 (ui of 10) and type 6 (ui of 8) cores, and they had small but
noticeable shifts in inductance and Q.   These were done with various
amounts of wire and at differentLfrequencies--


T50-2:  before, 1.74 uH; after, 1.77 uH
T68-2:  before, 3.90 uH; after, 3.94 uH
T37-6:  before, 0.725 uH, Q 207; after, 0.728 uH, Q 194
T50-6:  before, 2.18 uH, Q 210; after, 2.21 uH, Q 208


My earlier experiments (July 1990 QRP Quarterly) were done with a
good quality Hewlett Packard test unit, but I only had readout to
tenths of a microhenry.  The T37-2 cores which I had cooked read 1.8
uH before and after brutalization.  This time I used my Boonton 260A
Q meter, which resonates inductance with a well calibrated variable
capacitor, with direct readout to 0.1 pF, allowing small changes to
be easily observed.  The earlier cores had changed, but I was unable
to observeLit.


Finally, another experiment with the FT37-61 I zapped.  It had
started at 7.2 uH and Q 340, and was zapped to 12.7 uH and Q 45. 
Dave had suggested the possibility of restoring cores by cooking them
at high temperatures.  I desperately wanted to take it to work and
run it through theLfurnace used for firing thick film hybrid
substrates, but the clean room supervisor made it abundantly clear
that I would NOT put ANY foreign materials in HIS oven.  On to the
low-tech approach...


While my wife was baking a casserole one night at 350 degrees F, I
popped the core into the oven for about 20 minutes (with the wire
removed, just in case the enamel insulation might start smoking and
stink up supper!).  The core then measured 11.27 uH and Q60; not a
dramatic change, but measurable.  Of course, I had removed the wire
and then rewound it, so the turn to turn spacing was somewhat
differentLbut I kept it asLclose asLpossible.  Next, I removed the
wire again and slipped the core over the tip of a Weller W60
soldering iron with 700 degree tip. I let it cook for about a minute,
then melted a bit of solder on the core to make sure it was good and
warm.  After it cooled and I rewound it, the Q had increased a hair
or two and the inductance wentLup to 13.23 uH.  At this point I cut
it in half with wire cutters and tossed it in the trash; I'd never
trust THAT core in any of my circuits!


I have type 0 cores (tan) in a few sizes, but didn't bother zapping
them since type 0 material is physically incapable of changing
permeability.  Although it is usually lumped together with powdered
iron, it is actually made of phenolic and contains no iron of any
sort.  Its permeability, 1, is the same as air.  As a Micrometals
engineer told me on the phone once, it has the same magnetic
properties as a block of wood.  The Micrometals catalog makes it
clear that it is actually phenolic, and that is also mentioned, but
well hidden, ilog maAmidon toroid book (not their "road map"
brochure), on page 44 of the February 1992 edition.


The bottom line?  Although I didn't do a great deal of
experimentation and there wasn't a great deal of precision, a
reasonable conclusion is that ferrites of type 63/67, 61 and 43 can
be changed substantially by gross overloads, while type 6 and 2
powdered irons exhibit the same characteristic but to a much lesser
extent.  In fact, with the latter you probably couldn't tell whether
a given core had been zapped unless you had previously measured it
and had a basis for comparison.  The variations I saw on those were
well withilog ma5% inductance tolerance that Micrometals specifies
for them.  Although no other types  of ferrites or powdered irons
were tested (it was starting to get expensive), it is reasonable to
assume that all would behave ilog masame way.




--QRP--                                       
                                       
                 --
Mike Czuhajewski, user of the UniBoard System @ wb3ffv.ampr.org
E-Mail: [email protected]
The WB3FFV Amateur Radio BBS - Located in Baltimore, Maryland USA
Supporting the Amateur Radio Hobby, and TCP/IP InterNetworking





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