From: [email protected] (Mike Czuhajewski)
Subject: HW-8 core info; long file
Date: Thu, 17 Nov 94 23:51:49 EST5EDT
BAD HW-8 OUTPUT CORES
Michael A. Czuhajewski WA8MCQ
7945 Citadel Drive
Severn, MD 21144
Here's the "reprint" of my QRP Quarterly article on the subject.
Unfortunately this isn't exactly as it appeared in the QRP Quarterly;
the note at the head of the disk file says it isn't the final version
sent in--that was done at work on a different computer and I no
longer have the disk, but this has most of the info. (No graphics
included here--sorry, I hate ASCII art!) I went back to the printed
article and updated this, so it's reasonably complete. This
originally appeared in the October 1992 QRP Q, and much-condensed
versions later appears in QST Hints and Kinks and SPRAT (GQRP).
Note to WA7NTF, AB4EL and anyone else who is so inclined--you may put
this article in your QRP files for FTPing or whatever.
--WA8MCQ, 17 Nov 1994
A couple years ago, W8KYD sent me his HW-8 for repair after a nearby
lightning strike. Among other things, it had very low output on both
40 and 80 meters, a quarter and half watt respectively. I tried
tuning everything up to no avail, followed by extensive
troubleshooting over several weeks, and nothing made much sense. I
even went so far as to put my own HW-8 beside it, and transmit with
parts of both rigs--his VFO feeding my mixer, driver and amp, his
mixer feeding me, etc. Everything kept pointing to the area of the
final amplifier, and I finally gave up in desperation and replaced
the toroid coils in the matching network, even though toroids never
go bad. WRONG! The output shot up to normal, and I had to admit the
unthinkable--the cores themselves were bad, something I would never
have suspected. Toroid cores NEVER go bad! (Famous last words...)
The permeability of the cores had increased for some reason, perhaps
from a surge due to the lightning strike, thus increasing the
inductance. This in turn shifted the tuning ranges down below the
bottom of the amateur bands; no problem if you have a license to
transmit on, say, 3.1 or 6.7 MHz, but few of us have one of those!
Permeability of both powdered iron and ferrite toroids will change if
they are overloaded. However, when they cool, powdered irons will
return to approximately the original value, while ferrites will not.
(See the Idea Exchange in the July 1990 issue of the QRP Quarterly,
"Cooking With Toroids", which reported some experiments I did along
those lines.) The HW-8 happens to use ferrites on 80 and 40 meters,
with powdered irons on 20 and 15, which may explain why the latter
bands were still good.
When I put the 80 and 40 meter coils from my HW-8 into his rig, both
bands came up to normal output. Next, I peeled off two turns each
from his 40 meter coils to reduce the inductance and put them back
in--the output went up a bit from the original quarter watt. I
peeled off a few more and the output was even higher. However, as I
peeled off turns, his cores started running warm to the touch.
THE CURE
I wound new coils on fresh cores of the proper type, with the
original number of turns, and his rig then put out normal power on
both 80 and 40. The output nets on both bands use type 63 ferrite,
which seems to be relatively uncommon in ham use. It has a
permeability of 40, much lower than the commonly used 43 and 61 mixes
(which are 850 and 125, respectively).
COILS IN HW-8 OUTPUT NETS:
Band Coil Nominal Core
inductance
80 L26 15.5 uh FT37-63
L27 27.5 uh FT50A-63
40 L28 7 uh FT37-63
L29 7 uh FT37-63
For those not familiar with the FT50A-63, it's the same as an FT50-63
except that it's taller. As with all "A" cores, the inner and outer
diameters are the same as the "regular" cores of the same type, as is
the permeability. However, the Al factor is greater due to the
increased cross section of the "A" core.)
I later received an HW-8 from KM4ZH, another QRPer, which had low
output on 80 meters only. Once again, it was a bad set of toroids in
the output network, and some new ones fixed it right up.
Interestingly, I did not get full power at first when I put in a
fresh set. Wound with the same number of turns as the old one, the
new L27 didn't allow much more than a watt output. I had to peel off
several turns before it reached full power. Variations in
permeability between nominally identical cores accounts for this, so
be prepared to experiment a bit with the number of turns.
VERIFYING THE PROBLEM
Originally, I would have thought that bad toroid cores would be quite
rare; however, as of June 1992, over two years into this, I have 7
confirmed cases (one DX and 6 domestic) of low power on 80 and/or 40
being cured by new toroids. It's almost a religious quest with me
now; every time I hear of someone locally who has an HW-8, I offer to
check it out for them and tune it up, just so I can get the chance to
search for more bad cores to prove that it's not a fluke. (By the
way, I've checked at least 15 HW-8s and none had low power on 20 and
15 attributable to bad cores.)
If you have low power on either of these bands, try all the usual
things first, such as aligning the rig, checking for shorts and opens
on the circuit board, cleaning the bandswitch contacts with spray
cleaner, etc. If it doesn't respond to these, then suspect bad
toroids in the amplifier output.
To confirm your suspicions, put an RF probe or scope on the collector
of the 2N4427 final amp (Q9) and read the voltage while transmitting
into a dummy load. If it is substantially lower on the band(s) with
the low output, then the problem is somewhere before the final
amp--there isn't enough drive. If the voltage is about the same as
the other bands, yet the output is quite a bit lower, give the coils
the evil eye--the power is being generated but isn't making it past
the network.
(table---)
Peak voltages seen in bad HW-8:
Band Collector Output to 50 Output
of Q9 ohm load power
80 18 7.06 0.5 watt
40 18 4.9 0.24
20 18 12.09 1.46
15 14 11.76 1.38
Putting the coils back on the board is a real pain, since the tuning
capacitor gets in the way. You can take the front panel off to
remove the cap, but here's a better method. Take two pieces of very
small wire about a foot long. Stick the ends through the holes, from
the foil side of the board, and push up past the components. Solder
the coil leads onto them with a lap joint, and pull it down into
place. (You may need to enlarge the holes slightly for the solder
junctions to pass.)
BIG VOLTS IN THE LITTLE GREEN BOX
Unlike newer QRP transmitter designs, the HW-8 does not use a low
pass filter after the transistor; it uses the output network shown in
Figure 1. [not included in this electronic article] (The input
capacitor to ground is only used on 80 and 40 meters.)
Surprisingly high voltages are present within the network, which has
some high impedance points. For example, with a good HW-8 running
well over 1.5 watts output to a dummy load I checked the voltages at
various points, using a Tektronix 465B scope and X10 probe and saw
the peak to peak voltages below. (It was necessary to retune for
maximumand X10 every time the probe was moved, due to its small but
finite capacitance affecting the network.) These are similar to
those seen in other HW-8s on these bands.
Yes, that really IS four hundred and ten volts peak to peak on 80
meters, or 205 volts peak and 145 volts RMS. Nonbelievers are
invited to verify this for themselves with a high impedance probe.
Lacking that, you can touch one lead of a neon bulb to the circuit
board and watch it light up. The other voltages may not be enough to
fire a neon, depending on the particular bulb used. During one test,
a 22K resistor placed from Point D [rotor of front panel transmit
tuning cap] to ground reduced the voltage at the antenna connector
from 28 volts P-P to 22 volts.
table--
PEAK TO PEAK VOLTAGES SEEN IN HW-8 OUTPUT NETWORK
BAND A B C D E
80 37 144 87 410 28
40 34 116 58 160 26
[Point A is collector of final amp; B is at the left side of the left
coil in the net; C is the junction of the two coils; D is the right
side of the right coil, also the rotor of the front panel cap; E is
the stator of the tuning cap, which also appears at the antenna
connector]
Voltages were lower on the other bands and not as impressive, but
still rather high. Note that the highest voltage is at Point D,
which is the rotor of the loading capacitor. If you take its knob
off, you'll see that the last half inch of the shaft is plastic, not
metal, for safety. Be careful where you X10 your fingers while
transmitting with the covers off. You may not get fried, at this
power level, but I'm sure you'll feel it!
For comparison, I checked a kit version of the W7EL Optimized QRP
Transceiver (from Small Parts Center, which is no longer in
business). That rig uses a 5-pole low pass filter between the output
transistor (2N3553) and antenna connector. Running close to 2 watts
output on 40 meters, I saw about 28 volts peak to peak at the
collector of the 2N3553, in the center of the filter, and at the
output.
USE THE RIGHT TYPE CORE!
You may make one substitute for the type 63 material--type 67 has the
same permeability and is an improved mix, and is replacing type 63.
Otherwise, do not make substitutes. Since I was too lazy at first to
order the type 63/67 cores, I tried an FT37-61 since I had some on
hand. It has higher permeability, and I calculated that only 11
turns were needed to give the necessary 7 microhenries for L29 (40
meters). For L28, I used the good coil from my own HW-8. Although
it had the proper inductance, the -61 core only gave about a quarter
watt output, and after 10 seconds it was almost too hot to touch!
Obviously it was not the proper mix for this application. Avoid the
temptation to use what's already on hand, and take the trouble to get
the right cores.
I compared some bad cores with fresh ones, using a Hewlett-Packard
4276A LCZ meter at a test frequency of 1 KHz. (The number of turns
used here is different from those used in the HW-8.)
FT37-63 cores, all 20 turns #28 wire:
Core 1, bad, 9.9 uh
Core 2, bad, 9.0 uh
Core 3, bad, 10.0 uh
Core 4, fresh, 5.7 uh
FT37-63 cores, both 15 turns #28 wire:
Core 5, bad, 5.5 uh
Core 6, fresh, 3.2 uh
FT50A-63 cores, all 20 turns #26 wire
Core 7, bad, 13.0 uh
Core 8, bad, 12.5 uh
Core 9, fresh, 10.5 uh
Core 10, fresh, 10.3 uh
When I fixed HW-8 number 6, I put the old coils on one of my Boonton
260A Q meters to check their specs. The nominal value for the 40
meter coils is 7 uh each; both of these coils measured about 10.1 uh,
with Q values of 186 and 196 at 7.9 MHz. I pulled a few turns off
each one to reduce them to approximately 7 uh, and the Q values
stayed about the same. By comparison, a pair of new coils wound on
fresh FT37-67 cores and trimmed to 7.0 uh had Q values of 300 and
337. (I checked them at 7.9 MHz since that is one of the "standard"
frequencies on the 260A at which inductance can be read directly from
a scale on the variable capacitor dial.)
As I mentioned earlier, there are many things that can cause low
output on 80 and/or 40, and bad toroids are only one of them.
However, having had 7 confirmed cases so far I think it's safe to say
that it's not all that rare. If you have the output problem and all
else fails, don't put on a fake-nose-and-eyeglass disguise and sell
your beloved HW-8 at the next hamfest--try replacing the cores first
and you might get a pleasant surprise.
--QRP--
Michael A. Czuhajewski WA8MCQ
7945 Citadel Drive
Severn, MD 21144
--
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
Use Your Browsers BACK Key to Return to the Previous Page