Back in December I posted a question about ways to receive LF/VLF
radio signals. Based on the suggestions made by a number of people
here I decided to use my existing Heathkit Mohican receiver and add
this upconverter kit from Jackson Harbor:

http://jacksonharbor.home.att.net/lfconv.htm

The kit arrived and was half assembled before I turned on the
Mohican, its first power-up in some years; the horrible squeal that
erupted from the speaker put a bit of a damper on things. It now
appears that replacing the two output transistors (Germanium, no
less!) with NTE102As from Mouser will fix that, so I'm thinking
about an antenna that might be a little more snesitive to LF signals
than the Mohican's built-in whip.

Along those lines, I have a couple of (what I hope are) simple
questions that I'm hoping someone can help me get started with.

First, the need for impedance matching between an antenna and a
receiver. My understanding is that a resonant halfwave dipole will
have an impedance around 73 Ohms; unfortunately, unless I can obtain
research funding from the just-passed Congressional Economic
Stimulus bill I'm going to have trouble paying for 2.5km of copper
wire, some towers, a crateload or two of porcelain insulators,and
the land to build it on. (Hey, I promise to dump it back into the
economy ASAP. Really! grin!)

So any non-loop antenna I can construct will necessarily be a "short
wire" or "electrically small" antenna (two useful search terms).
But how does one go about calculating the impedance of a coat hanger
or an extension cord ("short piece of wire")?

I've done Google seaarches and read what seemed like the relevant
sections of the 2004 ARRL Radio Handbook and their Antenna Book;
unfortunately, most authors restrict their discussion to quarter-
wave or longer antennae. Any starting points, hints, or references
on impedance calculations for less-than-1/10-wavelength antennas
will be appreciated.

My other question has to do with how to interpret signal strength.
The first "standard reference" transmitter I'll be attempting to
receive will be WWVB out of Fort Collins, Colorado (60kHz/5000m).
Per the NIST documentation at:

NIST Special Publication 250-67: NIST Time and Frequency Radio
Stations: WWV, WWVH, and WWVB
http://ts.nist.gov/MeasurementServices/Calibrations/
Upload/SP250-67.pdf

figure 4.5 seems to say that I could reasonably expect to see a
signal of at least 100uV/m. Does this mean that I should expect to
see 100uV from any one-meter hunk of wire strung out horizontally in
the optimum direction? Or is there something more subtle going on I
need to be aware of?


Frank McKenney
--
One of the ways to give the impression of an aesthetic performance
to those lacking the organ of taste is indeed to put into a work
of art the political, religious, or other extraneous satisfactions
popular with one or another audience. Particularly, of course, if
strongly held. -- Robert Conquest, "The Dragons of Expectation"
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney
wrote:

First, the need for impedance matching between an antenna and a
receiver. My understanding is that a resonant halfwave dipole will
have an impedance around 73 Ohms; unfortunately, unless I can obtain
research funding from the just-passed Congressional Economic
Stimulus bill I'm going to have trouble paying for 2.5km of copper
wire, some towers, a crateload or two of porcelain insulators,and
the land to build it on. (Hey, I promise to dump it back into the
economy ASAP. Really! grin!)

So any non-loop antenna I can construct will necessarily be a "short
wire" or "electrically small" antenna (two useful search terms).
But how does one go about calculating the impedance of a coat hanger
or an extension cord ("short piece of wire")?

Hi Frank,

One doesn't try.

The simple solution is the conventional one - you use a tuner.

The tuner provides the matching (providing it has sufficient
inductance and capacitance - you will have to investigate designs) and
adjacent signal rejection (which could seriously de-sense your
received signals).

Your antenna doesn't need to be very big, but it might help to have it
very remote, if there are noise sources nearby (like motors, aquarium
heaters, bottle style TVs, ...); and the line sufficiently choked. A
good ground too, tying into the service ground through a separate wire
to reduce coupling of noise from shared grounds. This last may
introduce a ground loop if your Mohican is so vintage as to have had
relaxed design standards. A little research online reveals it is
battery operable. You may want to fully exercise that option.

73's
Richard Clark, KB7QHC

Richard,

Thank you for posting back.

On Mon, 16 Feb 2009 23:07:37 -0800, Richard Clark wrote:
On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney
wrote:

First, the need for impedance matching between an antenna and a
receiver.
--snip complaint about the size/cost of 60kHz h-w diploes grin!--

So any non-loop antenna I can construct will necessarily be a "short
wire" or "electrically small" antenna (two useful search terms).
But how does one go about calculating the impedance of a coat hanger
or an extension cord ("short piece of wire")?

One doesn't try.

So Yoda was right: "Do, or do not. There is no 'try'." grin!

What has puzzled me is that I have run across designs that use
(e.g.) a JFET isolation amplifier hooked to a whip or hunk-o-wire
with the statement (or implication) that this is done to ",atch the
antenna's impedance". So I;ve been trying to figure out how to
calculate/estimate what it would be, without much success.

The simple solution is the conventional one - you use a tuner.

The tuner provides the matching (providing it has sufficient
inductance and capacitance - you will have to investigate designs) and
adjacent signal rejection (which could seriously de-sense your
received signals).

I imagine that the inductance of a 6' extension cord (not plugged
in, just dangling from a planter hook grin!) is down in the
uH-or-less range, which would mean that most of the "tuning"
inductance would have to be supplied to achieve 60kHz. I have this
image of a big (tens of mH) inductor in series with a moderate
capacitor and my (electrically) short wire; all of the surrounding
EM sets the electroncs in the wire to dancing, but the series RC
blocks those which are wiggling "off-key" (e.g. not dancing at the
"proper" rate of 60kHz).

Your antenna doesn't need to be very big, but it might help to have it
very remote, if there are noise sources nearby (like motors, aquarium
heaters, bottle style TVs, ...); and the line sufficiently choked. A
good ground too, tying into the service ground through a separate wire
to reduce coupling of noise from shared grounds.

Well, there's no question that I have EM in the area. I hooked my
DVM -- set to ACV -- between the radiator and my 6' extension cord;
would you believe 8-10V??!! Not much current, though: feed it
through a 1k resistor and measure the voltage across it, suddenly
it's down in the mV range. grin!

... This last may
introduce a ground loop if your Mohican is so vintage as to have had
relaxed design standards. A little research online reveals it is
battery operable. You may want to fully exercise that option.

The Mohican came with two 12V power "modules" which plug into the
back of the unit. The AC power module has a transformer with a
12V-12V center-tapped secondary, which is good, but then they run
the line voltage out of the module and down into the receiver's
volume control's on/off switch. The module's 12V power and 120V
switching connections are done through a 9-pin tube socket with
mating connector/cable; remember those? grin!

73's

Thanks for the hepl.


Frank
--
There is one thing even more vital to science than intelligent
methods; and that is, the sincere desire to find out the truth,
whatever that may be. -- Charles Sanders Pierce
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

On Wed, 18 Feb 2009 12:06:09 -0600, Frnak McKenney
wrote:

What has puzzled me is that I have run across designs that use
(e.g.) a JFET isolation amplifier hooked to a whip or hunk-o-wire
with the statement (or implication) that this is done to ",atch the
antenna's impedance".

Hi Frank,

Matching provokes heated debates that in times past ran to 600+
postings - few knew what they were arguing (but enjoyed arguing
nonetheless) and little was offered.

A JFET at these frequencies does satisfy the naive requirements of
"matching," but that giving you a reception solution doesn't always
follow.

So I;ve been trying to figure out how to
calculate/estimate what it would be, without much success.

The Mohican schematic says quite planely Hi-Z input. This is borne
out by the antenna connection feeding a tank circuit in the front end
where the input stage is fed from a low tap into a 470 Ohm resistor.
This would be your JFET feed Z, but you could choose any suitable
close value. As for the input Z, the JFET input resistance is
perfectly capable of mismatching horribly high - although this is not
about optimal power transfer at these impedance levels. What is at
risk, is the JFET input capacitance which could present a low Z at
some frequency. Naturally, you select your JFET against this to
optimize. It will be in some ratio to the antenna capacitance (if it
is bare, short wire) and that will establish the proportion of signal
that gets in by divider action.

I imagine that the inductance of a 6' extension cord (not plugged
in, just dangling from a planter hook grin!) is down in the
uH-or-less range, which would mean that most of the "tuning"
inductance would have to be supplied to achieve 60kHz. I have this
image of a big (tens of mH) inductor in series with a moderate
capacitor and my (electrically) short wire;

That is one way, other ways work too and are electrically equivalent.
Loops help tune and match by a slightly more elaborate means, but
still fairly holds to simple requirements.

You don't need wire to build an inductor. At these frequencies you
can use a capacitor in a Gyrator design.

all of the surrounding
EM sets the electroncs in the wire to dancing, but the series RC
blocks those which are wiggling "off-key" (e.g. not dancing at the
"proper" rate of 60kHz).

And this responds to the filtering capacity (selection AND rejection).
This is called "Q" which also serves the yeoman's task of matching as
well (observe the input tank design for the conventional bands).

Well, there's no question that I have EM in the area. I hooked my
DVM -- set to ACV -- between the radiator and my 6' extension cord;
would you believe 8-10V??!! Not much current, though: feed it
through a 1k resistor and measure the voltage across it, suddenly
it's down in the mV range. grin!

Still a lot of power. However, those are probably 60Hz fields because
DVMs rarely have the AC BW to go much above 1Khz.

The Mohican came with two 12V power "modules" which plug into the
back of the unit. The AC power module has a transformer with a
12V-12V center-tapped secondary, which is good, but then they run
the line voltage out of the module and down into the receiver's
volume control's on/off switch. The module's 12V power and 120V
switching connections are done through a 9-pin tube socket with
mating connector/cable; remember those? grin!

Remembering isn't difficult. I broke into electronics through
TV/Radio repair during high school. If I could fix it, I got paid.

Anyway, power connections from that era brought "ground" notoriously
close to lethal if you plugged the radio into the wall wrong. Some
used AC noise reduction circuit design that could almost guarantee
your chassis was floating at 70V if things went wrong.

I had an ET striker (Navy parlance for a student electronics tech) who
connected a TV antenna input to ground, and the insulation melted off
of the wire in a heartbeat. This was in the day when we called TV
power line interlock replacements "suicide adapters."

73's
Richard Clark, KB7QHC

On Feb 18, 3:12*pm, Richard Clark wrote:
On Wed, 18 Feb 2009 12:06:09 -0600, Frnak McKenney

wrote:
What has puzzled me is that I have run across designs that use
(e.g.) a JFET isolation amplifier hooked to a whip or hunk-o-wire
with the statement (or implication) that this is done to ",atch the
antenna's impedance".

Hi Frank,

Matching provokes heated debates that in times past ran to 600+
postings - few knew what they were arguing (but enjoyed arguing
nonetheless) and little was offered.

A JFET at these frequencies does satisfy the naive requirements of
"matching," but that giving you a reception solution doesn't always
follow.

So I;ve been trying to figure out how to
calculate/estimate what it would be, without much success.

The Mohican schematic says quite planely Hi-Z input. *This is borne
out by the antenna connection feeding a tank circuit in the front end
where the input stage is fed from a low tap into a 470 Ohm resistor.
This would be your JFET feed Z, but you could choose any suitable
close value. *As for the input Z, the JFET input resistance is
perfectly capable of mismatching horribly high - although this is not
about optimal power transfer at these impedance levels. *What is at
risk, is the JFET input capacitance which could present a low Z at
some frequency. *Naturally, you select your JFET against this to
optimize. *It will be in some ratio to the antenna capacitance (if it
is bare, short wire) and that will establish the proportion of signal
that gets in by divider action.

I imagine that the inductance of a 6' extension cord (not plugged
in, just dangling from a planter hook grin!) is down in the
uH-or-less range, which would mean that most of the "tuning"
inductance would have to be supplied to achieve 60kHz. *I have this
image of a big (tens of mH) inductor in series with a moderate
capacitor and my (electrically) short wire;

That is one way, other ways work too and are electrically equivalent.
Loops help tune and match by a slightly more elaborate means, but
still fairly holds to simple requirements.

You don't need wire to build an inductor. *At these frequencies you
can use a capacitor in a Gyrator design.

all of the surrounding
EM sets the electroncs in the wire to dancing, but the series RC
blocks those which are wiggling "off-key" (e.g. *not dancing at the
"proper" rate of 60kHz).

And this responds to the filtering capacity (selection AND rejection).
This is called "Q" which also serves the yeoman's task of matching as
well (observe the input tank design for the conventional bands).

Well, there's no question that I have EM in the area. I hooked my
DVM -- set to ACV -- between the radiator and my 6' extension cord;
would you believe 8-10V??!! Not much current, though: feed it
through a 1k resistor and measure the voltage across it, suddenly
it's down in the mV range. grin!

Still a lot of power. *However, those are probably 60Hz fields because
DVMs rarely have the AC BW to go much above 1Khz.

The Mohican came with two 12V power "modules" which plug into the
back of the unit. *The AC power module has a transformer with a
12V-12V center-tapped secondary, which is good, but then they run
the line voltage out of the module and down into the receiver's
volume control's on/off switch. *The module's 12V power and 120V
switching connections are done through a 9-pin tube socket with
mating connector/cable; remember those? *grin!

Remembering isn't difficult. *I broke into electronics through
TV/Radio repair during high school. *If I could fix it, I got paid.

Anyway, power connections from that era brought "ground" notoriously
close to lethal if you plugged the radio into the wall wrong. *Some
used AC noise reduction circuit design that could almost guarantee
your chassis was floating at 70V if things went wrong.

I had an ET striker (Navy parlance for a student electronics tech) who
connected a TV antenna input to ground, and the insulation melted off
of the wire in a heartbeat. *This was in the day when we called TV
power line interlock replacements "suicide adapters."

73's
Richard Clark, KB7QHC

Its not uncommon to have a high impedance input into a preamp. This is
the one-size- fits-all
approach. While its not good engineering for the purist it works quite
well to make a casual user happy and may be the practical solution for
even the professional installation..
Ive had some experience limited working with VLF and it always seemed
the thing that made the difference between a good and bad VLF
antenna was the quality of the ground network

Jimmie

Jimmie,

Thank you for your comments.

On Wed, 18 Feb 2009 13:19:12 -0800 (PST), JIMMIE wrote:
On Feb 18, 3:12*pm, Richard Clark wrote:
On Wed, 18 Feb 2009 12:06:09 -0600, Frnak McKenney

wrote:
What has puzzled me is that I have run across designs that use
(e.g.) a JFET isolation amplifier hooked to a whip or hunk-o-wire
with the statement (or implication) that this is done to ",atch the
antenna's impedance".

Hi Frank,

Matching provokes heated debates that in times past ran to 600+
postings - few knew what they were arguing (but enjoyed arguing
nonetheless) and little was offered.

A JFET at these frequencies does satisfy the naive requirements of
"matching," but that giving you a reception solution doesn't always
follow.
--snip--
Its not uncommon to have a high impedance input into a preamp.
This is the one-size- fits-all approach. While its not good
engineering for the purist it works quite well to make a casual user
happy and may be the practical solution for even the professional
installation..

Well, I think of myself as a "casual user", and _I'd_ like to be
happy. grin!

I don't mind throwing in a high-impedance (JFET) front end to my
antenna simply on the basis that (a) people who seem to know what
they're talking about recommend it and (b) I associate "high
impedance" with "sensitive" (which seems like a desirable quality
when you're working with microvolts). Someday, though, I'd like to
have build up a framework in which _I_ can see why it's appropriate,
or at least "does no harm". grin!

My brother Bruce is working on the same problem from a slightly
different angle; his experience is in software and digital stuff,
and I find myself unintentionally assuming the role of "RF expert"
without an EE degree or years of circuit design to back it up. Left
to myself, I'm perfectly capable of pushing stuff around on the
breadboard until it seems to work, but when I'm offering advice to
someone else I'd prefer a better response to his questions than
"someone else said so". grin!

Ive had some experience limited working with VLF and it always
seemed the thing that made the difference between a good and bad VLF
antenna was the quality of the ground network

Thanks for the suggestion. Do you think that my current "ground", a
30x60' 4-way pipe-loop network (mixed copper and cast iron) with
thermal radiation elements might be... um, "less than
satisfactory"? grin!


Frank
--
"What one writer can make in the solitude of one room is something
no power can easily destroy." -- Salman Rushdie
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

On Wed, 18 Feb 2009 12:12:53 -0800, Richard Clark wrote:
On Wed, 18 Feb 2009 12:06:09 -0600, Frnak McKenney
wrote:

What has puzzled me is that I have run across designs that use
(e.g.) a JFET isolation amplifier hooked to a whip or hunk-o-wire
with the statement (or implication) that this is done to ",atch the
antenna's impedance".

Matching provokes heated debates that in times past ran to 600+
postings - few knew what they were arguing (but enjoyed arguing
nonetheless) and little was offered.

What I think I'm looking for would be a point of reference that
would let me, if not exactly evaluate the facets of such an
argument, at least be a foundation for forming a testable opinion of
my own.

A JFET at these frequencies does satisfy the naive requirements of
"matching," but that giving you a reception solution doesn't always
follow.

In my current state of ignorance of the subject, this sentence has
the appearance of a Zen koan: something that sounds non-sensical at
first glance, but which, after sufficient time and effort studying,
will undoubtedly become so blindingly obvious as to appear trivial.
Thank you... I think. grin!

So I;ve been trying to figure out how to
calculate/estimate what it would be, without much success.

The Mohican schematic says quite planely Hi-Z input. This is
borne out by the antenna connection feeding a tank circuit in the
front end where the input stage is fed from a low tap into a 470 Ohm
resistor. ...

Ah! You have a Mohican? Or just access to the manual? Mine is missing,
burioed somewhere in my basement; I was fortunate enough to locate a
copy of a GC-1A PDF some kind soul posted online.

... This would be your JFET feed Z, but you could choose any
suitable close value. As for the input Z, the JFET input resistance
is perfectly capable of mismatching horribly high - although this is
not about optimal power transfer at these impedance levels. What is
at risk, is the JFET input capacitance which could present a low Z
at some frequency. Naturally, you select your JFET against this to
optimize. It will be in some ratio to the antenna capacitance (if
it is bare, short wire) and that will establish the proportion of
signal that gets in by divider action.

I imagine that the inductance of a 6' extension cord (not plugged
in, just dangling from a planter hook grin!) is down in the
uH-or-less range, which would mean that most of the "tuning"
inductance would have to be supplied to achieve 60kHz. I have this
image of a big (tens of mH) inductor in series with a moderate
capacitor and my (electrically) short wire;

That is one way, other ways work too and are electrically equivalent.
Loops help tune and match by a slightly more elaborate means, but
still fairly holds to simple requirements.

You don't need wire to build an inductor. At these frequencies you
can use a capacitor in a Gyrator design.

"Gyrator"? I thought that was the rooftop dance that follows an
antenna adjustment in mid-thunderstorm. grin!

Google led me to the AAVSO site (www.aavso.org) which led me to the
Yahoo VLF_Group. Aaaaaaaaaaaaaaauuuuuuuggggh! A circuit that can
replace capacitors or inductors? My first reaction is "technology
at a level indistunguisable from black magic".

I don't think I'm in Kansas any more.

(On the other hand, I have lots more toys to play with. grin!)

all of the surrounding
EM sets the electroncs in the wire to dancing, but the series RC
blocks those which are wiggling "off-key" (e.g. not dancing at the
"proper" rate of 60kHz).

And this responds to the filtering capacity (selection AND
rejection). This is called "Q" which also serves the yeoman's
task of matching as well

Um. I don't think I ever got past the simplistic "High Q = Good,
Low Q = Evil" stage. Looking back, I can now see cases where an
excessively high Q might be... counterproductive, but as always, it
depends on what one means by "high" or "low" in a given context.

Noted as something else I need to review and not depend on instinct
for.

... (observe the input tank design for the
conventional bands).

Of the five bands (A-E, SW3 positions 5-1), the only one which seems
different is "E", with an additional 130pF cap between the antenna
and the tank circuit.

Is that what you're referring to?

--snip--
... The module's 12V power and 120V
switching connections are done through a 9-pin tube socket with
mating connector/cable; remember those? grin!

Remembering isn't difficult. I broke into electronics through
TV/Radio repair during high school. If I could fix it, I got paid.

_You_ got _paid_!?? grin! Woody Maiden, WA4GMV, ran a radio/TV
shop near my home and didn't object to someone hanging around and
watching over his shoulder in the afternoons, but he was a bit
concerned about his liability insurance. (Don't know why. I didn't
get zapped by a "discharged" CRT until my college years. grin!)

Anyway, power connections from that era brought "ground" notoriously
close to lethal if you plugged the radio into the wall wrong. Some
used AC noise reduction circuit design that could almost guarantee
your chassis was floating at 70V if things went wrong.

"We don' need no steeken' transformers!" I think I saw a 50C5 in one
of my basement boxes a few months back. grin!

I had an ET striker (Navy parlance for a student electronics tech)
who connected a TV antenna input to ground, and the insulation
melted off of the wire in a heartbeat. This was in the day when we
called TV power line interlock replacements "suicide adapters."

I learned about them from my father, a power company EE who wasn't
above swapping the odd tube or building a color TV from a kit with
my little sister's help.

73's
Richard Clark, KB7QHC

* * *

If you're getting bored, please feel free to skip the following; on
the other hand, someone with access to a GC-1 manual might enjoy my
story...

A few months back I pulled my old Mohican out from under a pile of
magazines and blew the dust off it. It had been buried so long that
I had forgotten why I never built an outdoor antenna for it or
showed it to my nephews an nieces; I remembered the moment I powered
it on: there was a loud buzz coming from the speaker.

This wasn't _hum_ mind you, which even when loud has a sort of
even-ness to it; this was an intense, jagged, and metallic "I'm
shredding your speaker cone and I don't care!" kind of
low-frequency buzz. Worse, it went _away_ when I turned the volume
_up_, and reappeared when I turned it (the volume control) down.

I was able to isolate it to the audio section by unsoldering the
capacitor connecting the volume control wiper to the base of the
first audio stage. It went away (blessed relief!), and when I hooked
up a RadioShack "utility" amplifier to the volume control wiper the
sound was clear and crisp. I considered this A Clue. grin!

NTE listed their NTE102A as a replacement for the Mohican's
2N407s... at nearly $7 each (apparently Germanium is on the
Endangered Elements List). I replaced all three AF transistors, and
the audio level was much better than it has been. Unfortunately,
the BUZZZZZ was still present, and also louder. I did notice that
turning the ANL ON (diode and resistor connected between the base of
X7 and ground) eliminated it.

I tried all sorts of things. I jumpered the volume control wiper to
ground to make sure the resistance arc hadn't cracked, leaving the
wiper floating near its low end. I tried swapping the transistors
around. I tried paralleling the electrolytics I could reach, but no
luck.

As you've seen (or know personally) the Mohican's IF and AF stages
are all on one printed circuit board which is bolted down to the
chaissis with ?2-40 machine screws around its border. I couldn't
see much of the underside of the AF section of this board because my
vision -- and fingers -- were blocked by a large AF transformer, so
I unbolted it and let it hang free (but not shorting anything; thank
&deity for stiff transformer leads).

With that pushed out of the way I could see all three AF transistor
sockets and even reach them with my test leads. I jumpered a 100uF
electrolytic from the chassis to the emitter pin of X7, the AF stage
1 transistor, parallelling it with the existing 150uF emitter bypass
capacitor (C55(?)), and a miracle occurred: the buzz disappeared!
I can't tell you how good it felt to hear normal-sounding hiss,
static, and the odd "sqrgrl" coming out of the Mohican's speaker.

No 150uF in the parts bin, so I wired two caps in parallel for a
147uF, shrink-wrapped them, unsoldered the decades-old 150uF and
wired the glob-cap in its place. Didn't even burn my fingers in the
confined spacegrin!. And then I turned it on.

BZZZZZZZZZZZZZZ!!

I'm sure you recall the feeling: "What did I do wrong?" Bad solder
joint? No... Overheated the component? Not as far as I could
tell. Dropped a solder glob across two traces frying by $7
transistors? No... in fact, although the buzz existed over a
larger percentage of the volume control's span, the louder end of
the audio worked just as it had before. My "proven cure" had made
the problem slightly worse.

It was late, I was tired, so I decided that, since my previous
jumpering had created 250uF total, I needed to hit the problem with
a bigger hammer: I jumpered in a _470uF_ this time. And the buzz
went away. Again.

Okay. Now I was _sure_ I had fixed the problem, so I unsoldered my
obviously-too-clever glob-cap and soldered in the 470uF, this time
triple-checking the polarity markings. I inspected the traces for
possible solder bridges. I inspected the new solder joints under a
large magnifier and wiggled the new leads; the joints looked good,
so I hooked up the power supply again and turned it on.

BZZZZZZZZZZZZZZ!!

By now I was tired _and_ cranky, so I jumpered in the original
decades-old 150uF cap. No buzz.

At that point I had two options: go down in the basement and bring
up a sledge hammer, or quit to go eat supper. It was a close call,
but supper won out. There's something very weird about watching the
news, eating supper, and looking over a schematic and trying to work
what I was doing differently when it worked.

If the emitter of X7 is wired to R37 and C55, and if the other ends
of R37/C55 are wired to ground, how on Earth could a new capacitor
jumpered from X7's emitter to the chassis cause different behavior
from the _same_ capacitor soldered to a trace going to the same
emitter and the PC board's ground trace?

All I could think was "this makes no sense, but it does appear to be
happening". R37/C55 were clearly grounded. The trace was unbroken.
If they weren't grounded, I'd be getting _no_ audio. So how could
_soldering_ to ground be any different from _jumpering_ to ground?
Even stranger, how could a jumpered connection be _better_ than a
soldered connection?

* * *

You've probably figured it out by now. The answer is that when I
was jumpering, it was to the metal chassis; when I was soldering, it
was to the PC board ground trace. The "ground" trace on this PC
board includes a wide band around its outer edge; the strip makes
contact with the chassis when the board is bolted down, creating a
really solid round-the-board ground. Somehow, over the past
half-century, the contact between the chassis and trace had become
slightly less than perfectly conductive, and was "floating" in the
Never-Never Land between zero and infinite resistance.

Loosen all ten machine screws slightly, spray some contact cleaner
in the extremely narrow gap, tighten the screws, and guess what? NO
BUZZ!

Re-mount the transformer(*). Reconnect C54 to the volume control
wiper. Check for accidental shorts, dropped bits of solder and
component leads, and make sure the transistors haven't fallen out of
their sockets while I had the chassis upside down, Turn it on.
STILL NO BUZZ!

By then it was 2230, so I went to bed. Next step will be to improve
the PC board/chassis contact by running a thin bit of fine steel
wool around a bit to get rid of any residual crud; with luck, it'll
be another couple of decades before this problem reappears.

As for the 470uF capacitor, I think I'll leave it in place. The
original is likely good, but 470uF should work as well, and all I
need now is to have that portion of the trace lift off from the PC
board due to overheating.

* * *

What's interesting here is the contrast between the schematic and
reality. On paper, or on my CRT, those components were clearly
connected to ground, and, had I hand-drawn my jumper wires, their
wiring lines would have looked identical. It took me a long time to
find the cause of the buzz because I was stuck thinking about the
lines on the schematic, even though my hands were working on
physical components, wires, and traces; how do you know when to stop
trusting your compass and check it against the sun and stars?

Definitely a "Learning Experience". grin!


Anyway, thanks again for your comments. My head is about to explode
from all the new ideas, but please don't take that amiss. If I
don't expand it occasionally, it starts collapsing into a dull,
super-dense mass (think neutron star grin!).


(*) GlueStic(tm) is great for assembling hardware in confined
quarters. It's much better than spit for making lockwashers and
nuts stick to your fingertips so they don't drop off into the
chassis interior.


Frank
--
"If language is not correct, then what is said is not what is
meant; if what is said is not what is meant, then what ought to
be done remains undone." -- Confucius
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

On Sat, 21 Feb 2009 19:37:34 -0600, Frnak McKenney
wrote:

Matching provokes heated debates that in times past ran to 600+
postings - few knew what they were arguing (but enjoyed arguing
nonetheless) and little was offered.

What I think I'm looking for would be a point of reference that
would let me, if not exactly evaluate the facets of such an
argument, at least be a foundation for forming a testable opinion of
my own.

Opinion is, after all, what powers the Internet.

A JFET at these frequencies does satisfy the naive requirements of
"matching," but that giving you a reception solution doesn't always
follow.

In my current state of ignorance of the subject, this sentence has
the appearance of a Zen koan: something that sounds non-sensical at
first glance, but which, after sufficient time and effort studying,
will undoubtedly become so blindingly obvious as to appear trivial.
Thank you... I think. grin!

I could reduce it to the classic "take two aspirin and call me in the
morning."

Ah! You have a Mohican? Or just access to the manual? Mine is missing,
burioed somewhere in my basement; I was fortunate enough to locate a
copy of a GC-1A PDF some kind soul posted online.

It didn't take much effort to scour the web for one (schematic).

You don't need wire to build an inductor. At these frequencies you
can use a capacitor in a Gyrator design.

"Gyrator"? I thought that was the rooftop dance that follows an
antenna adjustment in mid-thunderstorm. grin!

Google led me to the AAVSO site (www.aavso.org) which led me to the
Yahoo VLF_Group. Aaaaaaaaaaaaaaauuuuuuuggggh! A circuit that can
replace capacitors or inductors? My first reaction is "technology
at a level indistunguisable from black magic".

I don't think I'm in Kansas any more.

Gyrators have been around for a very long time, and can be found in a
billion telephones, one probably within reach of you at the moment.
They use telephones in Kansas don't they, Toto? You can build one
with four components (none of them an inductor) to make an inductor
more precisely than you could winding one.

If you truly want to be overwhelmed with the dark arts, try googling
for "magnetic amplifiers." (Art would go ballistic knowing such a
topic was in practice looooong before he left second form.) No tubes,
no transistors, and the orginal "solid state" design. As this may
sound as if it wanders from the subject of RF, add the name Ernst F.
W. Alexanderson to any search.

(On the other hand, I have lots more toys to play with. grin!)

all of the surrounding
EM sets the electroncs in the wire to dancing, but the series RC
blocks those which are wiggling "off-key" (e.g. not dancing at the
"proper" rate of 60kHz).

And this responds to the filtering capacity (selection AND
rejection). This is called "Q" which also serves the yeoman's
task of matching as well

Um. I don't think I ever got past the simplistic "High Q = Good,
Low Q = Evil" stage. Looking back, I can now see cases where an
excessively high Q might be... counterproductive, but as always, it
depends on what one means by "high" or "low" in a given context.

Noted as something else I need to review and not depend on instinct
for.

... (observe the input tank design for the
conventional bands).

Of the five bands (A-E, SW3 positions 5-1), the only one which seems
different is "E", with an additional 130pF cap between the antenna
and the tank circuit.

Is that what you're referring to?

I am merely pointing out the obvious application of a tapped inductor
of the tuned front end serving as impedance match to an high-Z antenna
(the topic of your choice). The schematic abounds in examples. One
need only substitute values to serve the right frequency band - a
simple exercise in reverse engineering employed since Hertz drew a
spark across a gap at the base of a loop.

If you're getting bored, please feel free to skip the following; on
the other hand, someone with access to a GC-1 manual might enjoy my
story...

Yes, your story was/is classic with a beginning, middle, and end. Very
few chroniclers here manage to write with as much clarity. (We get
mostly cheesy attempts with "cliff hangers" serving as examples of
neo-scholarly writing.) Your learning lesson of maintaining the
chassis ground with the trace is classic too. The discovery of
corrosion brings up the common practice of taking ALL the tubes out
and putting them back in to solve problems. Tightening ALL screws is
another hard learned lesson that bench techs either get or don't get.

You probably could have got away with cheaper transistors by also
substituting the bias diodes (56-7s) - but as events bore out, the
transistors were good. If you note the difference between the base
and emitter voltages, there is only about a tenth volt there. If I am
to presume the diode call-out is for an 1N56, it is germanium too.

What is more amazing is this wasn't about the decrepitude of the
electrolytic capacitors which usually suffer with time if they are not
used for a long while.

73's
Richard Clark, KB7QHC

Frnak McKenney wrote:

In my current state of ignorance of the subject, this sentence has
the appearance of a Zen koan: something that sounds non-sensical at
first glance, but which, after sufficient time and effort studying,
will undoubtedly become so blindingly obvious as to appear trivial.
Thank you... I think. grin!


As witnessed by the great Zen master, Yogi Berra......


Sorry, catching up on the groups and couldn't resist.

- 73 de Mike N3LI -

"Frnak McKenney"


http://ts.nist.gov/MeasurementServic...d/SP250-67.pdf


** Just how big is this file - eh ??

Why did you limit replies to ONE newsgroup while posting to TWO ???

What sort to total ****ING ASSHOLE are you ????


You ****ing ASININE YANK ****- head.




...... Phil