Hi, Tim. grin!

On Wed, 18 Feb 2009 02:03:44 -0600, Tim Wescott wrote:
On Mon, 16 Feb 2009 22:22:20 -0600, Frnak McKenney wrote:

Back in December I posted a question about ways to receive LF/VLF radio
signals.
--snip--
First, the need for impedance matching between an antenna and a
receiver.
--snip--
This may be a duplicate answer: I _know_ I wrote one, but it seems
to have fallen into the bit-bucket.

Received and replied to, but your rephrasing is also appreciated.

In short:

For receiving you don't need to couple well enough to the ether to
overwhelm the receiver's noise with the Faintest Possible Signal. You
only need to overwhelm the receiver's noise with atmospheric noise.
Given the amount of atmospheric noise at 60kHz, that ain't hard.

When you get to the point where you hook up the antenna to the rig and
you heard static over the noise of the receiver, you know your antenna is
good enough.

Well, I'm getting static with the built-in whip. On the other hand,
I haven't hooked up the downconverter yet.

(Transmitting is a different story, but try transmitting at 60kHz and
after the FCC gets done with you antenna size will be the least of your
worries.)

Yeah, but the picture of all the local "Atomic Clocks" changing at
once _is_ rather appealing. grin!

Whazza matta widda loop? They work fine, they provide some welcome
selectivity (well, at 60kHz one may provide _too much_ selectivity),
they're easy to construct, they're reputed to reject sky waves -- what
more could you want?

Laziness? A Scot's instinct to thrift? grin!

If you don't want to use a loop, the last time I did anything at MF a
short (1m) whip going to a JFET source follower was considered the bee's
knees to solve this sort of problem. The whip will pick up atmospheric
noise just as well as it'll pick up the intended signal, the JFET will
impedance match from that low-capacity whip to your receiver input
(I assume, I don't know what the nominal input impedance of your
rig is), and all will be well.

Well, from my point of view you just justified the effort you put
into this second post; you got me digging into the Mohinca's manual
which lists sensitivity (10uV) and Selectivity ("3 kc wide at 6 db
down"[sic]), but no specific antenna impedance. According to the
_schematic_, the two screw-lug connections on the rear of the
chassis are "HI-Z" and "GND" (apparently the Heathkit designers
thought of a "short wire" antenna as high impedance as well). The
HI-Z line runs through a "12uuF"[sic] to the whip, and then both are
connected through a 22pF capacitor to the Main Tuning and Antenna
Trim capacitors.

Fortunately the LF upconverter comes (IIRC) with 1MHz and 4MHz
crystals, so I won't be trying to force a 60kHz signal past those
itty-bitty little capacitors. grin!


Frank
--
Ninety-Ninety Rule of Project Scheduling:
The first ninety percent of the task takes ninety percent of the
time, and the last ten percent takes the other ninety percent.
--
Frank McKenney, McKenney Associates
Richmond, Virginia / (804) 320-4887
Munged E-mail: frank uscore mckenney ayut mined spring dawt cahm (y'all)

Joe,

Thanks for replying. I wan't entirely certain which of the four I
should respond to, but I'm assuming this was the vinfal version.

On Wed, 18 Feb 2009 06:11:29 -0800 (PST), J.A. Legris wrote:
On Feb 16, 11:22*pm, Frnak McKenney
wrote:
Back in December I posted a question about ways to receive LF/VLF
radio signals.
--snip--
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")?
--snip--
My other question has to do with how to interpret signal strength.
--snip--
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?

A field strength measured in 100 uV/meter is just that, but the
problem getting the energy out of the air and into a receiver.

Yes. It's not like I can just hang a bucket out the window and
bring it back full of electrons wiggling at just the right speed.
grin!

A short linear antenna has a very low radiation resistance ( 1 ohm)
which is a poor match to a practical transmission line, whose
characteristic impedance is typically 1000's of times larger. The
radiation resistance of an antenna is the component of its complex
impedance that is associated with the power captured. Balanis (Antenna
Theory Analysis & Design (1997), p.137) gives a formula for the
radiation resistance of a short dipole:

R = 80 * pi^2 * (W/L)^2 ohms

where W is the length of the antenna and L is the wavelength. The
value for a monopole is roughly half as much again.

Um... 1.5 * 80 * (%pi^2) * (1/5000)^2 is... 471 micro-Ohms? That 's
pretty low; why would anyone match that to a JFET input?

Why do you request a non-loop antenna?

I started there, ran into some questions, and wanted to clear up the
confusion in my own head before moving any further. It's not as
though I'm prejudiced against them; heck, some of my best friends
have radios with loop antennae. grin!

... A small circular loop antenna
also has a low radiation resistance but it can be increased by adding
turns. Balanis (p.209) gives a formula for the radiation resistance of
a small loop:

R = 20 * pi^2 * (C/L)^4 * N^2 ohms

where C is the circumference of the loop, L is the wavelength and N is
the number of turns.

Better still is to use a ferrite loop antenna. You may be able to get
one out of an old AM radio and adapt it to your receiver. The
resulting formula is identical to the above, multiplied by the
relative permeability of the core, u (SQUARED !), so you can use a
very small-diameter loop and/or fewer turns, getting improved
selectivity and sensitivity (i.e. high Q) in a tuned circuit:

R = 20 * pi^2 * (C/L)^4 * N^2 * u^2 ohms

Oddly enough, I now have ten old transistor radios that I picked up
at FrostFest a few weekends back for $1 each. I was looking for
ferrite and wide-ratio tuning capacitors, as they seem to be in
scarce supply these days. I don't know where today's kids are
getting their crystal radio parts from these days; it certainly
isn't Radio Shack.


Frank
--
"A man should never be ashamed to own that he has been in the
wrong, which is but saying, in other words, that he is wiser
today than yesterday." -- Jonathan Swift
--
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

In article ,
Frnak McKenney wrote:

Big Snip.

Go find Radio-Electronics magazine for 1983, and read the five(?)
articles by Ralph Burhans about receiving VLF.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

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

On Wed, 18 Feb 2009 20:57:00 GMT, (Mark Zenier)
wrote:

In article ,
Frnak McKenney wrote:

Big Snip.

Go find Radio-Electronics magazine for 1983, and read the five(?)
articles by Ralph Burhans about receiving VLF.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

Is that set of articles available on the web? That'd be an
interesting read.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food

In article ,
Jim Thompson wrote:
On Wed, 18 Feb 2009 20:57:00 GMT, (Mark Zenier)
wrote:

In article ,
Frnak McKenney wrote:

Big Snip.

Go find Radio-Electronics magazine for 1983, and read the five(?)
articles by Ralph Burhans about receiving VLF.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

Is that set of articles available on the web? That'd be an
interesting read.


Some of them. I remember on of them coming up in a discussion
in one of the radio newsgroups, and someone posted a URL.

They are practical hobby stuff mostly. A VLF converter, how to match
longwire antenna, an active whip antenna, a balanced loopstick. The most
useful non-hobby one was how to derive time/frequency references from
Loran-C.

I'm reluctant to go to the effort because this was back in the era when
R-E was paranoid about photocopying, with colored backgrounds for the
figures and charts, and it would take a day or two to photoshop the
scans so that they are of decent quality. Also, they were, near the end,
one of the more Internet savvy publishers and there may still be somebody
around to enforce their IP.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

"Jim Thompson" wrote in
message ...

Is that set of articles available on the web? That'd be an
interesting read.

...Jim Thompson

The 5 parts were later combined into a separate reprint, sold by R-E.
Sections for theory and circuits, construction, testing. Then building a
passive tuner VLF-LF, and a final section on loop antennas. One of the five
sections is a Loran-C receiver!!
I have an original copy in perfect condition, 30 pages, glossy paper,
copyright 1984 by Gernsback Publications, Inc.

On Thu, 19 Feb 2009 20:12:06 GMT, (Mark Zenier)
wrote:

In article ,
Jim Thompson wrote:
On Wed, 18 Feb 2009 20:57:00 GMT, (Mark Zenier)
wrote:

In article ,
Frnak McKenney wrote:

Big Snip.

Go find Radio-Electronics magazine for 1983, and read the five(?)
articles by Ralph Burhans about receiving VLF.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)

Is that set of articles available on the web? That'd be an
interesting read.


Some of them. I remember on of them coming up in a discussion
in one of the radio newsgroups, and someone posted a URL.

They are practical hobby stuff mostly. A VLF converter, how to match
longwire antenna, an active whip antenna, a balanced loopstick. The most
useful non-hobby one was how to derive time/frequency references from
Loran-C.

I'm reluctant to go to the effort because this was back in the era when
R-E was paranoid about photocopying, with colored backgrounds for the
figures and charts, and it would take a day or two to photoshop the
scans so that they are of decent quality. Also, they were, near the end,
one of the more Internet savvy publishers and there may still be somebody
around to enforce their IP.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)



Send them to me. I can work magic with a scanner ;-)

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

I love to cook with wine Sometimes I even put it in the food

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)