Meat Plow wrote:
On Thu, 05 Aug 2010 20:14:34 -0400, Michael A. Terrell wrote:

Meat Plow wrote:

On Thu, 05 Aug 2010 14:44:11 -0400, Michael A. Terrell wrote:

Meat Plow wrote:

On Thu, 05 Aug 2010 01:09:04 -0400, Michael A. Terrell wrote:

Meat Plow wrote:

On Wed, 04 Aug 2010 20:49:47 -0400, Michael A. Terrell wrote:

Meat Plow wrote:

May be of interest to some of the group RF nuts like me.

http://hawkins.pair.com/radio.html


I told you about that site a couple years ago.

Well ****, I don't memorize these things.


Here is one about the different types of broadcast towers in
use, in
the US.

http://www.fybush.com/

Did you watch the VOA videos or had you seen those before?


Yes, when I first found the site. I visited the VOA Bethany
(Mason,
Ohio) facility in the late '60s when it was being upgraded. The
contractors were modernizing the controll rom, and had just
finished installing the 10 new National Radio 50 KW transmitters.
I also visted the WLW site the same day and got a good look at the
500KW transmitter.

I going to try to arrange a tour of something that has that gear
still in operation. Like a few Continentals some Marconis, some
digital PSM/PWM gear. Listen to the hum and buzz, smell the rarefied
air.


It would be hard to match the WLW or an old VOA site with so many
working transmitters.

The VOA site had ten new nationals, and all the original Crosley
transmitters they were replacing. The Crosley were built before
there was television, and they caused so much TVI that they were to
be scrapped. The engineers said they had tried multiple
modifications to fix them, but nothing they could do without a
complete shutdown had helped.

The Crosley transmitters were cool, art deco styling with a
beautiful
fine grained green metal flake finish, and big glass windows. They
were about 25 years old, but looked brand new. That's what a station
can do, when they have a team of full time engineers.

That site could produce a total 1 million watts on any frequency
from 2 to 30 MHz. They had a huge east west curtain antenna they
used to hit Russia & Europe. The new National transmitters were
continuous tuned, so all you had to do was dial in the desired
frequency, and servo motors at each stage would tun the transmitter.
I was kidding with one of the engineers if he had ever though about
putting it on 27.185 MHz any yelling, "Hey 18 wheeler, there's a
Smokey on your tail!" He turned pale as a sheet. Can you imagine
thousands of semis slamming on their brakes, all over the country?
He quickly changed the subject!


Modern transmitters look more like a server room than a transmitter.
A solid state 5 KW AM transmitter isn't much more than a modulated
switching power supply in a single 19" enclosed relay rack. The
earlier Gate, Harris GE, RCA etc. were three or more racks covered
with knobs & meters, along with a few buttons for LV & plate
controls.

I find the newer pulse width and pulse step digitals fascinating not
really knowing much about them. One Marconi 500kw does pulse step
modulation. Raises the efficiency from 70 to 90 percent. No need for a
150kw audio amp just to modulate. Takes more brains then I have to
design a smart switching supply and amp to do that like the Marconi
does. And hell that's 1975 technology to boot.


Harris Broadcast had some white papers on their website a few years
ago that described the operation of their solid state AM transmitters,
but they had pulled them all the last time I looked. I have copies on
one of a dozen small fat 32 hard drives. I haven't had the time to move
all of the old files to one large NTFS drive yet. I just hope that they
are still readable. Maybe that idiot 'General Failure' hasn't messed
with the drive I need. ;-)

I'm going to google the two methods of digital AM transmitters. There are
some that still use a tube array final and some strictly solid state. One
I was looking at had an 8 devices on a card, maybe TO-3PBL type case and
each one put out 1Kw. These output modules in sync with the power
supplies are modulated by switching them on and off. I guess that's PSM.

what sort of maintenance is required to keep those giant tube monster
walk-in transmitters running for decades?