The AM Forum

Had a great time this morning. Dave, W1DJG, an engineer at WBZ gave us a tour of the WBZ AM transmitter site in Hull, MA. Afterwards we went to the Red Parrot restaurant along the beach in Hull and had a great lunch.
WBZ is a clear channel station and has two antennas with a cardoid pattern. It is on 1030 KHz and was putting out about 60KW.
Ah the monkey was swinging! Inside the little shack at the base of the driven antenna we could hear the program from the coils! They must have been moving with the modulation.
Check out all the pictures at : http://home.comcast.net/~w1qwt/wbztour.html (http://home.comcast.net/~w1qwt/wbztour.html)

Here I am infront of the main Harris solid state transmitter.
(http://home.comcast.net/~w1qwt/images/wbzxmt7.jpg)

Regards
Q, W1QWT

Strapping!  Very nice photos.  Thanks for sharing! 

The DX 50 is a cool rig. Giant D/A changing bits on the carrier zero cross.
I would like to see a picture of the output transformer. 

Not sure if this is the output transformer but it was in a box he called the tunning unit.
Looks like a still to me.
Regards
Q, W1QWT

(http://home.comcast.net/~w1qwt/images/wbzcoil2.jpg)

Sweet.

Thanks for sharing.

THanks for the nice pics.
The beauty of solid state...........if it's working right.........just turn the knob and the box might hum a little more......but the output meter effortlessly climbs up.
60KW??? 10KW to overcome losses or the DA??? Betcha it sounds like a dream!!!!

Fred

Nice pix, thanks. What you called the spare transmitter appears to be actually the phasor, with a Delta CPB-1 impedance bridge mounted in it. This is what adjusts the phase and amplitude to the towers to get that cardoid right. Wonder what spare TX they used? 4CX35,000 is used in the Continental 317 series I believe.

Fred the specs on that rig 85% efficient power line to RF, if I remember.
D/A has very low loss because when a module is off its primary is shorted as to not add inductance to the combiner transformer. This is a very cool method I have played with at much lower frequency operation. An old boss held a patent on the same waveform generator for power supplies. If you don't short the primary of each module turned off the efficiency and linearity goes way down. I think the DX50 has 126 modules and claims 1 or 2 % distortion. Audio goes into an A/D then the output of the A/D drives a prom with a look up table to select the necessary number of modules when the carrier goes through zero crossing. Since there is no power at the carrier zero cross there is low switching loss. The D/A generates a square wave of different alplitude based on the modules selected so it just takes a LC network at the output to clean it up. No worse than any class d final. (actually up to 126 of them in series)  That is the view of the last picture.
I think at carrier 25 or so modules are selected at each zero cross. when no modules are selected 0% and all modules selected positive peak. Imagine the digital dance at each zero cross of the carrier.
Very cool rig if you could build one for the ham bands...you could if you had a lot of time and hardware. 

I think I have it right now. Your the second person to tell me I had it wrong.
This is the picture of the spare Harris transmitter:
(http://home.comcast.net/~w1qwt/images/harris2.jpg)

The spare xmtr and the tunning unit were both in blue boxes so I got confused.
I think that picture of the coils were in the back of that blue box that was called the tuning unit?

Regards
Q, W1QWT

The spare tx is a Harris MW50. Vacuum tube pdm rig.

I have always been fascinated with the MW50, as that is the main full carrier TX at WBCQ that was modified by The TimTron, esq......for shortwave.
Are there any diagrams of the circuitry available on-line?? What final tube is used??

Back to my earlier observation of the DX 50 running 60KW......Why is WBZ running in excess of 50KW?? I understand that the station can increase power to maintain the radiated field strength that was calculated by the technical consultant. Usually it's plus or minus a few hundred watts to meet spec.
Thanks for any info

Fred

Solid State? What on earth is that!!


Nice pics OM

A solid state is one that spends less than it takes in

I think they're 4CV50000A finals
shelby kb3ouk

Wbcq's MW50a isn't blue, it's gray in the pictures I saw. It's also missing the doors. And I thought I recalled hearing Allan say that it used 3CX2500A finals, or maybe that was one of the other transmitters.
shelby

Its an MW50, not a. It uses a pair of 4cx35000a's. One in the modulator, one in the final.

As far as running above 50 Kw, 47CFR73.51, they can run at minimum 5.8% above licensed power to overcome losses in the DA and transmission system. What thier exact loses are, I don't know whithout looking at the DA schiz.

That's what I figured for the 60KW reading. It's amazing that the TX puts out what you need without stress or strain.

And Thanks Shelby for oyur input on the MW50 finals. Tubes with handles are awesome

Fred

the loses must be high if they have to run an extra 10 kw to make up for it.

that 4CX35000 is a little rusty, isn't it?

YUP something doesn't click with 10KW to make up for losses. Usually it's not that much between what's on paper and the real World.

Good eye Shelby. 'BCQ has a grey cabinet.

Fred

Yeah, I've seen them in both blue and gray, but the blue cabinets are more common, probably because they are newer than the gray cabinet ones.

I beleive that would be the Collins. I forget the model, but it's got "pizza oven" doors to the tube compartments. The finals sit inverted (anode-down) on a plate which is risen up by a giant threaded ring to engage the tubes in the sockets above. It was on 17.495 when I was last there.

The 7415 transmitter is a Gates 50S. Typical Gates big ugly gray cabinet with meters at the top. That's a PDM transmitter, if I recall correctly. Very similar in outward appearance to the 10G, but not that putrid green color Gates was fond of.

No idea what 5105 is. Never saw it operational.

5110 and 15420 are rigs that Timtron put together. The 9330 rig is a collins, it looks a lot like an AN/FRT-22.