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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Sat 04, 2020 1:42 am 
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I've followed this thread with interest. It's a bit difficult to judge scale from your photo, but those fuses appear to be 3AG fast-blow types. Given some of the posts I've read on ARF about bad experiences people have recently had with "no-blow" fuses sourced from some off-shore manufacturers, I trust the fuses used in those transmitter modules are sourced based on known quality and characteristics. Or are the modules tolerant?
R/ John


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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Sat 04, 2020 5:46 am 
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Greetings to John and the Forum:

The fuses are standard 3AG types. They are there to protect the traces on the board. The only reason for that fuse to blow is if the FET shorts source to drain or the electrolytic cap shorts.... which usually results in the destruction of the cap as you can see from the photo. The module in the close-up is a 1.5 KW module. That's the Peak RF power out. The drain voltage is 50 volts. I believe the modules run something like class AB1 or 2.... so the efficiency isn't all that great... perhaps 60% at best and probably less. 60% efficiency would require 50 amps at 50 volts to the module. There are two power supplies in the center cabinet; each one supplies two shelves of modules. The standard transmitter runs six visual PA modules, one aural PA module and one driver module per shelf. Normally, all the modules are 1 KW for a transmitter of the type we had, but in our case, additional aural power was required, so we had 1.5 KW PA modules from a bigger transmitter in the aural position. A 1 KW visual module would require about 33 amps. So, not counting the driver module (about 500 watts), there are 12 loads at 33 amps each and two loads at 50 amps each per cabinet... and each cabinet (of two) has its own power supply. That's roughly 500 amps at 50 volts available from the power supplies. That makes a pretty good arc welder in anyone's book.

If that current were not limited someplace, it could destroy the entire PA module in the event of a short by burning up the board. So... we weren't so much concerned about the FETs as we were about limiting collateral damage. Plus the fuses make a good place to interrupt the drain supply for test purposes. Normally, any fault condition that causes the FET to draw enough current to blow the fuse has already destroyed the FET.

Regards,

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Sat 04, 2020 7:00 am 
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Joined: Nov Fri 30, 2012 3:35 am
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Location: Phoenix, AZ
Jim and the Forum,
The PS would be a good arc welder indeed! At first glance I never would have imagined the 3AG fuses are there to protect the board traces ... but it makes good system engineering sense. Thank you for the additional insights ... very interesting.
R/ John


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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Mon 06, 2020 4:08 am 
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Has anyone calculated the cost of conversion at both end of the transmission-reception chain.

Would have be a huge number!


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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Mon 06, 2020 4:40 am 
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I think that the cost to TV stations of the repack is about the same as the analog to digital conversion was. Huge. I was curious enough to check into what two of the Detroit stations had to do. One had to build a new transmitter building and buy two new transmitters, a primary and a secondary. And two new antennas to go with them. The transmitters they had were obsolete models and could not be modified to run on the new channel. The other also had to buy two new transmitters and two new antennas. In that case the manufacturer of the old transmitters was no longer in business.

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Mon 06, 2020 6:27 am 
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Greetings to algermond and the Forum:

There was a complicated paired set of auction / reverse auction of the spectrum space. Supposedly, the people moving into the vacated spectrum will be the ones laying out the large amounts of cash; the moving stations are supposed to be reimbursed to some extent.... I never did follow it very closely, but the costs to the stations won't be as bad as direct out-of-pocket costs for the move.

Regards,

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Mon 06, 2020 6:51 am 
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So you still get to use Band 1 for digital in the US? No longer the case for us in Australia.

Band 1 channels can go extraordinary distances, guess they'd be desirable for regional stations as they were here. Channel 3 Newcastle used to cover half the state!

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Mon 06, 2020 1:22 pm 
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Tom Albrecht wrote:
After researching a little...

It's interesting to read through this RCA catalog of TV transmitters: https://www.americanradiohistory.com/Ar ... g-1956.pdf
...


Tom, thanks for the link. When I looked at those prices, Holy Cow, in 1956 dollars, lots of bucks!

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Mon 06, 2020 5:17 pm 
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irob2345 wrote:
So you still get to use Band 1 for digital in the US? No longer the case for us in Australia.

Band 1 channels can go extraordinary distances, guess they'd be desirable for regional stations as they were here. Channel 3 Newcastle used to cover half the state!

Well this is getting off topic, but I will post this:
In the US a side effect of the digital transition was that most stations ended up in the UHF band with a few stations in the high VHF band. There was almost nothing in the low VHF band (54 to 88 MHz). This was not by design, it just happened. As a result many companies sold "digital antennas" that were either UHF only or UHF and high band VHF. Now that we are repacking the TV stations to free up the higher UHF channels some stations are now being shoved down into the low band VHF. But hardly anyone can receive them because their antenna does now work on low band VHF.

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Thu 23, 2020 5:11 pm 
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Since we can't let this thread die . . . . .

I found it interesting to learn that the Inductive Output Tube (IOT) was invented and Patented in 1938. RCA acquired the patent and made an IOT.

The RCA 825 IOT was used in the Television transmitter at the 1939 Worlds Fair.

I am assuming on a VHF frequency as I recall the RCA TRK-12 TV could only tune part of the VHF band.

I am including a photo of an RCA TTU-110 transmitter which I believe was the last RCA UHF transmitter installed in 1985 by Me and RCA.
It was operating at full power (3 Million Watts Effective Radiated Power ERP), when I hit the OFF button in 2009 for the Digital Transition. It uses three 5 cavity Klystron tubes, two for visual, and one for aural. The rack to the left is a non-RCA modulator (Exciter) which was used in the final years.

This transmitter is still connected to its antenna but has no High Voltage Beam supplies or cooling system.

The station now transmits 1 Million Watts ERP using two IOT's


Attachments:
825.PNG
825.PNG [ 417.19 KiB | Viewed 252 times ]
RCA TTU110.jpg
RCA TTU110.jpg [ 127.48 KiB | Viewed 252 times ]
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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Thu 23, 2020 6:53 pm 
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To me a tube named Inductive Output Tube sounds like it should be the final high power tube. But at 35 watts output I see that it is not.

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Thu 23, 2020 7:11 pm 
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Tom

It would be interesting to see some info on the 39 Worlds Fair TV transmitter that suposedly used that 825 tube.

I was led to believe it was located on the Empire State Building at the time.

They would have likely been in low band VHF. this tube claims UHF to 300 MHz.

I owned a 39 Worlds Fair TRK-12 way back when and I believe these had a five channel tuner (Low V).

I believe they were originally AM audio and were modified to slope detection for FM after the fair.

There would have been some kind of cavity structure and magnetic focus coil set up as that 825 is a Beam type tube. Maybe electrostatic focus.

35 Watts would probably get some coverage around New York City.


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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Thu 23, 2020 10:50 pm 
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In 1938 300 MHz and up was considered UHF territory - pretty rarified atmosphere for most commercial RF functions. The War would of course push RF technology forward an order of magnitude or more in a few years.
I wonder if any of those 825 tubes (or whatever equipment they were used in) has survived?

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A collector of TV signal boosters and UHF converters -- God help me!
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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Thu 23, 2020 11:37 pm 
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Greetings. All!

I learned something new.... I thought the IOT was a fairly recent development. The data sheet for the 825 is interesting: It states that the tube can achieve 60% efficiency... yet the power output is given as 35 watts and the collector input power is given as 100 watts max. I don't quite understand that.

I would be very interested to see the output coupling cavity for the 825. The EEV / Marconi IOT's that I am familiar with have the coupling "shoes" built into the tube structure and they are clamped onto by an external resonant cavity.

Here's a generic article about IOT's:

https://www.researchgate.net/publicatio ... -_a_review

... and here is a data sheet on an IOT similar to the ones I worked with... can't find the exact one any more:

https://www.teledyne-e2v.com/shared/con ... TD2130.pdf

Regards,

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 Post subject: Re: First generation TV transmitters
PostPosted: Apr Fri 24, 2020 12:01 am 
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Location: Phoenix, Arizona
that e2V D2130 will make 30 kW, I use the D2100 in a Television transmitter and that tube makes 22 Kw. The two versions fit the same trolley with modifications for cooling. A factory fresh one will cost close to 50 Grand.

Varian turned into a company called CPI however they may be out of business.

I have two duds waiting to go to recycle.

As stations repack there are parts gathering dust and used tubes can be almost free plus shipping.

Now we build transmitters with lots of Field Effect Transistors running im parallel through combiners. Some are water cooled. No high Voltage, No Beam supply, No Crowbar Tube

I would like to see some data on that 39 Worlds Fair transmitter, must be something somewhere. There would have to be some cavaties to use that 825 IOT in the original transmitter.

RCA was big on publicity, where is it?


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