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Broadcast Transmitter conversion thoughts and ideas




 
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Author Topic: Broadcast Transmitter conversion thoughts and ideas  (Read 1960 times)
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K8DI
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« on: July 28, 2021, 04:02:05 PM »

So… in about six weeks, I will be picking up my first broadcast transmitter..RCA BTA-1R1. I’ve been looking at the manuals and schematics, and thinking how am I going to turn a thousand pounds of obsolete equipment into a functional ham band transmitter. I’ve got a couple extra challenges, though, so I thought I’d see what you all think.

The transmitter will have to live in my garage. There is no way, without major cutting, sawing, and demolition, that I can get it into my basement shack, nor is there room for it there anyway. One good thing: The shack shares a wall with the garage, albeit at a different height (i.e., if you squashed the house like a pancake, the garage and the shack would be back to back on a shared wall), so cable runs are short and simple between the two spaces. Along with that, the antennas come into the house by way of the garage then through that shared wall, so they’re there too.

Besides reworking the output coupling network and the oscillator/driver coupling/matching, figuring out VFO drive, and assembling the transmitter (iron and tubes are pulled now), then getting it running, here’s my thoughts:

Antenna: use a coax relay in the garage before the line comes into the shack, tie it into my control system.

Power level (less than 1kW/legal/etc.): The power cutback for night time operation is a bunch of resistors that get unshorted by a contactor in the HV line. What about if instead of this, I run the 240 v plate transformer off 120v, giving me half the plate voltage, just like the resistors accomplish? The rest of the transformers would still get fed 240, as designed, just the HV plate transformer on 120v. A side benefit is less heat and power consumption, another is better regulation of the HV under modulation. Perhaps I could use that power cutback contactor to interrupt the HV line for PTT? I wonder how it would hold up to the frequent use? Speaking of PTT, how unhappy will the oscillator and driver be if I remove the VFO drive? Do I want to work up some kind of plate (or cathode) circuit-opening relay setup instead?

Tuning: I’ve been thinking of a couple possibilities:  One, use relays to select preferred frequencies/bands, and not be able to tune it remotely. If I move 5 or 10 kHz, how bad is it likely to be mistuned?  Two, add one remote controlled variable inductance and live with the small range it will provide. Three, full remote control of the pi network and range switches. How much tuning do I really need to provide for if I am on say 3870-3885 only? Can it be set-and-forget???  As far as remote control, what have people done and how well has it worked?

Metering:  How can I get remote analog style metering? Digital meters with remote sensing are easy, but harder to use for tuning (looking for the dip). What can one get away with extending metering lines? Besides getting accurate measurements, what about safety?

Finally, I live in a four-season climate. How hard is it on an old transmitter to be at 15 degrees F and then turned on? Or in a closed garage with an inside temp of 95F in the summer, before I turn on the transmit-o-heater? On that, those who’ve had transmitters in garages in northern climates, is corrosion/rust a big issue?

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
W1ITT
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« Reply #1 on: July 28, 2021, 05:12:14 PM »

Ed...
I can't comment on the conversion issue.  Anything out in the garage is a potential home for mice and they will drag all kinds of shredded material in to build nests, then pee in the cabinet.  Seal openings, however small, very carefully.
One thing you might do to prevent condensation and later corrosion is to keep a heat source running inside the cabinet.  The pricey gun safes, even for use inside the house, offer a heating element to keep the air in there above the dew point.  Of course this will make the cabinet even more attractive to mice, so seal it even tighter.  Supposedly, dryer softener sheets will discourage mice.
Make friends with a cat.  Or at least allow yourself to think that you are his friend.
73 de Norm W1ITT
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WB4AM
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« Reply #2 on: July 28, 2021, 09:26:46 PM »

Hello,  I am certainly not the guy for the conversion,  but perhaps for remote metering,  maybe push comes to shove,  use a dvr camera to observe the metering.

Sorry that's all I have, it was the first thought that came to mind!

Ken
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K8DI
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« Reply #3 on: July 28, 2021, 09:32:07 PM »

Make friends with a cat.  Or at least allow yourself to think that you are his friend.
73 de Norm W1ITT

I already have one radio cat...  the other cats don't seem to care about radios...

Ed


* IMG_5302.jpg (712.49 KB, 2016x1512 - viewed 149 times.)
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w9jsw
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« Reply #4 on: July 29, 2021, 11:58:31 AM »

Consider putting a big variac in one leg of the HV primary. I did that on my 813 rig that is using the iron from a Gates BC1T. Works very well. I usually run at 2kv but it can go from 1.5k to over 2.5k (don't know how much more, my HV meter only goes to 2.5k).

Also consider a step start on the other leg. They usually do not have them on BC equipment because they do not get turned on/off very much.

I used a 115V 15A Superior Powerstat. Got it for $75 plus shipping. See pic.

BTW. I turn my HV on only during TX thru a PTT sequencer.

Be glad to share my schematic if interested.

John





* IMG_0502.JPG (175.79 KB, 703x937 - viewed 142 times.)
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KB2WIG
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« Reply #5 on: July 29, 2021, 03:24:00 PM »


Somewhere in the West Coast Handbooks is a discussion for running big iron at redu ed voltagws.... i,m not at home, so I don't know which edition, or even if it is applicable to yer transmitter..... maybe a update for later PM.

klc
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Patrick J. / KD5OEI
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« Reply #6 on: July 29, 2021, 03:32:12 PM »

Cold should be OK. "for -20 deg C to +45 deg C operation, use xenon rectifiers) or solid state instead of the original 8008 MV rectifiers.


Metal screening can be put over holes in the TX to exclude insects and the like The screen should be of tougher wire than old fashioned window screen, such as steel 'hardware cloth', and the holes larger maybe 3/32" to 1/8" to admit air well.

Louvered metal can be put over larger screened holes to exclude mice and reptiles. Some mice will chew the screen wire. The louvers have to be small enough to prevent mice from squeezing through. I used a HVAC register intake sort of panel.

I have had good luck with louvered panel with 1/8" louver slots backed by small hardware cloth. Nothing has ever gotten in except a spider or two in all these years.

Adding the screening to my similar size TX meant I also had to add a blower to keep the cabinet cool.
Large diameter low-speed blowers worked well without excessive noise. Fans would have been much noisier.

Looks like in the manual, the set comes with some kind of screening over the two top vents. On this transmitter, it looks like there are no other vents. Does cold air enter the rearmost vent and hot air exit the frontmost vent?
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KB2WIG
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« Reply #7 on: July 29, 2021, 09:40:31 PM »



Radio Handbook

 15th Edition

p389

It's online here....

http://www.tubebooks.org/Books/orr_radio.pdf


At a cursory glance, it seems to follow your thoughts.



On the critter access, copper screen doesn't taste that good... snakes maybe..

https://www.pestmall.com/stuffit-rodent-proofing-copper-mesh-100-ft.html

I've used this type of stuff to stuff.

https://www.homedepot.com/p/Scotch-Brite-2-8-in-Copper-Coated-Scouring-Pad-3-Pack-213C-CC/207015608

klc
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Patrick J. / KD5OEI
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« Reply #8 on: July 30, 2021, 12:05:40 AM »

Referring to the BTA-1R and BTA-1R1 manuals on BAMA, If I understand what was said properly, the 1R1 to be had uses taps on the HV transformer for power selection.

The BTA-1R  changes power by relays which add and remove series resistors in the HV DC supply to the tubes.
It has two settings, hi and lo, and you pick the power levels from 250, 500, and 1000W.
https://bama.edebris.com/download/rca/bta1r1/bta1r1-copy2.pdf
NOTE it is mis-named as this is the BTA-1R manual!

The BTA-1R1 changes power by changing primary (240V side) taps on the plate transformer for 250, 500, and 1000W. It's much easier. A large variac is not needed.
https://bama.edebris.com/download/rca/bta1r1/bta1r1.pdf
THIS is the BTA-1R1 manual.

If you are truly getting a 1R1, then it should already have solid state HV rectifier and taps for 250, 500, and 1000W on the HV transformer primary.

Then you could pick the 500W tap and perhaps reduce screen volts on the 4-400s a bit to trim the power.

Or better yet use the 500W tap and buck the 240V by only a little, maybe up to 15%. (36V)
Bucking should take only a small variac like 600VA (5A @ 120V or 2.5A @ 240V) if you follow it with a transformer having a 40-50V @ 15A-secondary.
This secondary would be connected in series with the HV transformer primary to buck the 240V mains going to the 500W tap.
You would have the range to adjust the carrier from 500W down quite a bit in case you are looking for the '375W' carrier figure.

Just thinking that instead of a 5KVA variac on the whole plate transformer or a 120V 2.5KVA one on one of the 120V Lines, that using the 500W tap and a small variac and transformer with only the required voltage to buck the mains would save money and space.

I hope this makes sense.
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IN A TRIODE NO ONE CAN HEAR YOU SCREEN


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« Reply #9 on: July 30, 2021, 04:30:56 PM »

They never sound as good on low power as high... Cool

From years of beating on them for a living.

73DG
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K8DI
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« Reply #10 on: July 31, 2021, 02:21:45 PM »

NOTE it is mis-named as this is the BTA-1R manual!

Thanks for the catch, Pat.  I was looking at the larger file (bigger is better, right?) and did not realize I was not looking at the 1R1. I think I am getting 1R1, due to the described tube complement,

I was interested not so much in the magical 375w number, but less power/heat/electricity. I expect that I will have to pull my antenna down and re-do the balun at the center at 1kW carrier, and of course, while finalizing a new matching network, the finals will be a little more forgiving of mistakes at 250w than at 1kW..

Thanks for the feedback, gents. 

Ed

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ka1tdq
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« Reply #11 on: August 10, 2021, 09:43:09 PM »

Not to beat a dead horse, but class E is much lighter. My cat Fluffy prefers it. I have mine all built and I'm testing it now.

I've never converted a broadcast transmitter but I've been tempted because they are sexy. Best of luck with yours. I'm sure once you get it bolted down you'll have lots of help here.

Jon


* Fluffy.jpg (1108.77 KB, 2016x1512 - viewed 89 times.)

* E Testing.jpg (3004.27 KB, 4032x3024 - viewed 126 times.)
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W3GMS
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« Reply #12 on: August 24, 2021, 06:24:25 PM »

Dave - K2DK had an entire series of articles in Electric Radio on the conversion process of many of the popular AM BC rigs.

Joe-W3GMS   
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K8DI
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« Reply #13 on: August 25, 2021, 08:23:31 AM »

Dave - K2DK had an entire series of articles in Electric Radio on the conversion process of many of the popular AM BC rigs.

Joe-W3GMS   

Any Idea when/which issues? I've only been getting ER for a few years, so I'd have to find the right back issues...

Ed
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KA3EKH
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« Reply #14 on: August 26, 2021, 09:07:58 AM »

Not much to do with the conversion although I did run a RCA BTA 1MX for some time on 160 see the pictures at:

http://staff.salisbury.edu/~rafantini/RCABTA.htm

But what I wanted to talk about is cats and broadcasting. One of the smaller stations I did work for back in the day was a 6kW FM/ 1kW AM that also had a RCA-1MX as a backup for a solid state BE but the thing is they had a name for there stations “Cat Country” and on all there billboards and vehicles they had a black cat logo.
They had a black cat that had the run of the station, but their general manager was always on me about not letting the cat into the back room where all the transmitters were because he was concerned that it can get into the equipment and get fried.
I just did part time stuff for those people so that meant that I was there on weekends or nights when no one was around, at that time think they still did a live morning and afternoon drive but ran automated the rest of the time so nobody would ever be in the building, except the cat.
One day I was there for something and when I opened the door the cat ran out and into the parking lot. Never did tell them that I let the cat out but a week or so later I was there and they had that cat or another just like it so figure it was all good.
Another animal story, years before this I was doing some work for a crapy little 5kW AM only and this was when AM was already dead in the commercial world. One day when I was there I saw the GM had a fish tank in his office with some goldfish or something like that in it. Said I see you got some fish, he told me that having the fish tank was good for relieving stress.
A week or so later I was back there and saw the fish tank was empty and turned upside down in a corner of the office. I asked what happened to the fish? And the GM told me there must have been too much stress and that wore them out.
And one last story about WKEN, has nothing to do with animals but have time this morning so here goes. This was back in the eighties and they had this syndicated radio commentator named Paul Harvey.
They use to receive his daily rants along with the news on a first-generation Scientific Atlanta C band digital receiver, a horrific monster that occupied almost an entire rack that was often broken between its direct C Band input and transmission line from the dish back, the failure of the multipole power supplies that were all over the unit or any number of other things.
And worst of all was the Paul Harvey show because the receiver would cut in and out at different times and think that may have been one of the only things they had sold was his commentary so you never knew if the receiver was cutting in or out or if it was just his cadence.
One winter we had an ice storm and that’s unusual around here, the ice was all over the dish and feed horn so the receiver was cutting in and out worst then normal. The GM called me and wanted to know what I was going to do about it? I told him that the ice would eventually melt so I was not going to do anything about it. In order to keep receiving Harvey and the news the GM drug out a three-drawer metal file cabinet and built a fire in it below the dish to get rid of the ice. That burnt out file cabinet stayed out under dish for at least a couple years.
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K1JJ
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"Let's go kayaking, Tommy!" - Yaz


« Reply #15 on: August 26, 2021, 12:33:28 PM »

When it comes to BC transmitter conversions, there are few better than Robert, W0VMC.  He has it down to a science and has paid his dues for many years.  Contact him with your ideas and I'll bet in an hour's telephone conversation he can tell you how to do the job right; no matter what transmitter you choose.  His projects sound great and look fantastic when completed.

Search the web for many of his projects.

http://w0vmc.com/pages/index2.html


T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed. 

Nothing like a new homebrew rig. Come into the shack, flip on the switches and everything works perfectly.

And, nothing like an old dog.
W3GMS
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« Reply #16 on: August 26, 2021, 04:59:19 PM »

Dave - K2DK had an entire series of articles in Electric Radio on the conversion process of many of the popular AM BC rigs.

Joe-W3GMS   

Any Idea when/which issues? I've only been getting ER for a few years, so I'd have to find the right back issues...

Ed

Information about searching back issues can be found on the ER website:  https://www.ermag.com/

Also, you may want to send Dave - K2DK an email.   He did an excellent job with his ER articles on converting many different BC rigs.

I would say his articles were at least 10 years ago.

Joe
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