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RCA Broadcast TX to 80m

Discussion in 'Home Brew' started by Captain Kilowatt, Jan 6, 2009.

  1. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Well after having started and stopped about a dozen times I am finally back on track with converting my RCA BTA-1S transmitter to 80m. I did have a problem with unbalanced modulation where it appeared on a scope that the top half of the carrier was modulated less than 30% while hitting 100% on the bottom half. Yeah, looked strange to me too. :blink: I found R23 which is a 124 ohm 1% resistor in the 2E26 first AF amp stage was 12+% low. I replaced it and now things look much better.Things still will not hit 100% pos mod but I think I have soft tubes and can live with it for now.All in all it sounds much better than it did when I started.Most of the distortion is gone and all that is left can be attributed to weak modulator tubes.I am feeling better now that I have a decent sounding TX to work with. Next I have to strip the PA section and rebuild it as a regular Pi network with a pair of vacuum variables I have on hand and a coil recycled from the existing output circuit.Not sure what to do about the 6146 RF driver stage yet. I may rebuild it for 80m along with an external VFO or I may eliminate it altogether and drive the 4-400's directly with a few watts of carrier from my DX-60.The latter has more appeal at this point but I may go the VFO route. As a side note I got a fright this afternoon while working on the TX. I had just solved the mod problem and had the tx running a full kilowatt into a dummy load when I turned around to plug in a Harris Solidstatesman audio limiter/amp I planned to used that was sitting on the floor. As I bent over my butt touched the TX cabinet that had a lower access panel removed at the same time I plugged the Harris in. KABANG!! :eek: Fire flew out of all the vent holes in the Harris followed by lotsa smoke. I had to change my underwear after that. Amazing what goes through your mind when you have your back turned on 3000 volts. The TX is fine but the limiter amp is toast I believe.So much blackened innerds in so little time. :cry: I have no idea whatsoever what happened. It worked fine the last time I used. Maybe caps but that's not all from the looks of things. I think it was actually on fire while I was trying to get the cover off.I started to tell the wife about it and she said "Yeah I heard a loud bang and you yell." I told her it was nice to know that she considered knitting a sweater more important than seeing if I was dead on the basement floor.I figured since she was a nurse she should have at least tried to give me mouth-mouth afterwards. [​IMG] Now I need to find a new limiter. :cry:

    I'm making this a sticky post so I can easily find it as I add updates and pictures as the project continues. It will take quite a while to complete.


     

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Here are a couple pictures of the TX inside.

    [​IMG]

    PA tuning section. The smallest coil is about the diameter of a regulation baseball on the inside.
    It consists of silver plated copper ribbon edgewound on a ceramic form.The tuning coil is tuned by a silver plated copper slug that moves in and out of the coil.This coil is the same as the other but larger.The antenna connection is a ceramic feedthru connector that exits the top of the cabinet.


    [​IMG]

    You can just see the modulation transformer to the right of the cabinet. It weighs in at 65 pounds and is mounted on the cabinet floor which is all copper plated BTW. Notice the rusty red color? Also mounted on the cabinet floor and not seen is the modulation choke. It weighs in at 85 pounds.The plate transformer is rated at around 3000 volts at 1.3 amps continous duty.The result of a well filtered power supply using choke input is a nice and stiff 3100 volts on the plates of the 4-400's. It uses a pair of them in the modulator stage and another pair in the RF output stage. I will get some more photos of the front of this beast in a few days.
     
  3. cyclops1970

    cyclops1970 Gay Nazi Liberation Party

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    Post pics of the RF deck...with power on!!!

    Re your wife: I'm sure by now she knows you're playing with 3000 volts. And she's a nurse to boot: if she hears a yell after the bang, that usually means you're OK and her services aren't needed. If she hears silence, then you're dead...and her services aren't needed!!!!
     
  4. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Pix in a few days. I finally found the digital camera again. Next time something goes bang I'll do my best to remain quiet and see what happens. :D
     
  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    As promised. Here are a couple pix of the RF deck showing the 4-400's with filaments lit.The two on the left are the modulator tubes and the two on the right are the RF power amplifier tubes. The high voltage is off in these photos as I would have to defeat two safety interlocks in order to have it on with the door open like that.One inhibits the HV primary from coming on and the other shorts out the HV buss when the door is opened. Both features are handy to have if one wants to live a long and happy life.The two small tubes in the lower part on the left are 2E26's and are audio amplifiers feeding the 4-400 modulator tubes.The single one on the right is a 6146 RF driver tube feeding the RF 4-400's.The small metal box between the tubes is a homebrew oscillator replacing the solidstate oscilator that was factory installed. It burned up long ago and a homebrew unit was installed. The red can on the oscillator unit is the crystal BTW. It is set for operation on 720 KHz right now.


    [​IMG]




    Here is the meter panel while making 1000 watts output. The HV is 3100 volts,the RF grid drive is around 125 mA, and the plate current is 350 mA. There is no power meter on these transmitters and power is typically calculated by measuring the RF current delivered into a known impedance.I tried to take a pix of the RF ammeter reading 4.4 amps but it was too out of focus to use. 4.4 amps into the 50 ohm load equals 968 watts output. (current squared times impedance)These are the last pix of the transmitter in it's current state. Next I will begin to strip the PA section and rebuild it for 80m.


    [​IMG]
     
  6. cyclops1970

    cyclops1970 Gay Nazi Liberation Party

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    Very pretty!!

    I love 4-400s.

    Just curious about your RF ammeter. I used to have one in my amp switchover box, but blew the thermocouple (it was an old Westinghouse--no replacement available). What do you use?
     
  7. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    I have a couple Sangamo meters in my stash good for about 1.5 and 2.5 RF amps as well as a Simpson 13163 RF ammeter that is good for 8 amps of RF. I have it wired inline going to the dummy load for testing this transmitter.
     
  8. wy0mn/7

    wy0mn/7 Member

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    Jeez! I've owned smaller refrigerators. Me, jealous? Heck yeah! I wanna see a vid of your utility meter!

    This morning I started ordering parts for OTB - Build an Early 1920s One-Tube Transmitter
    Many of the components are home-made.

    Ciao
    Lex
     
  9. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    :D Well from the AC line it consumes 2900 watts at dead carrier and 3900 at full peak modulation. The average power used under normal voice transmission is only 3200 watts according to the specs.To put it into perspective, the clothes dryer uses around 5000 watts. (y)

    I almost forgot,I need to switch it over to 250 watts before I tear it apart and rebuild.Best to start off with lower power when entering the world of the unknown. All I need to do is move a couple taps on a couple transformers.
     
  10. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Famous last words.:angry: This thing is taking a lot longer to get around to than I thought. Frustration is a wonderfull thing for inhibiting motivation. Everytime I try changing the power taps on the transformers to reduce power it trips the overloads and will not power up. :headbangThere are taps marked for 1000 watts, 500 watts, and 250 watts. I want the 250 watt setting for testing and will increase the output when it is all done. This was a simple matter the first couple times I did it,just move two taps,one on the plate transformer and one on the bias transformer. Simple enough to do. It worked before but not now. :confused: Maybe I can get to it tomorrow again. I am off for a few days but need to take care of some plumbing changes in the basement today.
     
  11. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    OK, well I have been looking at some changes in my conversion project and that is what has taken me so long to get back to it. I decided not to use the big Jennings vacuum variables in this project because they are so valuable to me it would be a shame to put them into this project that will see very limited use.They would be much better suited in an amp or high power tuner and see much more use.As a result of that I have been looking for suitable "breadslicers" to use instead.I have had a suitable tuning capacitor all along but finding a large breadslicer type with enough capacitance to use as a loading capacitor at a decent price (I'm cheap) is not easy. I finally found a dual section variable with 1000 pF in each section making for 2000 pF total capacitance at the ham flea market last weekend. The best part is it cost me the total sum of $10. (y) Here it is below.Note the twelve inch ruler.


    [​IMG]


    This will make mounting of the components easier as well as the breadslicers are not nearly as heavy as the vac variables, are easier to mount, and do not require the addition of turns counters either.The vac variables required 20 turns to cover the min-max range wheareas the breadslicers require a simple 180ยบ rotation. Things are looking up now for getting this thing done. Good weather will see me outdoors while wet weather will see my off days spent in the basement working on this transmitter. I think I will forego converting the 6146 driver stage for now and simply drive the finals with a carrier from either my DX-60 or my Kenwood TS-820S. I will use the DX-60 in all testing stages until I feel comfortable driving it with my baby, the 820S. I'm still looking for a good broadcast quality limiter/amp for the audio input after the Harris went supernova on me :eek: a while back but that is easy to do without for a while. More updates and photos as the project continues. Please be patient. This is a labour of love and not a "git'r done!" kind of thing. :D
     
  12. HiDef

    HiDef Active Member

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    Nice score. You might want to beef that thang up with the addition of some wire braid between the rotor shaft and ground. That cap will see plenty of circulating current. It looks like there isn't already a high current type contactor on the rotor. If so then forget what I said.

    Best audio processor for a handful of bux is the Behringer DSP-9024. It has a 6 band compressor limiter with adjustable band edges. Thet show up used for 100 bux.

    See you next fall on the low bands.....
     
  13. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Well gentlemen I am sorry to say that this will be my last update in this thread. Two weeks ago my transmitter suffered a major fault when the DC blocking capacitor that keeps the 3100 volts DC from the secondary of the modulation transformer apparently failed. As a result the modulation transformer developed a short when the 3100 volt main power supply rail was fed back through the secondary windings.The secondary windings have , or had, :headbang a DC resistance of around a thousand ohms which allowed the plate supply to deliver a substantial current before the overload tripped. Not only is the transformer shorted in the secondary it also has a primary to secondary short. I'll tell you, when I blow something up I do it in fine style. :cry: Not sure really which went first, the blocking cap or the windings shorting out that caused the cap to fail but the end result is that the mod transformer and associated modulation reactor, which is a large ,huge really at 85 pounds, choke are both toast and are made of unobtainium. Anything I can get from my contacts is either too small or wrong impedance. I have :headbang over this the last two weeks and have decided to forego the conversion and in all likelihood will dismantle the beast for parts. I can make a nice multiband amp from the parts and will buy either a matched pair or a matched quad of 3-500ZG's for it.The sockets are the same as the 4-400's. The 3-500ZG's require zero bias and no screen voltage as the 4-400's do. The screen supply in the TX will make a wonderful plate supply for some 811A's that I have kicking around here and I just may end up making a small transmitter using a pair of 811A's in the RF stage modulated by another pair of 811A's or 807's which I also have. I have a small modulation transformer that will handle about around 75 watts making it ideal for a 100 watt carrier transmitter. It should drive the home brew amp nicely as well. So in the end what appeared as a major let down may in fact be the push to get back into the roots of real ham radio and build something from the ground up. Wish me luck. Apparently I need some. (y)

    Oh yeah, here is a picture of the mod tube with vapourised elements. It must have been a firestorm inside when the mod transformer went critical.


    [​IMG]



    [​IMG]
     
  14. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    I have moved this thread back to this forum for a while due to several requests form some members that have seen the TX pix in my album and were wondering what happened to it. It may stay here or it may be deleted in the future.
     
  15. office888

    office888 Active Member

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    Thanks for posting this!

    It was a really enjoyable read, although I'm sad that it didn't work out.

    I'd love to cobble together an AM cabinet transmitter, but unfortunately, I just don't have the know how at this point in time.

    Maybe in the future...
     

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