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Hey QRN (or anyone else) Little AMBC TX- Want to move it to HF Bands

Discussion in 'Home Brew' started by Bow, Mar 4, 2009.

  1. Bow

    Bow Bastard Modulation Engineering

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    So I acquired a AMBC Transmitter, not a monster by any means, but not quite a PW either...

    A J.R. Cunningham CM 30-50 Transmitter along with the LPB ATU-100SF Antenna Tuner

    A friend of mine is in the AM radio business and he had it stuffed in storage for any number of years. He had removed it from a Transmitter site in when they upgraded. It was being used as a backup Xmtr and currently has a 1050 kHz Xtal installed

    The Tube line up is:
    12BY7, 6V6, 6DQ5 (broken in socket)
    I found a 6DQ5 tube to replace the broken one...

    Interesting note, it has a IRF820/TIP100 Modulator.

    Photos:
    http://riley-music.com/BowsStuff/CM3050/01Front.JPG
    http://riley-music.com/BowsStuff/CM3050/04Rear.JPG
    http://riley-music.com/BowsStuff/CM3050/07Top.JPG
    http://riley-music.com/BowsStuff/CM3050/10IDTag.JPG
    http://riley-music.com/BowsStuff/CM3050/18GutShot.JPG
    http://riley-music.com/BowsStuff/CM3050/15AntTnr.JPG

    I searched for "J.R.Cunningham Transmitters" and found a site, which was recently closed... :(


    Anyone else ever heard of them? The original build, Mr. Cunningham, passed away 2 years ago... and no one seems to have Tech info on these things...

    If it is 50 Watts output (like the website stated), I am interested in trying to put on 160 or 75 meters...

    I have a FCC-1/FCC-2 NorCap QRP DDS that I should be able to use instead of crystals.

    Any suggestion, hints, tips, or suggestions on how to move it up to the Ham Bands (I am a General Class)?



    Thanks
     
    #1 Bow, Mar 4, 2009
    Last edited: Mar 4, 2009

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Never heard of that company. The TX looks as if it may have been a carrier current transmitter and possibly was meant to feed AC power lines to serve something like a campus. Maybe it was meant to be an exciter for a bigger TX not sure.A schematic would be MOST helpfull. From what I can see it looks simple enough but looks can be deceiving. The verticle coil next to the transformer is likely the output tank coil and will have to have the tap adjusted to bring it into 80m where I assume you want it to be. You will need fewer turns in the circuit,best guess to start off would be about half as many.There may be some capacitors that need to be changed as well as I see nothing in the form of TUNE and LOAD capacitors. Since this unit was designed for single frequency use most likely these were fixed values.This is where having an antenna analyzer can come in handy to tune the output circuit when you have no idea what the values it will take to resonate it.It's a tricky process but it is outlined in the MFJ-269 manual. If you could find and scan a schematic it would help tremendously.
     
  3. Bow

    Bow Bastard Modulation Engineering

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    Thanks for the reply!

    Yeah, I have a few feelers out for possible tech data, but nothing yet.

    These were actually transmitters and were mainly built for missionary service in remote locations, like South America.

    I have an MFJ-259, but I'll look for a 269 manual to see how to try and "adjust" it.

    If I find some tech data, I'll let you take a look.
     
    #3 Bow, Mar 4, 2009
    Last edited: Mar 4, 2009
  4. Bow

    Bow Bastard Modulation Engineering

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    Wow.. one of my feelers came through!
    I now have a Manual for this thing!

    Reading the Manual, it is actually rated for 36 watts, but can push 50 watts with out 100% modulation.

    Here is the Schematic:

    http://riley-music.com/BowsStuff/CM3050/14CunninghamSchematic.jpg

    Tuning information for the AMBX Band:

    http://riley-music.com/BowsStuff/CM3050/18CunninghamTuneUpChart.jpg

    And Bottom Layout Drawing:

    http://riley-music.com/BowsStuff/CM3050/20CunninghamWiringDiagram.jpg

    Any suggestion to get this thing on 75 meters, or possibly 40 meters... or both?
     
  5. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    The first thing you should is prove that the unit works as it is before making ANY changes at all. Fire it up into a dummy load and test it. If all is good then use a signal generator and inject a signal on whatever freq the TX is operating on now. Do this by removing the XTAL and connecting the sig gen output to the xtal side of C 1. This will allow you to move the TX freq up the band as you tune it. The idea is to be able to provide an RF signal at any freq so that you can determine where the unit is tuned for.The big change is the tank circuit. Start with about half as many tunrs on L5,6. The TUNE and LOADS caps, C 12 and C 13 respectivly should be replaced with variables otherwise it becomes a hit and miss nightmare.Ideally C 12 should be about 2-300 pF and C-13 should be about 1000 pF. C 13 can be an old cap from a broadcast receiver while C 12 should have a bit more spacing.I see another variable cap not labeled between the 6V6 driver and the 6DQ5 final that may not be necessary or at the least may have to be changed to a much lower value. This is where some know-how comes into play when dealing with something like this. It is hard,next to impossible actually, to talk someone thru something like this especially when I have no direct experiance with this model of transmitter and cannot actually be present. I just know what should get you close and I know what I would do if it wasn't close enough. That is where experiance comes into play.One thing to consider is if there are any old timers in your area that into amatuer radio AM and see if there are any old timers still working at the local AM radio station. Many of them are hams and would love the oppourtunity to take on a project like this. Probably wouldn't cost much either as some would do it just for the "fun" of it. At least with a 6DQ5 in the final an "oops" is not nearly as dramatic (or traumatic) as having a pair of 4-400's with 3100 volts on them. :eek: ;)
     
  6. HiDef

    HiDef Active Member

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    For the output tank take the present parameters plate voltage and plate current and plug them into the formula for class C plate impedance. The answer will probably be 1800-3000 Ohms depending on the circus design.

    You can look at the charts in the ARRL or Orr handbook for tank component figures based on the impedance figure.

    You can make a dummy resistor out of junkbox resistor combinations that equals the plate impedance figure just calculated. Old carbon types are OK for this even if they have drifted a little. Just measure to make sure the final value is OK. With the transmitter off, connect the dummy resistor across the final plate lead and ground. Pipe the MFJ box into the transmitter's R.F. output port on the frequency of interest and tune your new tank for lowest SWR. Loaded without even turning the thing on!. That will make dealing with interstage coupling much easier. This trick works great for linear amps too just us the right calculation instead of class C.

    The DDS will probably not be a good idea unless it's something I've not yet seen. They are OK for QRP. Check the specs to see what the spurious outputs are. Most likely there will be too much junk. One way around would be to just get a crystal for the new band. Another would be to hetrodyne the DDS and pass the output through a bandpass filter.
     

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