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Author Topic: Newbie: ... and then there was smoke  (Read 1067 times)
M0PXO
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« on: January 16, 2019, 12:05:43 PM »

I have an Icom IC-7200 transceiver (secondhand), MFJ-945E ATU (new), an Alinco DM-330MWII (new) and a HyEndFed antenna (new).  Cabling up the antenna directly to IC-7200 works OK.  I then ran the antenna through the ATU, which requires a 13.8v supply to power the meter backlight and I connected this.  I also have an RF ground from the transceiver to the earth tag on the back of the PSU and then to a ground stake outside.

I powered everything on for the first time.  There was a smell of burning plastic so I powered off the transceiver but noticed that 5A was still going somewhere.  Then there was smoke from the supply cable to the ATU.  I powered everything off.

I then tried without the backlight supply connected.  The current drawn was low/normal, but the meter needles on the ATU do nothing when I hit PTT.

Is the ATU duff?
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W1VT
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« Reply #1 on: January 16, 2019, 12:32:21 PM »

You need to hit the PTT and talk into the microphone with the mic gain properly adjusted to get power output on SSB.
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KD0REQ
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« Reply #2 on: January 16, 2019, 02:08:39 PM »

PTT has nothing to do with magic smoke in the air. what I'd do is start triaging. find out what made stink.  get a "carnival" light socket at the hardware store, Edison base with two wires, put a 100 watt bulb in it. hack a cheap RF cable to that and plug it in the radio. power up the radio, tune to a dead spot just in case (even a light bulb is a sorta antenna) and following the manual for the 7200, key up and watch for light from the bulb. my experience is with that setup, you are probably around a SWR under 2:1. light, radio is not dead yet.  check to be sure there is correct polarity and voltage on the wall wart powering the MFJ tuner, mine requires 12 volts, center pin positive. hit the red button and see if it lights. measure the antenna coax with an ohmmeter, any reading other than a short (under 2 ohms) is good. connect the antenna directly to the radio, and on low power, key up again and make sure it doesn't trip out from high SWR.

if all that works, I'd suspect the tuner. got an Elmer nearby? always helpful to have somebody with some experience nearby when you do something interesting the first time.
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KD0REQ
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« Reply #3 on: January 16, 2019, 02:14:51 PM »

as far as burning up the cable, if your tuner takes (or gets) center pin negative, you could have just put as many amps as possible from the radio supply into the meter power cord and that could be the whole problem. the tuner will work just fine without the backlight. assuming it's not been dropped too many times and developed shorts and opens.  again, a local Elmer is a nice thing to have.

right now, from what you wrote, my chief suspect is a short near the connector on the wall wart.
« Last Edit: January 16, 2019, 02:34:55 PM by KD0REQ » Logged
M0PXO
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« Reply #4 on: January 16, 2019, 02:40:49 PM »

Thanks all for the above.  I extended the power supply cable to the ATU.  I must have made a mistake somewhere as I've metered out everything and all looks OK.  I then connected the frazzled power lead up (without my extension) and powered it all on and it works without a problem.

Thanks for the guidance on the ATU SWR.  I was just being stupid.  So is the correct procedure for tuning the antenna to stick the transceiver into CW mode and hold the key down?
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N9AOP
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« Reply #5 on: January 17, 2019, 11:10:17 AM »

A CW carrier is fine for tuning but you should be aware of what the maximum tuning power is for your particular tuner and stay within those limits.  Otherwise you may see more smoke.
Art
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G3RZP
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« Reply #6 on: January 17, 2019, 02:54:17 PM »

Best to keep the power down to a level that just shows on the SWR meters until you have the tuner adjusted for a 1:1 SWR.
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WA7ARK
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« Reply #7 on: January 17, 2019, 08:20:03 PM »

How is the 12Vdc connected to the Tuner back-light? If it is through one of those coaxial DC plugs, make absolutely sure that you understand if the center pin is ground or +12V.

Remember that in a 12Vdc powered station, the negative side of the 12V power distribution travels on the coax shields as well as the black wire...
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Mike, WA7ARK
M0PXO
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« Reply #8 on: January 18, 2019, 11:43:16 PM »

Thanks for all the responses.  I have a UK foundation license and so my transmitter is limited to 10w. The manual for the ATU says tune at low power but it doesn't set what qualifies as low. I don't change the inductor setting while transmitting and so hopefully I'm not risking the ATU.

I think you are absolutely right about the common negatives being the cause of the short. I remember I tested everything before I connected the coax  between the transceiver and the ATU and there wasn't a problem. I must have swapped the polarity with the extension lead.
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