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Hammarlund HQ-129-X - Voltage on chassis - Any ideas?




 
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Author Topic: Hammarlund HQ-129-X - Voltage on chassis - Any ideas?  (Read 8254 times)
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W4LTM
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« on: January 05, 2005, 08:24:33 PM »

Hi gang,

I have a HQ-129-X receiver that I have finished restoring.  It picks up stations well and is real sensitive.  This 129 was completely unmolested when I went through it and recapped etc.

I have noticed that when the power is off I have 150 V between the chassis and either side of the antenna connections (not the ground connection).  When the power is on I have about 25 V between the chassis and either antenna connection.

When I disconnect the antenna I have no voltage between the chassis and connectors.

I have had this issue for awhile and actually posted about it in a different forum, but after all was said and done, I chalked it up to my Vertical with the big loading coil - I have large 250K towers real close to the QTH and figured that the coil was pulling the power from the air.

Now I am using a Inverted V for 80 and I am having the same issues.

I would not think that I could be pulling in energy without a coil of some type (I made sure my transmatch was set to direct).  After searching on google I saw a couple similar posts (wife got shocked touching sink and radio etc...), but there were no real answers as to why this issue is occurring.  

Any ideas?

Thanks in advance,

Matt
W4LTM
Kemah, Texas
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wavebourn
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« Reply #1 on: January 05, 2005, 09:19:27 PM »

Matt, it has couple of capacitors, they are connected in series, and middle point of them is grounded. It is a RF EMI filter. Indeed, when the set is powered on, you'll have a half of voltage on the chassis as the result, if a switch is open you have full voltage.
0.1 uF will not kill, but may give sensitive sensations. Wink

Just ground the chassis to have zero volts on it, that's it. But before check DC resistance between power socket and chassys, if it conducts it means some capacitor is shorted, or something else and you need to fix your receiver.
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W4LTM
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« Reply #2 on: January 05, 2005, 11:04:46 PM »

Thanks for the reply, but why does the voltage drop to 0 (on or off) when the balanced antenna is removed?  I just can't grasp that piece of it.  Maybe I am missing a simple thing do to a brain cramp, but it still eludes me none the less.

Thanks
Matt
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wavebourn
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« Reply #3 on: January 05, 2005, 11:17:41 PM »

Quote from: W4LTM
Thanks for the reply, but why does the voltage drop to 0 (on or off) when the balanced antenna is removed?  I just can't grasp that piece of it.  Maybe I am missing a simple thing do to a brain cramp, but it still eludes me none the less.


Did you measure AC voltage on your antenna to the ground? Did you measure AC resistance between antenna and ground?
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w3jn
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« Reply #4 on: January 06, 2005, 07:06:19 AM »

1.  Is the chassis of the radio grounded to the OUTLET (i.e. the AC service ground)?

2.  Is  the chassis of the radio RF grounded (i.e. to a cold water pipe or ground stake)?

3.  Is the transmatch RF grounded?

If the answer to any of these questions is "no", you really need to do all of the above.  As Tolly noted, the line filter caps are going to place 60 VAC on the chassis of the radio, with respect to the AC ground.  

You don't state whether you are seeing AC or DC voltage between the antenna and the radio.  To answer your question we really need to know which.

My balanced-line flattop picks up a goodly amount of static, especially during a rainstorm (it's enough to arc over the caps in a Junkston KW Matchbox).  Could be this is the cause.  And you don't need a "coil" in the antenna for electromagnetic waves to induce a voltage in said antenna (that is, after all, the purpose of an antenna!).  

One quick test would be to connect the antenna to the input of your scope (you DO have a scope - right?) and see what the P-P voltage of that radio station is.

73 John
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W4LTM
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« Reply #5 on: January 06, 2005, 10:04:00 AM »

OK - Let me go further into depth...  While I was trying to finish arranging the new shack (wife though me out of my old one) I remembered the 129 voltage issue and that prompted my question on the chassis voltage.  This answered my question on the chassis voltage: "Matt, it has couple of capacitors, they are connected in series, and middle point of them is grounded. It is a RF EMI filter. Indeed, when the set is powered on, you'll have a half of voltage on the chassis as the result, if a switch is open you have full voltage. "   I have always had it grounded so it never bothered me and I basically forgot about it until last night.  Thank you for the answer.

As to the other issue of AC voltage from my antenna: I had to run this morning so I quickly put the meter on the antenna and AC ground and I showed 1 V AC coming into the shack from the inverted V.  That was with my transmatch (Nye Viking) enabled (not sure of the settings), but when I changed the transmatch to "direct" the voltage dropped to 0.  This is happening 24-7.  Once again I suspect that the proximity of the 250K towers 300 + yards away is the cause.  (The only thing good about the towers is that they are quiet.  Well they are after I had to go around and around to get burnt insulators changed over.  Took awhile as the lines feed N. TX. and also LA.)

I will do a few more checks later to get to the bottom of everything.  Yes, everything is grounded and yes, I do have a scope.

73,
Matt
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WA1GFZ
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« Reply #6 on: January 06, 2005, 11:12:45 AM »

I wonder if it is the ratio of EMI caps to ground and antenna C to ground.
Antenna C much lower so all the voltage appears on the chassis.
Maybe you have a grounding problem in your house bringing the grounded chassis above ground creating a big loop.
A crazy thought but I would bond all your grounds and maybe add more.
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w3jn
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« Reply #7 on: January 06, 2005, 12:30:36 PM »

I'm not familiar with the Nye Viking transmatch.  Is it a pi-network, or balanced input/ouput, or what?  What is happening inside the transmatch when you switch to "direct"?

I still don't know if you're seeing DC or AC between the radio and the antenna, nor do I know if your transmatch is RF grounded.  If it is, and it is not bonded electrically to the chassis of the radio or to the AC service ground, then I would suspect a ground loop like Frank GFZ suggested.

You say you have 150V, then 20V, now 1V - what are the exact conditions under each circumstance?

73 John
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W4LTM
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« Reply #8 on: January 06, 2005, 01:23:16 PM »

Thank you for the input guys.

John - The Nye Viking Transmatch is a Pi Network.  

Info: Low Pass Pi Network tuning 1.8 to 30MHz. Heavy duty, silver-plated continuously variable inductor with 25:1 vernier dial, 7000-volt variable capacitor and 15,000V switch selected fixed capacitors on output side. Tunes 40 to 2000 ohm antennas. Also provides harmonic suppression.

The "direct" sitting I am referring is to bypass the tuner so the feedline goes direct to the antenna.

YES, I have RF ground on the Transmatch and I have that as the center grounding point as well.  All of the radios then make there way to that point thus feeding off each others AC service ground as well.  I may need to ensure all the chassis for my radios have adequate ground as well...  (Drake TR-3 with RV-3, 129-X receiver, Hammarlund Four-11 & 20 transmitter and modulator, Viking II, and Yaesu FT-1000MP Mark V)  I suppose anyone of them could cause an issue/loop at this point. -Thanks Francis


"You say you have 150V, then 20V, now 1V - what are the exact conditions under each circumstance?"

Well, the 150V AC is between the radio chassis and the antenna.  Measured from the chassis to either side of the pl-259.  This is now obviously a grounding issue along with MY IGNORANCE of not realizing that the filter caps were put to ground so if the set was not properly AC grounded then I would get voltage from the chassis to any ground source including the Transmatch - Duh on my part.  The 20V was when the switch  to the 129 was turned on.

The 1V (separate issue now) you asked about is coming in off my feedline from my Inverted V after my Transmatch.  When my Transmatch is set to bypass then I see no AC voltage (measured from AC ground to either side of the PL-259).  When the transmatch is engaged I am showing 1V AC.  This is with all radios disconnected.  Thus I am getting 1V from somewhere - thus I suspect the towers...

Thanks for the replies and for putting up with my long winded explanations.

Matt
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w3jn
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« Reply #9 on: January 06, 2005, 02:44:31 PM »

OK, now I see the situation a bit more clearly.  If' you're only 300 feet from said 250KV towers you could almost be getting voltage capcitively coupling to your antenna.  Why it goes away when the tuner is in bypass mode is interesting.  I suspect that you're using a digital VOM - which is very high impedance - and the least bit of resistance to ground will knock it down.  I wouldn't be concerned about it.

What you do need to ensure is that all chassis of all radios are electrically bonded together, and your RF ground is bonded to the AC service entrance ground.  Not to do so will invite ground loops as well as disasters in the case of a lightning strike.

73 John
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wavebourn
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« Reply #10 on: January 06, 2005, 02:48:34 PM »

Quote from: w3jn
I suspect that you're using a digital VOM - which is very high impedance - and the least bit of resistance to ground will knock it down.  I wouldn't be concerned about it.


Also an acid rain can do the job, leaving some conductivity over insulators.
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WA1GFZ
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« Reply #11 on: January 06, 2005, 02:50:49 PM »

now if you could just couple some useful power off the transmission line.
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