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Author Topic: Neutralizing Coil (Rewind?)  (Read 7697 times)
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Rick K5IZ
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« on: May 15, 2007, 09:48:12 PM »

Below you can see a picture of the neutralizing coil from the BT-20-A.  I hope the photo shows the windings well enough for you to see the insulation is off on a considerable portion of one side.  The coil (1L8 on the schematic) is placed within a few inches of one of the 810 finals and I'm sure the heat from that tube helped in the deterioration of the enamel insulation.  My question is, do you think it's possible to simply recoat the winding with a good laquer and hope the inside of each wire is still insulated from the other or do you think it would be a lost cause and I should rewind the coil?  I'm not hankering to rewind, but will if necessary.  The coil is easily reached and can be taken out without trouble at any time.  However, I don't want to take a chance on trashing my tubes or maybe worse.  I can't recall experiencing this before, so I am shooting in the dark.  Any advice?  Anyone had this problem with any coil with enamel coated wire?

Thanks for any assistance.

73,
Rick/K5IZ


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W8KHZ
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« Reply #1 on: May 16, 2007, 07:40:42 AM »

Rick,

If the loss of enamel insulation on the coil was caused by a light, superficial scraping of the windings, I think you might be able to get away recoating with coil dope or laquer. In this case, however, the deterioration of the enamel appears to be the result of heat, so if it were me, I wouldn't trust that the enamel between the windings would still be good. In this case, I would rewind the coil. Rewound, it will then be unlikely to give you problems for the forseeable future and it will look nicer too.

These are just my thoughts... no, I'm not an expert... but I did stay at a Holiday Inn Express last night...   Grin

Good Luck es 73,
Brian - W8KHZ
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Currently running a big homebrew transmitter (pair of 250THs modulated by a pair of 810s) paired up with a National HRO-50.  I also run a BC-610-I / NC-2-40D combo which is a lot of fun too.

Catch you on 75M AM!
Rick K5IZ
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« Reply #2 on: May 16, 2007, 09:39:32 AM »

Hi Brian,

The more I look at it the more I feel you are right and I should just bite the bullet and rewind.  I'll see if I can find my gauge to check the size of the wire and order a spool.  It has several taps, so it will be a bit more of a challenge, but I don't see any major problems.  If anyone has advice on how to to proceed with the winding, please let me know.  The last coil I wound of this size was on an old oatmeal box for my crystal set.  Trust me, that was a LONG time ago!

Thanks for the advice...
Rick
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k4kyv
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Don
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« Reply #3 on: May 16, 2007, 01:14:10 PM »

Unless it was close-would over a smooth ceramic coil form, there should be little grooves in the ceramic form to maintain some spacing between turns.  Look at it with a magnifying glass to see if the turns are actually touching.  If not, I wouldn't worry about it.  Bare wire would have worked just as well.  If you are concerned about the appearance, I would simply dope the entire coil with some kind of lacquer or varnish. It should have no effect on circuit operation at all.

If the turns are physically touching, then rewinding would be the only option.

It is not unusual for the enamel to flake off wire with age.  I have a large nearly unused roll of #12 enamelled wire, and you can scrape down to bare wire with your  fingernail.  About the only thing I use that wire for is ground radials or coils with spaced turns.  No doubt this is the cause of some transformer failures when adjacent turns short out due to deteriorated enamel insulation.  I  have an old Hunter ceiling fan that still works, but the enamelled wire that makes up the coils is so fragile that you can literally rub the insulation off with your finger.

I have never been a fan of inductive neutralisation, since each setting of adjustment holds for one frequency only.  That was fine for a broadcast station, but is totally unsatisfactory for amateur work where you will be moving around on the band.

Those who run broadcast transmitters, but insist on keeping them 100% stock and refuse to make any changes whatsoever so that they would work more satisfactorily in amateur service, and use one crystal to operate one frequency are doing AM a disservice.  This is no different from slopbucketeer "groups" that keep their digital displays set to one frequency and one  frequency only, where they have declared ownership, and never operate anywhere else.

How is the neutralisation adjusted?  By varying taps on the coil, or is there a parallel capacitor to resonate the coil to exact frequency?

I much prefer the circuit with a tapped grid or plate coil and variable capacitor.  You can usually make it work over at least one ham band without having to reset anything.  A carefully laid out circuit should work over the range of several adjacent bands.  Push-pull cross-neutralised finals theoretically should hold neutralisation from DC to daylight.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
Rick K5IZ
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« Reply #4 on: May 16, 2007, 02:16:32 PM »

Thanks for the reply, Don.  Unfortunately, the coil is close wound on a smooth ceramic surface.  Rewinding is my only choice, it appears.  The neutralization scheme uses a parallel variable capacitor and tapped coil configuration allowing considerable latitude within a single band.  I am going to put it 160 meters so I doubt it will be any problem having at least two fixed frequencies.  If I use a VFO of some type I still think it will neutralize within the 160 meter band.  I had considered going for 75 meters too, but I'm afraid I'd have to completely rework the neutralizing section to get there.  I'll know more about how far everything will stretch once I have it fired up on the original BC frequency.

Thanks again for the advice and assistance.

Rick/K5IZ
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k4kyv
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Don
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« Reply #5 on: May 16, 2007, 08:11:43 PM »

Probably the easiest way to get it to  work on 160/75 would be to replace the entire grid tank circuit with a split stator variable cap and  balanced coil, and go to a variable cap for neutralising.  Basically the same circuit as the Gates 1 kw series uses.  My BC1-T will stay neutralised across the entire 160m band.  Never tried it on 75.  My  homebrew rigs use pushpull triode finals and they stay neutralised regardless of what band they are tuned to.

The inductive neutralising might hold across a major portion of 160.  It works by including the tube capacitance as part of a parallel tuned circuit from grid to plate.  The extremely high impedance of the unloaded parallel tuned circuit blocks the feedback through the grid-plate capacitance.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

- - -
This message was typed using the DVORAK keyboard layout.
http://www.mwbrooks.com/dvorak
WZ1M
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« Reply #6 on: May 23, 2007, 03:55:39 PM »

Ricks coil for the BT-20-A arrived today. Its toast. I think Rick nailed it when he said the heat from the tubes was the problem. Looking at the pattern of insulation failure on one side, more than another side, heat was the problem. Should have it ready to go back to him in a few days.
All the best, Gary...WZ1M
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