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Old 3rd Oct 2009, 8:11 pm   #1
'LIVEWIRE?'
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Question Which capacitors to use as aerial isolators in an AC/DC FM Radio?

I need to replace the two isolating caps. between the aerial socket and the RF coils in a VHF Only AC/DC Radio, and was about to fit a couple of 470pf 1kV ceramics, when I happened to glance at the parts list for the radio in question, which stated that the original tubular ceramics, which I accidentally fractured when removing the aerial coil assy.!, were rated at 3kV.
Since the coupling winding is completely isolated from the radio chassis, I'm wondering if 3kV types are really neccesary, but don't want to compromise on safety. My feelings are that (modern)1kv caps. are more than adequate for mains isolation, but may not withstand a lightning strike if an outside aerial is connected, but in such an event neither would 3kv capacitors, surely?(or, despite working in the radio repair trade for almost 40 years, admittedly for much of that being only concerned with car radios, am I missing something??)
N.B. I have some NOS 500pf red tubular ceramics, but there is no working voltage marked on them, hence my choice of the modern 470pf ones.
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Old 3rd Oct 2009, 8:36 pm   #2
Al (astral highway)
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

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My feelings are that (modern)1kv caps. are more than adequate for mains isolation, but may not withstand a lightning strike if an outside aerial is connected, but in such an event neither would 3kv capacitors,
Indeed, they wouldn't withstand a direct strike. But that's not the point. A very big charge can build up on an outside aerial even without a lightning strike.

By way of illustration, my parents' TV was once completely destroyed by a static charge building up on an aerial inside the loft and well away from the apex of the roof.
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Old 4th Oct 2009, 3:19 pm   #3
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

I'd have thought so long as its a class Y cap you'll be ok
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Old 5th Oct 2009, 9:40 am   #4
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

The high voltage rating was used just to make it as unlikely as possible that the capacitor would fail short-circuit. Nowdays, you should always use Y-rated capacitors between mains-connected chassis and the outside world.
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Old 5th Oct 2009, 9:48 am   #5
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

use your 1K caps, and put a Neon bulb across the Ant and Gnd, it will work as a shunt for anything above 90v.
I do this with ALL my equipment that requires an outdoor antenna, or you can fit this to the Antenna itself.

if your set gets hit by Lightning it (and other things) are going to be toast anyway, no matter What caps you use.
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Old 5th Oct 2009, 10:01 am   #6
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

It doesn't need a lightning strike to cause a build up of static on an aerial. Even a distant thunderstorm will cause sparks an inch long to jump across the the feeder of my 80 metre diople.

See this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=36909
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Old 5th Oct 2009, 10:26 am   #7
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

So far as I know class Y capacitors are not available in values as low as 470/500pf hence my choice of 1kV disc ceramics. This is a radio I have been repairing for someone else, and I don't know if an outside aerial will be used with it, but I guess adding a neon would be a wise precaution. Thanks, YT2095UK, for that suggestion. Whilst I appreciate that large static charges can build up on outside aerials, this would also be the case when an AC only mains radio, fitted with an IDENTICAL VHF tuner minus the aerial isolating capactors, was being used, and, in that case there would be a direct path to earth via the coupling coil(These are BUSH valve radios, BTW- a VHF90A and VHF61)
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Old 5th Oct 2009, 10:31 am   #8
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

Yes, I got the impression that their sole purpose was safety mains-isolation in AC/DC sets like the VHF90A, hence their omission in the AC-only VHF61.

As you say, the tuner head is identical in both.

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Old 5th Oct 2009, 1:12 pm   #9
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

If you use a neon make sure it is connected to true earth.
You can't use the chassis as the neon would strike if the chassis were live.
The set side of the capacitor should have a DC path via the coupling coil.

If I remember correctly old TV isolators had high value resistors (2.2 M ohm with a suitable voltage rating) across the caps to drain the static.

Jim
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Old 7th Oct 2009, 11:23 am   #10
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

To correct an error in my original post(thanks, Jim!), there is a path to chassis through a centre tap in the aerial coupling coil, which is wound to match a 240/300 ohm dipole. You may be right about resistors being fitted accross caps in TV isolators-I don't have one to hand to check-however Bush, in their wisdom didn't fit them to the VHF90 Radio. As to fitting a neon I have no way of arranging a connection to true earth, other than via a terminal on the back panel, which, as it's not my radio, may not be used!
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Old 8th Oct 2009, 12:07 pm   #11
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

Quote:
Originally Posted by 'LIVEWIRE?' View Post
So far as I know class Y capacitors are not available in values as low as 470/500pf .....
Hi
I needed a 100pF Y rated capacitor and could not find one either.
I eventually got round it by using a 100pf 1KV disc ceramics and a 1nF Y rated capacitor in series.

The value wasn't exactly 100pF but close enough for what I needed it for and if the disc ceramics was to fail the Y rated cap will protect the user.

Frank
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Old 8th Oct 2009, 1:17 pm   #12
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Default Re: Which capacitors to use as aerial isolators in an AC/DC FM Radio?

Given that these caps are only there for isolation, the value won't be at all critical. If you really want to keep the original capacitance you can fit a standard low value cap in series with the larger class Y cap.

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