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Old 19th Jun 2024, 2:12 pm   #1
DonaldStott
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Default Hunts Capacitors

These Hunts Mouldseal Capacitors are usually change on sight for me as they are terribly unreliable and leaky - often as not they are cracked or showing other signs of damage!

They are normally replaced with modern polypropylene caps but I am repairing a Murphy A272C where there is a Hunts capacitor (0.005uF 350V DC) in the VHF tuner and its value is AFAIK quite important. Currently FM is completely dead on this set although MW and LW are ok.

So my question is what type should be used to replace the cracked Hunts capacitor in this sensitive location?
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Old 19th Jun 2024, 2:16 pm   #2
paulsherwin
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Default Re: Hunts Capacitors

I doubt if the value is critical if the designers specified a Mouldseal. At a guess without a circuit, it will be some sort of decoupler.
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Old 19th Jun 2024, 2:22 pm   #3
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Default Re: Hunts Capacitors

Snippet of circuit attached for reference.

Can you ID which circuit ref. it is, please, Donald?
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Old 19th Jun 2024, 2:44 pm   #4
paulsherwin
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Default Re: Hunts Capacitors

I imagine it's C7 in the snippet.
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Old 19th Jun 2024, 2:57 pm   #5
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Default Re: Hunts Capacitors

Yep - C7 it is.
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Old 19th Jun 2024, 3:03 pm   #6
paulsherwin
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Default Re: Hunts Capacitors

That doesn't look at all critical, though it will indeed cause problems if it's leaky.
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Old 19th Jun 2024, 3:31 pm   #7
HamishBoxer
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Default Re: Hunts Capacitors

.0047uF will be fine and if C7 is duff, that will stop FM.
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Old 19th Jun 2024, 4:58 pm   #8
DonaldStott
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Default Re: Hunts Capacitors

My question was really about what type of capacitor to use, I've plenty of 0.0047uF in my stocks - here is another snippet from the Murphy Service Manual:-

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What's the difference/impact if any between a 'paper tubular' and a 'metallized paper tubular' apart from different materials?
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Old 19th Jun 2024, 5:07 pm   #9
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Default Re: Hunts Capacitors

A metallised capacitor of any dielectric is likely to be smaller and cheaper than one made with separate foils and the same type of dielectric.

John
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Old 19th Jun 2024, 5:36 pm   #10
paulsherwin
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Default Re: Hunts Capacitors

I honestly don't think there's anything to worry about. Just use whatever you have to hand.
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Old 19th Jun 2024, 7:42 pm   #11
DonaldStott
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Default Re: Hunts Capacitors

Quote:
Originally Posted by jjl View Post
A metallised capacitor of any dielectric is likely to be smaller and cheaper than one made with separate foils and the same type of dielectric.

John
Thanks John - I note that your Avatar is a Murphy A272C.

Let me know if you have any hints or other inside information - PM me if more appropriate?

Quote:
Originally Posted by paulsherwin View Post
I honestly don't think there's anything to worry about. Just use whatever you have to hand.
Thanks again Paul - as usual I'm not worrying, just interested from an educational point of view.
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Old 19th Jun 2024, 7:49 pm   #12
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Default Re: Hunts Capacitors

I think you'll find that's a neutralising cap. Nothing critical. One of the Mullard mustards would go in there nicely or any other polyester type cap.
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Old 19th Jun 2024, 9:01 pm   #13
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Default Re: Hunts Capacitors

If you look around the circuit, you'll see that all the capacitors in resonant circuits are of the order of a few to several tens of pf. There is one in the 100pf region coupling two resonators together. At 5000 pf, that mouldseal is of trivial reactance compared to impedance levels in use in the tuned circuits. It's essentially a DC block. Given a free choice of all capacitor types, I'd probably opt for a 4700pf ceramic.

It's a bit big for NP0 dielectric, but X7R would be fine. Various plastic dielectrics are going to be better than the original paper part, anyway.

David
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Old 19th Jun 2024, 9:15 pm   #14
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Default Re: Hunts Capacitors

Quote:
Originally Posted by paulsherwin View Post
That doesn't look at all critical, though it will indeed cause problems if it's leaky.
I don't think it will - both sides are nearly at HT potential when FM is active.

If it's duff though, there might be unwanted oscillation which could kill FM. It looks like a sort of neutralising capacitor to me too, as others have commented.
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Old 21st Jun 2024, 10:11 am   #15
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Default Re: Hunts Capacitors

Quote:
Originally Posted by Radio Wrangler View Post
At 5000 pf, that mouldseal is of trivial reactance compared to impedance levels in use in the tuned circuits. It's essentially a DC block.
Thanks David - when someone uses the words 'reactance' and 'impedance' in the context of 'tuned circuits I'm obliged to go and look up my copy of Scroggie to try and undertsand this!
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Old 21st Jun 2024, 10:34 am   #16
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Default Re: Hunts Capacitors

It comes down to proportioning of values and the Q of a tuned circuit, not as a resonator thought of in isolation, but of the Q it has when loaded with all the other things it is in circuit with.

If I say 1uH and 100pF, you can calculate the resonant frequency from one over two pi root L C.

But 100nH and 1000pF resonate at the same frequency.

As a series tuned circuit they both approach zero ohms, but woesened by losses as ESR

As a parallel tuned circuit they both try to approach infinite ohms, but worsened by losses acting as an equivalent parallel resistance.

It's when you move away from the resonance that they look capacitive or inductive depending on which way you move off. The choice of values, the high inductance version or the low inductance version that scales the impedances OFF RESONANCE.

So the choice of how to get a resonance allows you free choice of L or C, but forces the value of the other one in order to hit the right frequency. If you know the losses and resistances presented by the circuit to the resonator, you can use the freedom of choice of L or C to pick a value that gives you whatever Q value you want, though the Q of the LC pair alone sets a maximum Q you can achieve.

So just in doing an LC tuned circuit you get to choose the centre frequency and you get to choose how sharp the resonance is. You have two components to choose, you have two dimensions of freedom and voila! you also have two parameter goals you want to hit. So it all fits together.

Q isn't just important in setting stage bandwidths, it's a handy mathematical tool that allows you to jump around between component values, equivalent loss resistances (series or parallel) and also bandwidths. It allows you to do things that ought to involve differential equations in your head.

Scroggie doesn't put things all that well. If you look at things from various different directions, you eventually find a view that makes things look a lot easier. So things are easier and simpler than they seem from reading Scroggie. Scroggie is right, but sometimes fails to show things the easy way. Don't let him put you off.

If something looks too difficult, then there is a good probability that you simply haven't found the right viewpoint to make it simple.

David
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Old 22nd Jun 2024, 8:29 pm   #17
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Default Re: Hunts Capacitors

It's ac grounding (decoupling) the top end of L6 at 10.7MHz. It's a wierd circuit, as it's in series with C13 (in order to connect to ground), which is smaller in value - that has a reactance of -j30 at 10.7MHz.
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Old 23rd Jun 2024, 12:07 am   #18
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Default Re: Hunts Capacitors

So that means that there is a significant high-frequency voltage on C13, fed to it by our capacitor under consideration.

That appears on L3 top end. It smacks of some sort of neutralisation.
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Old 24th Jun 2024, 12:17 pm   #19
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Default Re: Hunts Capacitors

C13 should tie the top end of L3 to gnd as regards RF there should be no RF signal here, C7 is probably to ensure L6 and L3 top end are always at the same RF low signal maybe avoids some parasitic oscillation due to gnd paths from C13 and any separate capacitor going to top end of L6. Linking them together ensures they will be at the same point for RF signals The RF signal path does not go through here at all and L6 and L3 top end need to be decoupled for RF which is what the capacitors are doing surely.

I can't see how they could be used to feedback any signal for neutralisation
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Old 27th Jun 2024, 8:00 pm   #20
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Default Re: Hunts Capacitors

C7 now replaced but appreciate that I now need to replace C13 which is another Hunts Mouldseal: -

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High voltage but low capacitance so what would be an appropriate type in this situation?

I'm assuming that a Radial Ceramic Disc Capacitor High Voltage, 1KV 470pF would be ok?
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Last edited by DonaldStott; 27th Jun 2024 at 8:27 pm.
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