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Old 25th Jan 2022, 10:13 pm   #41
Maarten
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Default Re: Hunts capacitors - always bad?

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Originally Posted by dazzlevision View Post
They are called Duolectric "Supermoldseal" metalled paper & film capacitors.

Construction: metallised lacquered paper and Polyester film interleaving, enclosed in "Supermoldseal" cast epoxy resin.
That construction (also called mixed dielectric) is typical for 'heavy duty' capacitors in the 1970's, but not impossible for the 1960's (though many manufacturers still preferred all-paper capacitors). I don't know about Hunts specifically.

It's less unreliable than all-paper construction, but the paper part of the dielectric usually breaks down just as in paper capacitors. The capacitor might not show leakage at reasonable voltages, but the capacity would often measure notably above nominal in that case.
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Old 25th Jan 2022, 10:50 pm   #42
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Default Re: Hunts capacitors - always bad?

I have bags of these. Brand new still in military packaging.
Have a look at my LCR meter readings. These were made in 1971.
I of course dont use them, I keep them to show other peeps that I "teach" what to look out for in old amplifiers/radios.

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Old 26th Jan 2022, 8:10 am   #43
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Default Re: Hunts capacitors - always bad?

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"Call for Hunts!"- old TV advert.Les
Yes, I remember them - mixers for alcoholic drinks (tonic, soda, ginger ale, etc.).
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Old 26th Jan 2022, 10:15 am   #44
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Default Re: Hunts capacitors - always bad?

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I have bags of these. Brand new still in military packaging.
Have a look at my LCR meter readings.
The problem is knowing what they would have tested like when new.

But how do you find your NOS capacitors on an insulation (high voltage) test?
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Old 26th Jan 2022, 12:09 pm   #45
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Default Re: Hunts capacitors - always bad?

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Originally Posted by joebog1 View Post
I have bags of these. Brand new still in military packaging.
Have a look at my LCR meter readings. These were made in 1971.
I of course dont use them, I keep them to show other peeps that I "teach" what to look out for in old amplifiers/radios.

Joe
Does the second photo show 27 ohm?
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Old 26th Jan 2022, 1:57 pm   #46
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Default Re: Hunts capacitors - always bad?

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Does the second photo show 27 ohm?
Looks to me like an ESR of 27R which is not great, but that is not testing leakage. Those kind of testers do not usually do that.

ESR testers can be useful but it is quite possible for an old paper capacitor to be quite leaky while still having a good ESR. The point is that it does not take much leakage to be very damaging in some parts of a circuit.
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Old 26th Jan 2022, 2:00 pm   #47
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Default Re: Hunts capacitors - always bad?

Thanks for that, I have never used one of those testers. Either an AVO or 100-200 volts and series meter with a current limit resistor.
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Old 26th Jan 2022, 2:00 pm   #48
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Default Re: Hunts capacitors - always bad?

I think we should have a 2 minute silence for Hunts. This service guy seems very happy to fit them. The company was 51 years old when this advert was published in the W.W. in 1952. John.
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Old 26th Jan 2022, 2:13 pm   #49
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Default Re: Hunts capacitors - always bad?

Hunts' little brown jobbies fed and clothed TV repairmen's families for around two decades.

Look what happened to the business after they'd gone!

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Old 26th Jan 2022, 3:18 pm   #50
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Default Re: Hunts capacitors - always bad?

Yes! J.
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Old 26th Jan 2022, 11:33 pm   #51
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Default Re: Hunts capacitors - always bad?

Yes to two of your questions. Yes its 27 ohms.
Yes I have done high voltage tests. A dc power supply a resistor and a VTVM to check leakage.
After reforming ( yes they do get a bit better ) my VTVM still reads some leakage. This varies cap to cap but overall I class them as useless for coupling/decoupling. I have changed so many over the years I simply don't bother making problems for myself.

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Old 27th Jan 2022, 11:15 am   #52
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Default Re: Hunts capacitors - always bad?

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After reforming ( yes they do get a bit better )
Paper capacitors do not "reform" in the strict sense of that. While powered they will electrolyse some water away but this usually destroys the metal film plates so the capacity goes down.

What does work very well I have found is heating them under high vacuum while monitoring the leakage. I have restored a few waxy capacitors to good working order (undetectable leakage at room temperature) albeit with some loss of capacitance which I put down to the loss of the polar OH groups that were in the paper, then as water and now sucked out of them.

Returning to the topic: I tested a couple more of the blue Hunts in a more rigorous way and they appear to be perfect*. So perhaps whatever date they are is the watershed moment in Hunts production?

* they appeared better than perfect so I have another puzzle: my Lafayette capacitor tester gave a tiny negative leakage reading! I can only think it is an effect of ripple on its supply so perhaps its own capacitors are failing!
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Old 27th Jan 2022, 11:56 am   #53
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Default Re: Hunts capacitors - always bad?

These caps may be superior to the Mullard mustards-only by dissecting one can we even actually know their construction.No one has said they have had failures with them If I had some I would use them wherever suitable,just as I might use a mustard. Les
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Old 27th Jan 2022, 1:28 pm   #54
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Default Re: Hunts capacitors - always bad?

When new, paper capacitors and especially mixed dielectric capacitors are indeed superior to polyester capacitors. The important word being "new". While mixed dielectric often lasts longer than paper only, it will fail.
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Old 27th Jan 2022, 3:39 pm   #55
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Default Re: Hunts capacitors - always bad?

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When new, paper capacitors and especially mixed dielectric capacitors are indeed superior to polyester capacitors.
That's interesting - in what way?
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Old 27th Jan 2022, 4:55 pm   #56
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"Call for Hunts!"
There's an anagram waiting... Back on topic, not all are bad, some are bloomin awful. I remember as a kid (aged about 10, yes playing with hundreds of volts, DC too) when you melted the outside of the black ones with a soldering iron it smelt like poo.
 
Old 27th Jan 2022, 5:03 pm   #57
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Default Re: Hunts capacitors - always bad?

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when you melted the outside of the black ones with a soldering iron it smelt like poo.
....thanks for the reminder
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Old 28th Jan 2022, 12:03 am   #58
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Default Re: Hunts capacitors - always bad?

I still use paper caps. BUT I use brand new ( maybe 40 years old though ) Paper In Oil capacitors.
I do have quite a collection of them. I have tested them vigorously and have not yet had a single failure.
Essentially they don't leak!! ( within reason ). The leakage that does exist doesn't change any biassing conditions on grids for instance.
These caps are in a soldered together can, that has a THICK plastic sleeve covering it. The end seals are glass. The plastic sleeving has the caps information printed thereon.
Most of you will have seen this type of cap on the inputs of CRO's and VTVM's.

These PIO are also ex Australian military and are individually packed in thick paper envelopes that are screened inside with aluminium foil and filled with dry air. The envelopes are sealed somehow and a pair of stout scissors is required to open them. They are extremely difficult to tear by hand, although some do. One capacitor to an envelope.
Each capacitor is rolled up in a piece of soft tissue. It's rolled about 10 times, the connecting leads are then bent over gently along the sides of the cap so that neither the leads or case are damaged. I wonder what the cost was to the military?

The photo shows what I am talking about. One problem I do have, is that I don't have many values.
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Old 30th Jan 2022, 10:57 pm   #59
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Quote:
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When new, paper capacitors and especially mixed dielectric capacitors are indeed superior to polyester capacitors.
That's interesting - in what way?
Higher dV/dt and I think also lower loss factor. Somewhere in between polyester and polypropylene (which suggests that polypropylene is always a good replacement). Also apparently easier to make capacitors rated for higher voltage ratings, as polyester capacitors for voltages above 630V are very rare.

This is the reason that even Philips, who were quite early to adopt polyester capacitors, still used paper or mixed dielectric capacitors in some critical positions for roughly 30 years.

Last edited by Maarten; 30th Jan 2022 at 11:03 pm.
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Old 31st Jan 2022, 12:21 am   #60
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Default Re: Hunts capacitors - always bad?

Thanks! I'd not heard that before, always good to learn...

Certainly polypropylene is not easy to process - metallising it is difficult, too. And I can imagine polyester sheets, thick enough to withstand 630V-plus, might be either too rigid to roll up.

The problems have been cracked eventually, but what you say does explain the existence of the mixed dielectric.
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