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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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12th Jan 2017, 10:19 am | #1 |
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Anatomy of a 1936 waxie
I am re-stuffing the capacitor box in a 1936 HMV gram deck, and I was interested to see the internal construction of the hysteresis motor's two 0.7uF phase-shift capacitors which I eventually extracted from the depths of the pitch.
The capacitor is formed from a 1" wide length of a sandwich of waxed paper and aluminium foil, rolled up to an overall diameter of about 7/8" (22mm) and I was quite impressed by the quality of construction and the materials used. From inside to outside, the strip comprises five elements as follows: Inner aluminium foil 0.4 thou' (0.01mm) thick Waxed paper dielectric 1.0 thou' (0.025mm) thick Outer aluminium foil 0.4 thou' (0.01mm) thick Waxed paper insulation 0.6 thou' (0.015mm) thick Outer waxed paper layer 0.8 thou' (0.02mm) thick The capacitors are rated at or close to mains voltage. The pictures hopefully show the construction. The connections to the inner and outer foils were made by very thin copper strips, one near the centre of the roll and one near the outer edge, wound up tightly within the roll. The entire component had been encapsulated in black pitch rather than wax. Phil
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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 12th Jan 2017 at 10:21 am. Reason: Dimensions clarified |
12th Jan 2017, 7:11 pm | #2 |
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Re: Anatomy of a 1936 waxie
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12th Jan 2017, 7:37 pm | #3 |
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Re: Anatomy of a 1936 waxie
The dielectric strength of waxed paper is quoted at 40-60 MV/m, so a sheet of waxed paper 0.025mm thick works out to have a voltage rating of 1,000 volts DC, quite adequate to withstand the peak voltage of the UK mains. I imagine the dielectric used in X2 caps is polyester or similar, but I've no idea of the detail. It must be thinner than the paper as the modern caps are smaller for a given capacitance and voltage rating, but I'm just guessing, although I'm sure someone will know!
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12th Jan 2017, 10:17 pm | #4 | |
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Re: Anatomy of a 1936 waxie
Quote:
This was posted on the thread about RIFA X2's by Kalee20: "And the capacitor itself is made from zinc metallised paper" So that is why I was interested to compare the insulation thickness. Last edited by Argus25; 12th Jan 2017 at 10:23 pm. |
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12th Jan 2017, 10:56 pm | #5 |
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Re: Anatomy of a 1936 waxie
I can't really comment, as I've never dismantled a Rifa X2 capacitor to find out.
My purpose for posting the images and dimensions of the vintage capacitor was more for historical interest, and to show the internal construction of a component that was in common usage in the 1930s and onward.
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13th Jan 2017, 12:20 am | #6 |
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Re: Anatomy of a 1936 waxie
Yes Phil, and very interesting it is too!
Steve.
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13th Jan 2017, 1:10 pm | #7 |
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Re: Anatomy of a 1936 waxie
Was the component actually defective?
TimR
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13th Jan 2017, 1:27 pm | #8 |
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Re: Anatomy of a 1936 waxie
Yes, low capacitance causing reduced motor torque. Someone had tried to repair it before, where the rubber insulation had perished at the exit point from the capacitor box.
I overhauled one of these motors a while ago, and new caps got it running like new.
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13th Jan 2017, 2:23 pm | #9 |
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Re: Anatomy of a 1936 waxie
When you stripped the component, was there any visible evidence of what had caused the low capacitance? As you noted in your original post it comprises only five elements.
I possibly sound as if I'm interrogating you about it but I'm just curious. I am either blessed or cursed with an inquiring mind. I can never decide which! TimR
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13th Jan 2017, 2:35 pm | #10 |
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Re: Anatomy of a 1936 waxie
A perfectly valid question! I don't mind being interrogated... well, sadly, in the process of melting and scraping out the pitch, then pulling out the two capacitors with pliers, I did some damage to the capacitors themselves. The lead-out wires also pulled out of the rolls. I unravelled some of the foil/paper layer but there wasn't any visible damage. However, the outer turns showed some evidence of oxidisation on the surface of the aluminium foil. My only suggestion is that this causes higher resistance at the copper terminations, which are only held in place by mechanical pressure. This could have caused an increase in the ESR. These capacitors are in series with the hysteresis motor windings, so I guess that an increase in series resistance would cause the cap to become more lossy.
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13th Jan 2017, 9:18 pm | #11 |
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Re: Anatomy of a 1936 waxie
Thanks for taking the trouble to answer to answer that so comprehensively. Do you think that intimate contact between dissimilar metals had anything to do degradation of the contact area or was it just, simply age related.
Tim
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13th Jan 2017, 11:34 pm | #12 |
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Re: Anatomy of a 1936 waxie
Not sure, Tim. The copper contacts were bright and clean enough when they pulled out. What I was thinking was that the aluminium foil itself had oxidised. Rather than being bright and shiny, the foil surface was dull grey in places. The presence of an oxide layer may or may not have increased the contact resistance, and may also have had an effect on the capacitance value.
It is interesting to speculate on the failure mechanism of waxed-paper dielectric capacitors. My opinion is that atmospheric moisture slowly gets in over many years, causing DC leakage across the dielectric, causing heat dissipation. Thus waxies become leaky, but do not necessarily lose capacitance, as I've discovered when testing NOS waxies. The well-known 'dripping wax' appearance is usually a dead giveaway. This particular component, being potted in pitch rather than wax, doesn't show any evidence of having been getting hot, hence my thoughts about oxide formation on the aluminium foil, reducing its capacitance and increasing its ESR. The truth is, I don't know and I'm only speculating! Either way, we can't really complain about a component that has deteriorated slightly after 80 years' service.
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14th Jan 2017, 12:24 am | #13 |
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Re: Anatomy of a 1936 waxie
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14th Jan 2017, 8:39 am | #14 |
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Re: Anatomy of a 1936 waxie
That looks very similar to my recollections of a smoothing capacitor that died in a radiogram I had about fifty years ago as a teenager. Unfortunately, I didn't think of giving it an autopsy - I simply replaced it with something about a quarter of the size and was pleased to have my radogram up and running again. All sadly long gone now.
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14th Jan 2017, 9:57 am | #15 |
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Re: Anatomy of a 1936 waxie
Kevin, well done on extracting those caps virtually intact! I made much more mess. The job really needed a blowlamp or heat gun to melt the pitch, but due to the Museum's Health and Safety policy, all I had was a fan heater.
Your HMV deck sounds excellent. Ours doesn't have a lid.
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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 14th Jan 2017 at 10:01 am. Reason: Addendum |
14th Jan 2017, 8:57 pm | #16 |
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Re: Anatomy of a 1936 waxie
A very interesting autopsy of a failed paper dielectric capacitor. I guess that we all come along lots of these in our sets. Their inherent leakage is something that I remember well when I used to buy new old stock ex WD components pretty much straight out of stores with protective wax paper wrapping, tropical packaging etc. In my experience, their leakage then was much as we find it today.
So my question is: Is there evidence that leakage occurs mainly in the first few years of life and then levels out, or does it get worse and worse as the decades pass? I'm often pleasantly surprised by wax capacitors that remain serviceable after 70 years or so, sometimes even in critical coupling applications, and certainly in decoupling positions. I prefer to leave these in place, but is leakage likely to increase over the next 20 years or so? Martin
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14th Jan 2017, 10:33 pm | #17 |
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Re: Anatomy of a 1936 waxie
I wish I could answer your question, Martin. Unfortunately, the evidence is mixed. I have come across brand-new old-stock unused Hunts Mouldseal capacitors which were more like 10 kilohm resistors, and in contrast I've found ancient waxies testing perfect in pre-war sets.
If you think back to the typical internal atmosphere and air quality in buildings in the decades when these components were being made, it would almost certainly have been moister and more laden with carbon-based smoke particles from both coal fires and tobacco than in the late 20th century. Some of this moisture and conductive contaminants would have been absorbed by the dielectric paper, and then sealed forever within the capacitor's encapsulation. I think it's more to do with the dielectric material, as paper capacitors routinely become leaky whereas wax-dipped mica dielectric capacitors (probably made under identical conditions) are virtually immune.
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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 14th Jan 2017 at 10:35 pm. Reason: Clarity |
15th Jan 2017, 6:12 am | #18 |
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Re: Anatomy of a 1936 waxie
I think its the acid in the paper, reacts with the aluminium foil when any moisture gets in.
The leakage is always paper/foil caps, as you say Phil, mica just doesn't do it. There is also the operatives finger sweat, especially females, to consider. I bet they didn't assemble the mica sheets by hand but rolls of paper and foil would be easier to handle. |
19th Jan 2017, 12:41 am | #19 | |
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Re: Anatomy of a 1936 waxie
Quote:
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19th Jan 2017, 12:50 am | #20 |
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Re: Anatomy of a 1936 waxie
Yes Phil I used a blowlamp on its lowest flame until the pitch started bubbling, then I just gave the wires a gentle tug. Then I turned the heat up and poured the rest of the stuff out of the tin into that pot, with a view to re-potting the new capacitors. However I decided to encase the new caps in aerated polyethylene foam sheets (from the packaging of some industrial IGBTs).
I got the procedure from forum members who had unpotted HMV's radio capacitor cans. I can confirm the pitch stinks when warm and I did it in the garage. The lid of the 'gram was home-made by a previous proud owner. Both capacitors tested about a quarter of their value, the motor only hummed. Startup torque on these motors is poor (once up to speed it's considerable) so freedom to move of all motor parts is paramount.
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