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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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11th Jan 2021, 5:43 pm | #1 |
Triode
Join Date: Oct 2020
Location: Newport, Gwent, Wales, UK.
Posts: 23
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Ekco AD76. Same capacitor keeps failing.
This is my first post on this forum, so I hope it is all OK. At the start of the lockdown last March, I decided to attend to some of my numerous radio collection that were in need of attention. I started with a “Round Ekco”, an AD76. Many hours were spent on addressing physical and mechanical issues before turning my attention to the electronics. This is where I may need some comments from the learned contributors of this forum. When I eventually got to powering it up, all went well for a while, a local station came in loud and strong on a 2 yard length of aerial wire. Then, after about 25 minutes, there was a pronounced “phut” and a large discharge of white smoke from under the chassis. The 8uf smoothing capacitor had spectacularly failed. This cap was part of an order of replacements that I had ordered from a company in Canada who seemed sympathetic to enthusiasts of vintage radios. I looked around the parts bin for another then remembered I had ordered 2 of the same cap. Installing the second one allowed the radio to perform very well, but only for another 25 minutes when that too failed in the same manner. Both caps were rated at 500v. I am now waiting for a delivery of various capacitors from RS and will try again. To round this off, all electrolytics, papers and waxies have been replaced but all the original micas have been left in situ. All valves apart from the output have been changed. Could it be that the 8/500 smoother could not tolerate the ripple current? Is there something else in the circuitry that could cause this? I would like to install the chassis back in the cabinet permanently, but my confidence has been a little shaken.
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11th Jan 2021, 5:57 pm | #2 |
Moderator
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Re: Same capacitor failing.
C29 8uF is connected across the field winding of the energised speaker. Have you checked that it's fitted the right way round polarity wise? Negative lead should go to chassis. Modern caps have the negative lead marked with a series of arrows.
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11th Jan 2021, 6:26 pm | #3 |
Nonode
Join Date: Jun 2008
Location: Coningsby, Lincolnshire, UK.
Posts: 2,814
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Re: Ekco AD76. Same capacitor keeps failing.
Interesting, it does sound like it was fitted backwards, if not then a small possibility that the capacitors were marked up wrong, were they axial leaded? Normally the end with the rubber bung in it is the positive end with those. Might be worth doing a sanity check when you fit another, connect your multimeter across it to make sure it’s receiving the voltage the right way round, and if not, switch off quick before you pop another cap!
Regards Lloyd |
11th Jan 2021, 6:29 pm | #4 |
Triode
Join Date: Oct 2020
Location: Newport, Gwent, Wales, UK.
Posts: 23
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Re: Ekco AD76. Same capacitor keeps failing.
Yes, the original 8uf was part of a group of 3 electrolytics encased in a wax block contained within a card wrapper. For the sake of originality, I carefully remove the enclosed wax block and housed the 3 caps in the remaining wrapper. All 3 negative sides terminated on a common chassis tag, so it was difficult to make a mistake. One other symptom I omitted to mention was that the audio volume is quite loud even with the volume control fully anti-clockwise. Is this a clue to anything? Many thanks for your reply.
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11th Jan 2021, 6:38 pm | #5 |
Rest in Peace
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 4,100
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Re: Ekco AD76. Same capacitor keeps failing.
Will the fact that this radio has a half wave rectifier have an effect on the ripple handling capability of the capacitor.
Mike |
11th Jan 2021, 6:43 pm | #6 |
Triode
Join Date: Oct 2020
Location: Newport, Gwent, Wales, UK.
Posts: 23
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Re: Ekco AD76. Same capacitor keeps failing.
The cap was definitely installed the correct way round. Station X suggested this could be the cause but as per my reply, it was difficult to make the same mistake 3 times as the old capacitor housing, which was retained, contained 3 electrolytics all going to a common chassis tag. Thanks Lloyd.
Regards, Ray. |
11th Jan 2021, 7:00 pm | #7 |
Triode
Join Date: Oct 2020
Location: Newport, Gwent, Wales, UK.
Posts: 23
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Re: Ekco AD76. Same capacitor keeps failing.
This is what I suspected. As a result, I THINK I have selected the version with the highest ripple current rating from the RS site although it is slightly off spec at 10/450 but given the purported robustness of modern components, I hope this won't make much difference. Many thanks.
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11th Jan 2021, 7:05 pm | #8 |
Octode
Join Date: Sep 2020
Location: Thetford, Norfolk, UK.
Posts: 1,731
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Re: Ekco AD76. Same capacitor keeps failing.
From everything that has been said, it would seem to me that the capacitor cannot handle the ripple current.
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11th Jan 2021, 7:14 pm | #9 |
Triode
Join Date: Oct 2020
Location: Newport, Gwent, Wales, UK.
Posts: 23
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Re: Ekco AD76. Same capacitor keeps failing.
I think the truth may emerge later this week on delivery of the new capacitors from RS. I'm really looking forward to putting it back together permanently! Thanks,
Ray. |
11th Jan 2021, 7:25 pm | #10 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Ekco AD76. Same capacitor keeps failing.
I would say as an approximation the ripple current rating should be at least 190mA at the frequency concerned.
Lawrence. |
11th Jan 2021, 7:40 pm | #11 |
Triode
Join Date: Oct 2020
Location: Newport, Gwent, Wales, UK.
Posts: 23
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Re: Ekco AD76. Same capacitor keeps failing.
Looks like there could be some more smoke signals in the house. The ones on order are rated at 90ma. Would two 4's in parallel handle the ripple current better? Thanks.
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11th Jan 2021, 8:28 pm | #12 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Ekco AD76. Same capacitor keeps failing.
Hi, if possible when the new caps arrive try monitoring the temp as they are running to see if it is a ripple current issue. If they start going over 40C it is time for a rethink
Ed |
11th Jan 2021, 10:34 pm | #13 | |
Dekatron
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Re: Ekco AD76. Same capacitor keeps failing.
Quote:
If you've got room, an 8uF 450Vac motor run capacitor might be more robust and possibly more like the physical size of the original. The rectifier can take up to 16uF reservoir with that series 50R resistor.
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12th Jan 2021, 8:30 am | #14 |
Dekatron
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Re: Ekco AD76. Same capacitor keeps failing.
RS do a 10uf 450vw 350ma ripple current.
https://uk.rs-online.com/web/p/alumi...itors/7258994/
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Frank |
12th Jan 2021, 9:34 am | #15 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
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Re: Ekco AD76. Same capacitor keeps failing.
^^^ Sorry that is not correct in the OP's case.
They have a ripple current of 350mA @ 100kHz. It is a third of that at 120Hz so likely a sixth at 50Hz... Filament, check the data sheets for 'frequency coefficient' (or factor) when looking at ripple current. Lots of HV caps are quoted at high frequencies (100kHz up) used in SMPS supplies etc. Their use at 50 or 100Hz seriously degrades the 'headline' ripple figure.
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12th Jan 2021, 9:38 am | #16 |
Dekatron
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Re: Ekco AD76. Same capacitor keeps failing.
Thanks, didn’t notice that.
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12th Jan 2021, 6:59 pm | #17 |
Nonode
Join Date: Jun 2007
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Re: Ekco AD76. Same capacitor keeps failing.
The Canadian capacitors may have the can connected to the positive layer of the internal capacitor.
These would be suitable for negative bias supplies. If you 'reform' the last remaining healthy Canadian capacitor you should see the current reduce over time if it is correctly polarized. Use a high value resistor, 50k or greater. If the current increases over time it would suggest that the polarity of the capacitor is different. |
12th Jan 2021, 11:30 pm | #18 |
Triode
Join Date: Oct 2020
Location: Newport, Gwent, Wales, UK.
Posts: 23
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Re: Ekco AD76. Same capacitor keeps failing.
I only had 2 of the 8/500s and they have both failed so I will have to find a suitable replacement. The brand name was MIEC. The order from RS arrived this afternoon and I am tempted to fit the 10/450 contained therein but as I suspect it could be a ripple current issue, I feel that I may have to source a more robust component. A motor run cap could perhaps suffice if I could find one small enough. Surely someone somewhere has replaced their smoothing/reservoir caps during a refurbishment. If so, what did they use?
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13th Jan 2021, 12:11 am | #19 |
Nonode
Join Date: Jun 2008
Location: Coningsby, Lincolnshire, UK.
Posts: 2,814
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Re: Ekco AD76. Same capacitor keeps failing.
I replaced them on my AC76, the AC only version of your set, can’t remember what with though, but I don’t think there was anything particularly special about it! It wasn’t physically large, it fitted into the cardboard box along with 2 other caps, if I remember there is 8+8+2uf in the box, mine had 10+10+2.2uf fitted and it’s still working fine 10 years later, I’ll have to dig through the archives and see if I noted down what caps I used.
Regards Lloyd |
13th Jan 2021, 2:59 am | #20 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
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Re: Ekco AD76. Same capacitor keeps failing.
Tried some simulations- these came up with about 200mA RMS ripple for the 8uF as in original circuit and about 235V average ripple DC on the 8uF and the LS field (about 190V p-p ripple!). This is clearly your capacitor killer!
After some iterations, I found that two 33uF 450V Vishay electrolytics (RS 324-5436) in series with 100k voltage divider chain across have a ripple rating of 260mA at 50Hz and give 16uF which the rectifier is happy with. To keep the HT DC similar to the original, the surge limit resistor needs to increase from 50R to 150R. Actual ripple current in this configuration is about 200mA, average DC V about the same and p-p ripple down to about 90V. Looks like a win-win The 100k dividers need to be rated at least 0.5W and the 150R surge limiter at least 10W. Food for thought, at least! A motor run capacitor might well be cheaper
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