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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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26th Sep 2021, 6:16 pm | #1 |
Tetrode
Join Date: Feb 2015
Location: Belfast, Northern Ireland, UK.
Posts: 83
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Capacitor leakage.
hi I have some old TCC metalpack PIO 0.1uf capacitors - rated 350v
I have tested them at 300v, the highest my power supply goes up to and I get 150uA. A new Vishay equivalent is around .5 uA This on A Fluke DVM, inline current limiting resistor I wanted to know if this leakage is acceptable in an audio circuit as I wanted to keep the authenticity of the amp (RDG1046) many thanks and sorry for the noobish question Last edited by mindburner; 26th Sep 2021 at 6:34 pm. |
26th Sep 2021, 6:34 pm | #2 |
Octode
Join Date: Dec 2018
Location: Llandeilo, West Wales, UK.
Posts: 1,092
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Re: capacitor leakage
I would not trust a Metalpack for coupling duties (''that cap'' duties).
Try another test. Put the cap in series with a 1MΩ resistor. Now feed 300 volts into the cap and ground the bottom of the resistor. Measure the voltage across the resistor. If you measure any volts the cap is U/S.
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26th Sep 2021, 6:51 pm | #3 |
Guest
Posts: n/a
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Re: capacitor leakage
Stick with the modern types, much better, if they where available when the amplifier was designed they would have been used. Paper In Oil, posh for the day, now rubbish.
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26th Sep 2021, 6:51 pm | #4 | |
Tetrode
Join Date: Feb 2015
Location: Belfast, Northern Ireland, UK.
Posts: 83
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Re: capacitor leakage
Quote:
I did the test you suggested .120v |
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26th Sep 2021, 6:51 pm | #5 |
Rest in Peace
Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
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Re: capacitor leakage
300V / 150μA = 2MΩ insulation resistance, definitely unusable for anything with that kind of voltage on it and disastrous for any output valves coupled from it. One would expect @snowman_al's test to produce around 100V across the resistor and 200V across the capacitor.
Even for decoupling duty they would be likely to drag voltages down significantly and might get worse with time as there will be some undesriable effects going on inside. I fear most Metalpacks are in this condition now, they held up better than some but no longer trustworthy in general. |
26th Sep 2021, 7:53 pm | #6 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
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Re: Capacitor leakage.
At 300V that is only 2Meg insulation resistance. That is not high at all. I'd put them in the duff category.
Craig
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27th Sep 2021, 8:34 am | #7 |
Octode
Join Date: Mar 2016
Location: Knutsford, Cheshire, UK.
Posts: 1,488
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Re: Capacitor leakage.
The 'Super Metalpack' version of these caps are far better and may be a suitable replacement. These were made for much tougher applications, higher temperatures and have ceramic end-seals.
They are usually in good electrical condition too - far more durable than the ordinary 'Metalpack' types. They are quite pricey though, as many collectors of vintage amps have the same idea to use 'SuperMetalpack' to replace the standard 'Metalpack' caps...
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27th Sep 2021, 10:05 am | #8 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Capacitor leakage.
These capacitors are now over 70 years old with a design life of a guaranteed 10 years. They are now untrustworthy and useless causing damage to expensive valves and transformers.
I don't care how you test them, they are now rubbish! To give you an example. I completely brought back to life a Murphy V114 television [1947]. I replaced all the usual paper caps but left the two .5uf 350V oil filled ones in the frame and line timebase. They were spot on value with excellent insulation but the lack of height that scanned 50% with the height control on maximum turned out to be one of the oil filled caps that tested 100%. They are not a perfect oil seal allowing tiny quantities of damp air to enter over a very long period. Oil filled line output transformers suffer the same problem. Just restuff them as suggested. Whatever caps you use they will be 500% better than the ones employed in the 1946 RGD 1046 radiogram. They only had paper, wax, pitch and oil as insulators all of which love to absorb moisture. If you are a BVWS member you can purchase excellent caps at moderate prices and there are of course many other suppliers of good quality components. No need for fancy expensive brands. John. |
27th Sep 2021, 11:08 am | #9 |
Banned
Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
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Re: Capacitor leakage.
I have military packed bags of Metalpack caps. I keep them to show beginners of what sort of caps to replace and not use.
The above comments are pretty obvious!! CHUCK em out!!! Sorry, mods, Joe |
27th Sep 2021, 2:35 pm | #10 |
Tetrode
Join Date: Feb 2015
Location: Belfast, Northern Ireland, UK.
Posts: 83
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Re: Capacitor leakage.
thanks all for the replies. Interesting about the 'supermetalpack' versions.
I think I will just replace all 3 of the blocking PIO caps with stuffed ones. I have spare 0.1 metalpack so can include those if ever selling the amp's. I will try and reforum the 15 and 30uf electrolytic caps Whilst not perfect the 'stuffed' caps (pictured) look pretty decent and have a good modern equivalent inside. Last edited by mindburner; 27th Sep 2021 at 2:42 pm. |
27th Sep 2021, 4:03 pm | #11 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: Capacitor leakage.
Yes definitely replacement is the sane thing to do, not only in coupling roles but decoupling too. 2Mohm of leakage can really mess up the behaviour of an AGC line....
When I see metalpacks or their smaller friends metalmites in always remove them and replace with some of mr. Vishay's nice yellow goodness. Doing this turned a Pye C12 transceiver from a deaf underachieving lump into a much more usable beast. |
29th Sep 2021, 5:27 pm | #12 | |
Tetrode
Join Date: Feb 2015
Location: Belfast, Northern Ireland, UK.
Posts: 83
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Re: Capacitor leakage.
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30th Sep 2021, 12:11 am | #13 |
Octode
Join Date: Nov 2013
Location: Penrith, Cumbria, UK
Posts: 1,993
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Re: Capacitor leakage.
As well as agreeing with the wisdom of replacing those PIO caps on sight, I'd also counsel NOT dismantling and restuffing them but to dispose of them in a hazardous waste repository as as far as I am aware the "oil" in them is a PCB and that's not good for you at all.
Is it worth risking your health for a cosmetic result? regards. Andy |
30th Sep 2021, 9:59 am | #14 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,548
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Re: Capacitor leakage.
You could be right about that. I have some PIO caps but they are labelled 'No PCB oil'....They are 1970's vintage and I've used a few in non-critical items but I've also had a couple leak and make a mess in the storage tray so likely the rest will be binned.
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30th Sep 2021, 11:21 am | #15 | |
Tetrode
Join Date: Feb 2015
Location: Belfast, Northern Ireland, UK.
Posts: 83
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Re: Capacitor leakage.
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