Some years ago I read a comment on "The Audio Amateur", written by Gary Galo, about correctly forming new electrolytic capacitors.
Does any one have any idea what was the issue?
I have the mags, but they are too many and the note was quite small, so I may not even notice it.
Searching in DIYAudio i found a thread from 2004, but they do not actually get to a how to do it:
Forming Cap's question..
Perhaps someone can put more light on this important issue.
A friend of mine has a Luxman power amp, one of the best of it's time, late '80s. The caps seemed to need some recovery, if it's possible, or replacement.
As they are four 15,000uF/65v, replacing for high quality ones, like Siemens or
similar ones, I think it's worth re-forming the caps.
Does any one have any idea what was the issue?
I have the mags, but they are too many and the note was quite small, so I may not even notice it.
Searching in DIYAudio i found a thread from 2004, but they do not actually get to a how to do it:
Forming Cap's question..
Perhaps someone can put more light on this important issue.
A friend of mine has a Luxman power amp, one of the best of it's time, late '80s. The caps seemed to need some recovery, if it's possible, or replacement.
As they are four 15,000uF/65v, replacing for high quality ones, like Siemens or
similar ones, I think it's worth re-forming the caps.
You can reform them individually by using a 65VDC power supply and a series 10k 1W resistor.
This limits the capacitor current to a few mA. Allow several hours for reforming each, when the
voltage drop across the 10k should be small, a volt or less. It's best to cover the
capacitor with a box while reforming, in case the worst happens.
You can reform all 4 capacitors at once with four 10k resistors, one in series with each capacitor.
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The electrolyte may have dried out, they may need to be replaced.
Pre-heat the caps in an oven at their maximum rated temp to re-distribute the electrolyte.
Form at rated voltage with a constant current source of no more than about 10mA or so.
Any that do not come up to rated voltage within a reasonable period of time, an hour or so, are best replaced.
Pre-heat the caps in an oven at their maximum rated temp to re-distribute the electrolyte.
Form at rated voltage with a constant current source of no more than about 10mA or so.
Any that do not come up to rated voltage within a reasonable period of time, an hour or so, are best replaced.
The key is monitoring the voltage across that 10k resistor
15000uF/10Kohm is an RC time constant of 150seconds.
So with a DC supply of c.65-70Vdc, the cap should charge to 99% of the supply voltage in 5RC = 750s = 12minutes, roughly.
Around this point, check the voltage across the resistor: it should be dropping like a stone. If you can find the spec sheet for the cap, it should give a value for max leakage value, but for a large -capacitance reservoir cap like this, c.100uA may well be acceptable.
So you should expect to see the voltage across the series resistor drop, down to c.1v or less, after say 20mins. This may take an hour or so to stabilise. If the leakage current remains larger on extended test (and by larger, I mean - significantly larger, 5-10x or more the datasheet-derived value) the cap is likely beyond recovery. It is leaky, will self-heat, and not provide useful service in-circuit as a low AC impedance.
NB. : Just because the cap reforms to acceptable leakage values doesn't mean it is perfect either: but for that you then need a way to test ESR. I'd make sure the cap passes this reform test, first.
15000uF/10Kohm is an RC time constant of 150seconds.
So with a DC supply of c.65-70Vdc, the cap should charge to 99% of the supply voltage in 5RC = 750s = 12minutes, roughly.
Around this point, check the voltage across the resistor: it should be dropping like a stone. If you can find the spec sheet for the cap, it should give a value for max leakage value, but for a large -capacitance reservoir cap like this, c.100uA may well be acceptable.
So you should expect to see the voltage across the series resistor drop, down to c.1v or less, after say 20mins. This may take an hour or so to stabilise. If the leakage current remains larger on extended test (and by larger, I mean - significantly larger, 5-10x or more the datasheet-derived value) the cap is likely beyond recovery. It is leaky, will self-heat, and not provide useful service in-circuit as a low AC impedance.
NB. : Just because the cap reforms to acceptable leakage values doesn't mean it is perfect either: but for that you then need a way to test ESR. I'd make sure the cap passes this reform test, first.
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I was looking at two videos by the same guy on reforming caps.
How to Reform Electrolytic Filter Capacitors on tube radio or amp. - YouTube
Reforming Old Electrolytic Capacitors - YouTube
I remember that Galo said in TAA that voltage should be applied slowly, and stay in half voltage some time. This guy seems to suggest the same.
My firend says the 15K caps were at 8K6uF when he last measured them.
How to Reform Electrolytic Filter Capacitors on tube radio or amp. - YouTube
Reforming Old Electrolytic Capacitors - YouTube
I remember that Galo said in TAA that voltage should be applied slowly, and stay in half voltage some time. This guy seems to suggest the same.
My firend says the 15K caps were at 8K6uF when he last measured them.
An externally hosted image should be here but it was not working when we last tested it.
I have access to a hundred (or more) of these.
Most regular caps are rated at 85°C (most mil-spec are 105°C).
With an ammeter between cap and 0V. http://www.angelfire.com/electronic/funwithtubes/Restore_cap.html
It's equal to the voltage across the series resistance divided by the resistor's value.
So, 1VDC across 10k is 0.1mA leakage (about the most you should accept).
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Why do you think I don't trust what you said? It seems logic that something like that could be effective.
Or I wouldn't have made the other questions about time on the oven, what would happen to the plastic and all that. I even thought it might be useful to use a silicone base to place the capacitor.
That's what I meant by to find more information.
Or I wouldn't have made the other questions about time on the oven, what would happen to the plastic and all that. I even thought it might be useful to use a silicone base to place the capacitor.
That's what I meant by to find more information.
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