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Old 6th November 2005, 10:31 PM   #1
gwyz is offline gwyz  United States
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Question Is "reforming" Electrolytic Capacitors necessary?

Hi,

----------------
Note: Before posting this thread I did a search and found a couple of posts which mentioned the use of a "variac." I don't have such equipment, and my question is a bit more specific. (Please read)
---------------

I bought a pair of Heathkit AA-13 Mono Amps. They need new capacitors, so I bought new stock with the exception of two, which are NOS ("New Old Stock").

The NOS is a 40-40-20 Twist Prong Capacitor which I feel very lucky to have found. I was about to give up and try rebuilding my old Twist Prong Caps with new Electrolytic "guts." Then I stumbled onto a web site in Nebraska that listed the very part I needed. They are the only place on Earth that had them (unless I just don't know where to look). The original stock was 40-40-20 at 350 volts (at 40 ohms) and 450 volts (at 20 ohms). The new Twist Prong caps are rated at 500 volts at all three terminals.

Anyway, while reading about replacing Electrolytic Capacitors, I read a couple of articles about the need to "reform" them if they have been sitting idle for any length of time; (which some define as little as 6 months).

Does anyone know the truth regarding the matter? Also, is there an easy way to do it? I don't have the kind of equipment necessary to produce 500 volts. It's just me, my soldering iron, and my multimeter. (I ain't got no steenking "variac"!)

I need to know if I am going to ruin these rather expensive capacitors ($25 bucks) if I install them without "reforming" them. If "reforming" *is* necessary, is there an "easy" way of doing it? For example, I read an article about reforming the quad caps in a Dynakit Mark III which involved removing the rectifier tube, removing the red wire and replacing the red wire with a 100k resistor, then plugging in the unit and using IT'S OWN JUICE to do the "reforming."

Doing it without a "variac" or other equipment is specifically what I am asking.

Thanks in advance...

smiles,

Gregg
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Old 6th November 2005, 10:49 PM   #2
AJT is offline AJT  Philippines
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yes, if the NOS caps have slept on the shelf for years! i would not risk a big bang, i will reform!
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Old 6th November 2005, 11:04 PM   #3
gwyz is offline gwyz  United States
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Quote:
Originally posted by Tony
yes, if the NOS caps have slept on the shelf for years! i would not risk a big bang, i will reform!
-------------------
Hi Tony,

The other half of my question was how to do it without equipment like a variac? Is there a way to do it similar to the method I stated for use with the Dynakit Mark III?

Thanks,
Gregg
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Old 6th November 2005, 11:34 PM   #4
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This is a useful site, and there is some more discussion here.
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Old 7th November 2005, 01:22 AM   #5
anatech is offline anatech  Canada
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Hi Gregg,
Reforming is absolutely necessary with NOS caps. Even then, they may not be good. Often, the electrolyte dries out and the cap is less than useless. You have no idea how many cans I've emptied and installed new units inside.

Pinkmouse has listed a good web site, the information is correct. I leave them forming much longer. Then I discharge them and measure the capacitance and dissipation factor. Where possible, buy new capacitors.

I see Tony has reformed (to what?? )

So you need an adjustable HV power supply and a resistor as stated on the site pinkmouse linked to. A variac is the least expensive way to do this. I use a regulated power supply as it's the easiest way. You get to use your meter. Watch the measurement across the resistor as it is at the high potential. Make sure there are no children or strangers that can startle you, or my favorite - what's this ? POW. Please be careful. Also, do this in the chassis. At least the caps aren't rolling around free and there is some protection from touching the terminals.

I Like SS rectifiers for this because the tube type don't get going until the voltage is up there. To drop the AC line voltage, you could run a light bulb in series and another in parallel to make a voltage divider. Vari the wattage ratios to control the voltage.

-Chris
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Old 7th November 2005, 02:21 AM   #6
AJT is offline AJT  Philippines
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hi Chris, you got me there!

my setup for capacitor reforming, psu is just hooked up from parts on my workbench.

use a supply voltage equal or greater than by 1.5 x printed wvdc on the caps.
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Old 7th November 2005, 05:02 AM   #7
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Start first with applying rated voltage and then measure leakage. If it's under 1 mA after while, nothing to worry about. With time they get reformed in normal usage.
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Old 7th November 2005, 06:16 AM   #8
AJT is offline AJT  Philippines
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of the two pairs of caps, one set, the cornell-dubiller formed to the rated voltage of 60volts, the other set, a japanese one was rated 63volts, but at 71volts the leakage was still going down, is this an indication of quality or what?
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Old 7th November 2005, 02:00 PM   #9
gwyz is offline gwyz  United States
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It looks to me like people do this (mostly) to capacitors that are much bigger than the 40uf I'm working with.

Not only that, but when I look at the prices on variable transformers in the 500 volt range (with most having an output max of 240 volts), it would be MUCH MUCH MUCH MUCH MUCH cheaper for me to just open the can and simply rebuild the twist prong cap with brand new electrolytics.

I found this site which is very helpful:

http://www.nmr.mgh.harvard.edu/%7Ere...ics/index.html

Thanks everyone...

smiles,

Gregg
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Old 7th November 2005, 03:11 PM   #10
anatech is offline anatech  Canada
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Hi Gregg,
You need to look at the CV product (capacitance X voltage) to get an idea of "size". I would probably not reform a large, low voltage capacitor. There are replacements available.

Old can type like yours are a different story. Firstly, the unit should be powered up to assess it. What you want to do is avoid damaging any parts. To do this, the capacitors need to be reformed and replaced if they have failed. This will allow you to see what needs to be done, even if the caps sort of work. At least you will not need to purchase a bunch of caps (possibly resistors or rectifier tube). If the unit is simply powered up, the power transformer may be damaged. End of unit.

So you can see there is good reason to follow some procedure for reforming the caps. What I would do is get a variac and temporarily tack some silicon diodes (or make a circuit with clip leads). You can then use the existing power transformer to generate the higher voltages. Yes, the first time it will cost some money. You will however use this over the years making checking these caps a lot easier. Because all the caps are in circuit, you need to troubleshoot which cap is drawing current. That is simple.

Remember, on a multiple section capacitor, if one unit is bad the entire can should be replaced as the other sections will also fail later.

-Chris
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