ELECTROLYTIC CAPACITORS: HAZARDS!
1/ Electrolytic capacitors are a problem with old equipment and failure of an electrolytic capacitor may not only make a mess inside an instrument but cause the demise of the power transformer, this author KNOWS! It is essential to bring old equipment up gently using a variac with an ammeter in series. Know what the correct current should be (power consumption rating divided by rated line voltage, usually 117V). Ensure that a fast fuse is fitted that is the closest standard value JUST greater than the correct current. (A slowblo fuse can be fitted for reliability later.) Watch the ammeter CAREFULLY, any bumps (that will occur when rectifier(s) start to conduct and the caps charge) should immediately diminish, if a bump does not diminish IMMEDIATELY there IS a problem. Turn it off and investigate. If the current exceeds the rating, there IS a problem, turn it off and investigate. Even if you do manage to get to full line voltage without a failure, sit with it and watch the ammeter for SEVERAL hours. The times I have failed to do this, I have lost power transformers. It is up to you, I simply want to help you to avoid disappointment.
2/ If there is a problem, you are faced with finding the offending capacitor. Start with the cap(s) nearest the rectifier(s), disconnect them. Then test each capacitor by attempting to charge it; I use a simple mosfet current limiter set at 5mA to charge them with a minimum danger of a burst, place a voltmeter directly across the capacitor and ensure that you do not exceed the rated voltage. COVER THE WEAK END OF THE CAPACITOR WHILE TESTING, you have been warned. On most old capacitors, the weak end is the terminal end. Some newer capacitors have a cross on the opposite end, intended to easily release pressure in the event of failure. Using an external power supply together with a series ammeter, the cap should charge easily to full rated voltage and hold at 1mA or less. 1mA is a very generous leakage tolerance, a new cap will be in the micro-ampere range*. Consider, a cap operating at 400V and leaking 1mA will be dissipating 0.4W. That is enough to cause the can to slightly warm enough to feel. Too much more than that can cause the cap to burst, make a mess, maybe injure you and destroy a rectifier and/or power transformer. WEAR EYE PROTECTION!
*After posting this, I found some official information on leakage allowance on this Heathkit SB-102 site: Sprague uses the following formula for the limit of “good”:
I = kC + 0.3
I = the max leakage in milliamps, C is the capacitance in mfd
k = .01 for 3-100 WVDC
k = .02 for 101-250 WVDC
k = .035 for 251-350 WVDC
k = .04 for 351-500 WVDC
E.G. for a 100µF capacitor rated for 450 WVDC, the leakage limit is,
0.04 x 100 + 0.3 = 4.3mA.
In my view this is generous; a unit having many capacitors leaking that heavily will also be loading the power transformer harder, maybe beyond the design limits.
3/ If the capacitor fails the above test and will not reach the rated voltage and hold at 1mA or less, it may be possible to re-form it. Turn the voltage down until the current falls to less than 1mA. Leave it and the current should fall further as the film re-forms. Then turn the voltage up until the current reaches 1mA again and leave it. If you can continue this process until the cap reaches the rated voltage at 1mA leakage or less, then you have re-formed your capacitor. The next comment is especially relevant to equipment containing re-formed capacitors:
4/ Remember, electrolytic capacitors need to be used to maintain the insulating film. Turn on every piece of kit you have REGULARLY, at least once a month.
6/ Here is information on capacitor can rebuilding, I have not tried this service and offer this for information only: “Rapid return of your rebuilt can. Any twist-lock can rebuilt for $30, up to four section. Maximum 450 volt at that price. Nut mounted cans $20 single section, for multi-section add $2 per section on nut mounted cans only. Shipping add $4 per order for Priority & insured shipping via PO. Rebuilt cans returned only after receipt of check, money order, or credit card info. Our guarantee on all rebuilt cans, 1 year. We will test any can for leakage and capacitance, at correct voltage, for $2. Frontier Capacitor, PO Box 218, Lehr, ND 58460 or 403 S. McIntosh, UPS. Toll free (877) 372-2341. Ph.: (701) 378-2341. Fax: (701) 378-2551, voice mail recording anytime”
7/ Some equipment, such as Tektronix will have a time-delay relay fitted. This means that the voltage will be applied suddenly once the relay element has warmed, this is not what we want when first powering up an unknown piece of kit. Short the delay contacts and then use the variac method. NOTE! Once the instrument has passed this test, REMOVE the short!
9/ In the event of a failed power transformer, here is a re-winding service that I have used, they provide an accurate re-wind. The process of de-constructing and re-winding a transformer is detailed so don’t expect to pay new transformer prices. I think it is good value. Gary at TRS.
10/ Here are some pictures of a dry “twistlock” electrolytic from a HP frequency meter.
There were two caps, one had failed and being dry was easy to pull out of the can. I re-stuffed them both since they are used in series. It is usually fairly easy to get the cardboard sleeves off because the glue has dried.
I then use a Dremel tool with a cut-off disc to cut around the aluminum can just above the base and pull the base away, tearing the hot connection out of the capacitor in the process. (Be careful to protect your eyes and do not breath the dust.)
I then clipped off the hot connection and drilled a hole through the base next to each terminal and soldered a wire to each terminal of the replacement cap. The holes are large enough (60mils) to fit firmly around each soldered joint so that the cap can be pushed onto the base. Then I soldered the wires to each terminal, it is a good idea to use colour coded wires to clearly identify the polarity.
I then wrapped the cap with foam mounting tape and was rewarded with a serendipitously firm fit into the cardboard sleeve! The sleeves were still a firm fit around the twistlock base.
Here are the re-installed caps: