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I have vintage Heathkit H89 computer from late 1970's and early 1980's. It uses linear type power supply and used Sprague 673D series electrolytic capacitors; two 10,000uF @ 15WVDC for +8V circuit and one 4700uF @ 25WVDC for +18v circuit. Due to age as 4700uF capacitor blew out and ceased +18v. I struggled to find a replacement because Sprague sold the capacitor department to Vishay years ago. Vishay said that one have no direct replacement nowadays because the 673D series had higher ripple current rating as much higher than today capacitors that almost 65 percent lesser.

I am aware that if I replace with exact same value but ripple current will heat up inner core and then burn up and blow the seal quickly.

Have to seek a solution for above problem. If I can use multiple capacitors in parallel and put resistors series with capacitors. As much as like LED circuit that use limit current resistor so I wonder if it possible to use divide method with five 1000uF in parallel that would bring total of 5000uF as resistors will limit the current into the capacitor to protect from heat buildup.

As I believe the original capacitor can handle up to 11A current.

Have you any idea to save vintage computer and also I have several Heathkit equipments may need to replace aged capacitors.

Forward to hear from you....

Thank you very much, Steven

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  • \$\begingroup\$ Many times power supply (and other) circuits are not designed at the limit of the component ratings. You may find that any modern capacitor with conservative voltage and capacitance ratings will work perfectly well. You may also find that modern parts are smaller and more efficient and you can replace with a higher value device into the same space that will be less stressed than the original. If it was me I would fit a close fit part and check the voltage drop and ripple under rated load to see if it is regulating ok. \$\endgroup\$ – KalleMP Feb 11 '15 at 11:57
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You can put multiple modern capacitors in parallel, as long as their combined ripple current rating exceeds the original specification. For example, you could use four 2500 µF capacitors that have a 3 A ripple current rating to replace a single 10,000 µF, 11 A capacitor.

Do NOT add any series resistors! The whole idea is to reduce the equivalent series resistance as much as possible, not raise it!

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Better etching of the aluminum foil has allowed electrolytics to be smaller than they once were, but smaller size has resulted in a drop in ripple capability. There may be a low ESR in the size you need, they use better electrolyte I think. If not, a parallel strategy might work.

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