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A large aluminum filter capacitor about 1" X 3" can, from a 1972 power supply, needs to be replaced. it is 4000uf/50V/85 degrees. Any reason why it can’t be replaced with a much smaller 4700uf/50V/85 degrees Electrolytic cap from Amazon? I don’t understand the difference, except for physical size and price.

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It probably could, but the devil is in the details. Ripple current, loss etc....

What is key to appreciate is the present cap is from the 1970's. Technology has moved on from there in not only in dielectric but also in manufacturing techniques.

Compare the two data sheets is the best advice

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    \$\begingroup\$ Good luck trying to find a data sheet for a 1970s-era generic part like an electrolytic cap! You might find the data in an ancient parts catalog, I suppose, if somebody has bothered to scan it and put it on the web. \$\endgroup\$
    – alephzero
    Commented Sep 3, 2019 at 8:57
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Adding to the answers: The cost of an Electrolytic Capacitor is nothing compared to any impact of a failed power supply, especially in the field of medical, manufacturing, military or other important usage.

But even when used only for private purposes, the time effort to change it again after failing a 2nd time is not worth to choose cheap caps.

There are many low-quality caps sold via internet that do not even match the ratings printed on the casing. The capacity could be less, the maximum voltage and temperature could be lower.

Like stated already in other answers, it is worth to purchase a high quality cap with higher ratings both for voltage and temperature (in that case e.g. 80V and 105 C degrees), and ESR and other datas should be compared via data sheets.

Since old caps anyway had a high tolerance of f.e. -20%/+50%, a higher capacity value should be no problem unless the ESR does not become too low. So 4700uF or 5600uF as replacement for a 4000uF should be fine.

Normally there is much heat produced in old power supplies since they work via series regulation. The caps should not be close to or touching heat sinks.

If the supply is anyway open, these points could be checked as well:

  1. Are all diodes/bridge rectifiers feeding that cap ok? Sometimes a broken cap is the result of bad diodes.
  2. All the screws pressing the power elements (f.e. 2N3055 transistors) to the heat sink(s) should be re-fastened. In many cases those screws are not tight enough anymore after half a century.
  3. Any resistors (or other elements) show burnt casings? Any burnt/colored areas on the PCBs?

If the power supply was broken and not used for a long time, the first test should be via feeding from a variac after repair.

The voltage should be slowly raised in order to enable all Electrolytic Caps to rebuild the oxyde layers without high surge currents - also the new cap could have been stored for a long time.

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Adding to JonRBs Answer:

Capacitors in the 70's were huge in comparison to current caps.

Advances in manufacturing led to a massive size shrink along with other benefits. Some effects from these improvements are contrary to each other. I try to give some thoughts on it.

Smaller caps have lower ESR. This is something which could be problematic within a intricate design, but the probability that in a 70's supply a high ESR might be crucial is rather low. You can figure out from the schematic, if a low ESR is acceptable. The inrush current might grow to high with a low ESR.

Smaller caps have a lower surface, making it more difficult to dissipate thermal power but on the other hand they might improve convection inside the housing.

You could choose a capacitor with slightly higher voltage and temperature rating to improve lifetime, because the ratings of parts from the 70's were a lot more conservative due to the greater variations in manufacturing processes.

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    \$\begingroup\$ And you could make up the difference between the old ESR and new ESR with an external resistor if need be. \$\endgroup\$ Commented Sep 3, 2019 at 8:42

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