On 9/6/2025 3:35 PM, M De Simone via Jukebox-list wrote:
I am rebuilding an AMI model I power supply and am replacing the 1000uF 50v capacitor. It has been suggested by Jay to increase this value to 2400uF. I have a new but older stock can capacitor that is 1500uF 50v on hand that will fit perfectly into the existing clamp but it measures 2800uF with my meter and esr leakage is almost zero. Why is the value almost twice what it should be and should I use it? It would save me a trip or ordering one. I’m more curious than anything else of why the value is so much higher. Thanks, Mauro
Reading way over the rated capacitance is very common with older electrolytic caps. I believe that a cap tester that can't charge a cap up to its rated voltage, yet tests for leakage, isn't worth using for testing leakage. ESR isn't leakage, it's Equivalent Series Resistance. Not important in a simple power supply like this. It is important in high frequency applications like in switching power supply's and many other high frequency circuits. Personally, I don't think increasing the capacitance is important in this circuit but Jay may have his reasons. I have tried it when motors run slow even after a good cleaning and lubricating but it didn't help. Anyway, I'd say go ahead and use what you have, it will work just fine.
Hold on, I decided to google it.
"Old electrolytic capacitors can read higher capacitance than new ones when the aluminum oxide dielectric layer degrades over time, especially from prolonged storage without bias voltage, causing the layer to shrink and effectively bring the plates closer. This phenomenon is known as "reforming". However, this increased capacitance is often a sign of degradation, as the layer can become thinner, which also reduces the capacitor's maximum voltage tolerance."
That's google AI and I think they have the "reforming" part backwards. To "reform" an electrolytic cap, you charge them up at a lower current until the voltage comes up to its rated voltage. That "reforms" the aluminum oxide layer and it then has less leakage. But I've been wrong before!