electrolytic measures good, acts bad?

[Jerryz1963]

Is it possible for an electrolytic to measure good but break down under high voltage? I have one that measures good, but when I inserted it in the circuit, i had a small lightning storm inside the 5U4. I took it out of the circuit and all was well. It comes after a pretty stiff inductor, and it's only 33 uF. the first electrolytic (after the rectifier) is below 50 uf, so the 5U4 shouldn't be stressing out.

[xtian]

Yes, definitely.

[rdjones]

Is it possible for an electrolytic to measure good but break down under high voltage? I have one that measures good, but when I inserted it in the circuit, i had a small lightning storm inside the 5U4. I took it out of the circuit and all was well. It comes after a pretty stiff inductor, and it's only 33 uF. the first electrolytic (after the rectifier) is below 50 uf, so the 5U4 shouldn't be stressing out.

Define "measures good".

A cap can be tested for capacitance value by many meters.
A proper leakage test must be done at the operating voltage.
A simple ohm test done by a meter (even a very good one, eg Fluke) will almost always eventually show infinite ohms and is of little real value.

rd

[Jerryz1963]

I used a capacitance meter. It seems to be pretty accurate, but doesn't do the leak test.

[Joe L]

Put a 100k resistor between the cap and the choke. Disconnect any load after the cap. Turn the amp on and measure the voltage drop across the resistor. It should take a moment or two to get near 0 volts. It should end up with less than a volt.

This test will not hurt the rest of the amp even if the cap is dead shorted.

..Joe L

[Jerryz1963]

Put a 100k resistor between the cap and the choke. Disconnect any load after the cap. Turn the amp on and measure the voltage drop across the resistor. It should take a moment or two to get near 0 volts. It should end up with less than a volt.

This test will not hurt the rest of the amp even if the cap is dead shorted.

..Joe L

Thank you very much.

[Stevem]

X2 with Joe there!
One tip on cap life is to never discharge them by shorting there + end right to ground as this can blow a hole in their dielectric so to speak.
Some times we do this by accidence when working on a amp, sometimes the cap is old or defective.
If the amp has a standby switch in the DC side of the power supply then turning that on with the main ac switch off is the safest way to discharge the power supply, if not than draining the main first filter and in turn all the others by means of hooking a 470k resistor across that filter will get the job done in 45 seconds.

[dorrisant]

Eico 950B... This piece of gear will allow you to see the capacitance as you crank the HT up. Some caps will fail just shy of the voltage ratings, this guy will let you see which ones fail. Also, it lets you compare the turns ratio of transformers and sort through resistors and coils so you can match the values when needed. It also uses a way cool seeing eye tube which gives it a cool factor. I found mine on eBay for about $60 shipped. I love it!

Tony