Issue Testing Large Audio Amp Electrolytic Capacitors with LCR Meter

Thread Starter

Amorelovensheimer

Joined Apr 1, 2022
7
I am attempting to test the electrolytic power supply capacitors in a vintage Forte Model 3 amplifier. There are four, each rated 50v 24,000µF and since they have screw terminals it’s easy to test them out-of-circuit. The LCR meter I borrowed is an East Tester ET4410 (available from, e.g., Amazon). Given the rated values of the capacitors, and since my understanding is that the capacitors function to smooth the 120Hz output from a rectifier, I set the LCR meter to do a series capacitance test, and set the test frequency to 120Hz (instead of the default 1kHz). The test leads from the meter to the capacitor are the four wire Kelvin type. What happens is that the displayed capacitance values slowly oscillate up and down on the meter in a range from about 18,000µF to about 26,000µF. The ESR values also fluctuate between about 2 and 3 ohms. I assume that this has something to do with the capacitor charging and discharging a bit (?). My questions are: why is the oscillation occurring, how can I get a stable (or at least meaningful) measurement result, and is it desirable to obtain the results at the frequency at which the capacitors will operate? Thank you.
 

DickCappels

Joined Aug 21, 2008
8,708
Side-stepping your question, why not just use a resistor, power supply, and DVM and measure the amount of time it takes the capacitor to charge up to 63% of the power supply voltage?

1648860501647.png
 

LowQCab

Joined Nov 6, 2012
2,049
LCR Meters are quite often not designed for measuring huge, "Bulk-Storage" Capacitors.

I would replace them simply for the fact that they are "Vintage".

How long the originals will last, and perform satisfactorily, is definitely a question to consider.
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crutschow

Joined Mar 14, 2008
29,792
The meter specs say it will measure up to 99,999µF so it should be able to measure your caps.

Did you add a DC bias with your measurement?
The DC bias should be greater than the peak AC measurement voltage.
Electrolytics may not measure properly if the peak AC measurement voltage goes negative.
 

MrChips

Joined Oct 2, 2009
26,099
Better still, determine the operating voltage and current of the power supply under load.
Test the capacitors with a similar 120Hz rectified voltage and load. Measure the ripple voltage.
 

Thread Starter

Amorelovensheimer

Joined Apr 1, 2022
7
Side-stepping your question, why not just use a resistor, power supply, and DVM and measure the amount of time it takes the capacitor to charge up to 63% of the power supply voltage?

View attachment 264074
Thanks. I can do that, and will. I need to do more reading, though, as I'm not clear on how to interpret the results, including whether if the caps "passed" whether they are functioning optimally. On the latter, one thing I wanted to determine was whether the caps could use some "reforming" (which I've never done, but figured I might as well if there would be some improvement to be had).
 

Thread Starter

Amorelovensheimer

Joined Apr 1, 2022
7
LCR Meters are quite often not designed for measuring huge, "Bulk-Storage" Capacitors.

I would replace them simply for the fact that they are "Vintage".

How long the originals will last, and perform satisfactorily, is definitely a question to consider.
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Fair point. I have, however, gotten a bad power supply capacitor (for a smaller amp) from one of the component supply houses that has a stellar reputation (which of course they replaced no-question), so I would want to test any replacements to make sure I was getting my "money's worth" since they aren't cheap.
 

Thread Starter

Amorelovensheimer

Joined Apr 1, 2022
7
The meter specs say it will measure up to 99,999µF so it should be able to measure your caps.

Did you add a DC bias with your measurement?
The DC bias should be greater than the peak AC measurement voltage.
Electrolytics may not measure properly if the peak AC measurement voltage goes negative.
Thanks. I did not add a DC bias, and will need to read up on how and how much is needed (I can't recall what the meter's auto-set test voltage was).
 

Thread Starter

Amorelovensheimer

Joined Apr 1, 2022
7
Better still, determine the operating voltage and current of the power supply under load.
Test the capacitors with a similar 120Hz rectified voltage and load. Measure the ripple voltage.
Thank you. I never would have thought of that, which does seem like a true operational performance test, but am not sure how I would find a rectified voltage source that is "unsmoothed" or that otherwise mimics the rectifier/rectified voltage in the amp itself.
 

MrChips

Joined Oct 2, 2009
26,099
Thank you. I never would have thought of that, which does seem like a true operational performance test, but am not sure how I would find a rectified voltage source that is "unsmoothed" or that otherwise mimics the rectifier/rectified voltage in the amp itself.
You have the test setup right there.
Do you have an oscilloscope?
Measure the DC voltages on the capacitors.
Measure the AC ripple on the capacitors.

Do you hear any hum from the speakers at low and high volume settings with no music input?

1649004443234.png
 

Thread Starter

Amorelovensheimer

Joined Apr 1, 2022
7
You have the test setup right there.
Do you have an oscilloscope?
Measure the DC voltages on the capacitors.
Measure the AC ripple on the capacitors.

Do you hear any hum from the speakers at low and high volume settings with no music input?

View attachment 264179
That’s the amp! By “right there” I assume you mean that I could connect a borrowed oscilloscope (?) somewhere in there to assess ripple? Being a Mechanical Engineer I’m not sure how to do that. Plus, fully powered caps make me skittish. I don’t hear any hum, so I assume the caps are performing their smoothing function OK, but I wondered - if they are losing steam - whether they’d affect the amp’s performance in other respects such as in the case of a transient in the audio content that would require a high power draw. That said I could admittedly be way off base. Thanks for your input!
 

Marley

Joined Apr 4, 2016
475
Notice the little white plug beside the capacitor terminals. This a kind of "safety valve" to release internal pressure should this occur. The fact that these are still in their original positions - not popped out - is a good sign.
If these capacitors have not been used in many years, they may be low on capacitance. But should improve with use - reforming.
 
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