AR-11 Collins Capacitance Increase, Needs Reforming

[Pete B]

I tested the capacitors in System #1 with no reforming or even playing music through the system. This system has been in its box unused for about 10 years. It is interesting that the 40 uF cap is actually physically smaller than the 10 uF. I believe that they were going for much lower ESR in the 10 uF cap to provide higher Q in the tweeter crossover section.

The 10 uF cap appears to have leaked, there was a small spot of white material on the outside of the case. I measured this 10 uF cap with the following results:

100 Hz 30.9 uF

500 Hz 25.8 uF

2K 21.2 uF

5K 18.5 uF

I then reformed the cap with current limiting on a 64V power supply, the capacitor did not come up to voltage quickly as most even 10 or 20 year old caps do. I find that most good quality electrolytic caps do not need reforming. This cap took about an hour to come up to 60V in one direction and 55 V in the other. This voltage reading is only approximate since we don't know the reverse voltage across the reverse biased cap. Most consider the cap reformed when the voltage reaches 10 % above the rated voltage, but again we don't know since this is a non-polar type. Here are the results after reforming:

100 Hz 12.6 uF

500 Hz 11.8 uF

2K 11.5 uF

5K 11.5 uF

Much better, but I still would not use this cap. I just wanted to demonstrate that these caps have become leaky since reforming builds up the oxide layer on the plates which acts as insulation. The caps leak at a much lower voltage and have the wrong capacitance because the oxide layer provides insulation. I would not trust a good tweeter to a cap this old, especially one that has leaked. A good film cap should be used on tweeters.

The 40 and 120 uF caps tested good as far as capacitance goes, but I did not test them for leakage.

Pete B.