Re: "Forming capacitors" - means what?


KB6NAX
 

I don't find anything to quibble with in this thread. I've
reformed 'lytics in numerous old radios and audio amplifiers and
monitoring capacitor temperature is the best indicator for
controlling the reforming process. I recently reformed the 'lytics
in a McIntosh MC75 amplifier. The power supply uses silicon diode
rectifiers in a voltage doubler arrangement which if full line
voltage is applied will blow leaky capacitors to kingdom come. I
controlled capacitor temperature by adjusting the variac to keep the
capacitors from becoming uncomfortably warm. A capacitor that
exhibits high leakage current will of course act like a resistor and
heat up. As the capacitor warms up the leakage current increases.
The increased current will increase the temperature and if reforming
does not act to limit the temperature rise the capacitor will go into
thermal runaway and blow its juices out (and in some cases the
capacitor may explode). The McIntosh caps ran warm for about 16
hours before they reformed sufficiently to cool down. To be sure, I
kept the process going for 48 hours going to full line voltage after
about 24 hours (don't be in a rush to increase voltage!). The friend
who I restored the amp for is happily listening to it daily. How do
I measure capacitor temperature? With my hand (only one hand though,
I follow the one hand rule when working on high voltage stuff and so
far it has saved me to write this - hi!). I remove the tubes in
vacuum tube gear to keep the capacitors from being heated by the
tubes. Use of a metered variac helps as you can see if power supply
current is increasing, a sure indicator that trouble is ahead. Not
all old capacitors will reform, they will continue to exhibit high
leakage current and as someone mentioned they will eventually short.

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