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Old 15th Dec 2008, 1:46 pm   #30
Oldtimer74
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Join Date: Apr 2008
Location: Worcester, Worcestershire, UK.
Posts: 330
Default Re: Test equipment for valve radio repair

HI. Whilst I would agree with almost all the previous comments I would like to add a couple. NEVER switch on an mains wireless that you have no knowledge of or has not been in use recently without first disconnecting the electrolytic smoothing capacitors. You can easily get a BANG and a nasty mess and a requirement for a new rectifier.
Having said that I always try to reform electrolytic capacitors that have not been in use for some time, even new old stock. This is easy to do and I reckon I have had 95 per cent success even managing some wet electrolytics from the 1930s.
You need a DC supply of 250-300V. It should be at the same voltage as the caps are going to be used at but I find if you can only get 250V then they will almost certainly be OK for a bit more.
Connect the cap in series with a 15k resistor and a multimeter, on suitable current range, with corerct polarity across the DC supply. Initial current will be several ma but will quickly drop and in time will eventually settle down at, hopefully a low value. Leakage current is always present in electrolytics but should not exceed (for plain foil types - these are likely to be the earlier ones) 0.15 X cap (in mfd) X voltage, microamps. For Etched foil types it should be 0.5 X cap (mfd) X voltage, microamps This equates to about 1 ma for a 450 V 16 mfd. If the cap is noticeably warm at the end it is probably doubtful.
Good luck
Pat
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