[NJARC] Bring it up slow on the VARIAC

[antqradio at sbcglobal.net]

Don't have the time to babysit an electrolytic capacitor while it reforms.  Much easier to make up a little power supply with a large value resistor to limit short circuit current to 10 mA or so.  Just hook up the cap, turn on the power supply and occasionally monitor the voltage across the cap with your trusty DVM while you do something else like cleaning the chassis or cabinet.  
If the cap will reform, the terminal voltage will rise to the approximate open circuit voltage of the power supply.  If it doesn't, then no harm is done since the maximum current that the power supply can charge the capacitor with is limited to well below what would cause heating in the capacitor.
I only bother to do this with canned electrolytic capacitors in higher end equipment.  Cost conscious consumer radios and the like use cheap wax covered capacitors which never seem to reform to the point where they have a low leakage current so I just replace these outright, just like Dave mentioned.  This is usually done with the cap needing reforming disconnected from the rest of set.
Once the cap is reformed and put back in the circuit, the reforming power supply does double duty by powering the set and I can then check non-electrolytic bypass and decoupling capacitors.  Any leakage in these capacitors is easily found with simple voltage measurements and the leaky capacitor is then replaced.  This makes quick work in finding those leaky screen bypass capacitors.Jim

      From: islandradiosvc--- via NJARC <njarc at mailman.qth.net>
 To: NJARC at mailman.qth.net 
 Sent: Monday, November 7, 2016 11:44 AM
 Subject: Re: [NJARC] Bring it up slow on the VARIAC
   
 The Variac is a nice tool to slowly bring up voltage on a set.. This alone does not equate to the electrolytic capacitors being slowly brought up to voltage. It also does not tell you much, about current draw. To start with, you want an ammeter on the load. The only Variacs that I have seen , that have this feature built in, are the Eico and the Sencore.  Even if we add an ammeter  in series with the load, it does not tell you what the capacitors are doing. Obviously if an 8 tube set, is beginning to pull 4 amps, we have an issue, but something that drastic is not usually the case. Also, bear in mind that because of the warm up characteristics of rectifier tubes, the filter caps may not see voltage until it is high enough to damage them without reform. If you choose to re from the electrolytics, I suggest using a bench B+ supply, capable of reaching working voltage, along with a digital multimeter to monitor the current draw of the capacitor being re-formed. In my experience, a healthy cap, (in the 5-60 mfd range) after re form, should NEVER pull more than 3 or 4 ma. You slowly bring up the bench supply, across the cap, with the meter in series, and watch the current, you will see it spike, then begin to fall. each time it falls below 4 ma, raise it again, say 60 volts at a time, but never let the cap pull more than 50 ma. When they pull huge amounts of current, is when they destroy themselves. Better still. Eico  and Heathkit, and others made capacitance bridges, with built in power supplies. These are very lightweight supplies, only capable of supplying a few MA.. They are IDEAL and rather foolproof for re-forming, Their lack of available current, prevents them from damaging the capacitor, and you can simply hook them up, and let the cap sit for 20 min. or however long it takes. Again, I place the MA meter in series, and watch the current fall. IF you find an electrolytic will not stop leaking current, within a reasonable amount of time, I say toss it. It is simply not worth the risk. Also... I make it a policy to re check current draw a day or 2 after reform. If the capacitor does not act like a  new capacitor (or at least close) and takes an extended period of time to stop pulling current, again.. toss it. I have seen too many "re-formed" capacitors fail suddenly and take a power transformer or costly rectifier tube (or both) with them.  We are reaching a point where this stuff is OLD.. I am finding more and more electrolytic capacitors , particularly those from the 1950s and before , that are reaching the end of their useful life. 15 or 20 years ago, these capacitors seemed much more willing to re-form and and stay formed. From what I have been able to tell, capacitor technology changed vastly in the 1960s, and hence electrolytic capacitors made from that point and later, seem to fair better, and they are also newer. Also Sprague, Mallory ,Astron and a few other electrolytic  capacitors seem to survive vastly better than some other brands.  Just my 10 cents on all this...  Used to be 2, but we can calculate for inflation... 
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