Using a Variac to revive power supplies

Nasty things exploding capacitors. Engineers dictum "If in doubt chuck it out" ie replace. We used to reform supposedly new electrolytics if they had been in the stores for a while. Regards ? Rod Smallwood ? ? -----Original Message----- From: cctech-bounces at classiccmp.org [mailto:cctech-bounces at classiccmp.org] On Behalf Of js at cimmeri.com Sent: 24 December 2010 15:13 To: cctalk at classiccmp.org Subject: Re: Using a Variac to revive power supplies Subject: Re: Using a Variac to revive power supplies From: Tobias Russell <toby at pdp11.co.uk> Date: Thu, 23 Dec 2010 19:06:04 +0000 To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> Ok, I suspected as much. So for a linear supply such as the 8/E's is a Variac a valid way to go or should I reform the capacitors individually as I've done previously? All the best, Toby ------------------------------------------------------------------------ Toby, I've found that removing every single electrolytic and reforming to full (or 10% over) capacity provides the best results. The Variac method -- while better than nothing -- doesn't limit your current... plus doesn't reform all caps to full capacity, nor tell you anything about their condition and leakage. 1. Remove cap. 2. Check ESR. If ok then.. 3. Reform with resistor and variable power supply while monitoring current... raise input voltage slowly to keep max current to no more than 1ma. Let current settle down every so often to check for leakage rates at various voltages... One method I use is halves... start out at 50% rated voltage, let settle, then 75%, settle, then 87%, 95, 100.... Checking leakage at each stage like this gives you an idea of cap condition.. so you can decide whether to replace or not. 4. Check ESR again. 5. Check capacity. 6. If capacity is within 10% of spec, ESR is what it should be, and your leakage rates are acceptable, then you're good to reinstall. Otherwise, replace! - John Singleton