Subject: Fw: Re: Variac and forming caps Date: Mon, 13 Mar 2000 19:37:03 -0600 From: nfmk@juno.com To: eengineer@erols.com --------- Forwarded message ---------- From: Roy Morgan To: n4dh@juno.com, baswaplist@foothill.net Date: Fri, 21 Jan 2000 09:58:46 -0500 Subject: Re: Variac and forming caps Message-ID: <4.2.0.58.20000121095832.00b4b8b0@sdct-sunsrv1.ncsl.nist.gov> At 07:02 PM 1/20/00 -0500, n4dh@juno.com wrote: >What's the proceedure for turning a an old rig, using a >variac? Times, voltages, etc. 1) study the schematic to see if there is any dc drain on the B+ supply (such as voltage dividers or VR tubes). Unhook/remove them if any are present. 2) apply re-forming current to the B+ system from a variable power supply through a 50K or 100K resistor. (The variac is useful to provide the variable supply if solid state rectifiers or separate plate and rectifier filament supply are present. The radios own B+ supply CAN be used if you put in solid state rectifiers AND insert the needed series resistance.) 3) monitor the supply voltage and the capacitor voltage(s). Compute or measure the charging current. 4) increase the supply voltage to keep charging current modest (say 5 to 10 ma per capacitor) 5) when the capacitor voltage stabilizes at an appropriate leakage current you know the health of the cap. This may take many hours. Do not be in a hurry. I use one to 5 ma as final leakage current. If the final cap voltage is above the operating voltage, and above the turn-on surge voltage, all is well. A capacitor whose leakage voltage has lowered due to age and deterioration will draw lots of current above that voltage and will not continue to re-form. Multi-section caps can be re-formed through the isolating resistors commonly found in multi-stage filter setups with out unhooking them. Be mindful if the later sections of such filters have lower working voltage rating. The best way is to unhook one end of any electrolytic from the rest of the circuit, then re-form it individually. This also allows for testing of loss factor or series resistance if you are able to do that. Some folks say that re-formed voltage, leakage current and series resistance must be measured before a cap can be properly judged. The problem with the "bring it up gently on a variac" approach is that a cap section can get to its limit and begin to draw lots of current and you will not know it. The rectifier or worse yet the high voltage winding on the transformer can be damaged and you will not know until the thing smokes or quits altogether. Another possibility is that the capacitor sits there getting hotter and hotter and finally blows up in your face, covering the insides of your radio with sticky, corrosive goo. Have you tried to find a replacement transformer for a Hallicrafters HT-32 lately?? Or a 1939 receiver? Peter Dahl will make one for you for about one hundred and fifty dollars plus shipping. - Roy Morgan Keep em glowing! K1LKY since 1959 7130 Panorama Drive, Derwood MD 20855 301-330-8828 ________________________________________________________________ YOU'RE PAYING TOO MUCH FOR THE INTERNET! Juno now offers FREE Internet Access! Try it today - there's no risk! For your FREE software, visit: http://dl.www.juno.com/get/tagj.