CV Transformer - Capacitor on separate winding ?

Hi Guys (well OK - Tony mostly :-), Started looking at the Compupro 8086 S-100 system this evening. As usual during initial power tests - pulled all cards, and began to power-up through variac and series light bulb - Bulb glows brightly as power comes up, suggesting system is drawing far more power than it should, and chassis voltages (+8, +16, -16) fail to come up hardly at all. After investigating, I discovered that there is a large capacitor attached to a separate winding off the transformer. This is a oval metal can capaciter (looks like a cylinder, except that end- on view would be oval). It reads: 4 MF 660 AC 60 HZ RONKEN P81A23405H01 Protected 900 AFC NO PCB'S CONTAINS FLAMMABLE FLUID 2483 Disconnecting this capacitor "cures" the excess current draw and the chassis voltages come up fine (still running through variac at reduced AC voltage with series light bulb as I expect this cap is part of a "line voltage regulator". The transformer is labled "C.V." (Constant Voltage?) I am reluctant to conduct a "full power" test as the light bulb glows and full intensity if I bring the Variac up to 120v with the cap connected - ie: the system is appearing as a very low impedance load - when it should be drawing virtually nothing. Ohm-meter tests of the CAP show that it is NOT shorted, and is functioning as a capacitor (brief current flow when leads reversed). It looks to me as if this Cap is acting as a low impedance load at 60hz and effectively "shorting" the winding that it is connected to, which causes the transformer to draw excessive power. Can anyone explain to me what is going on, and why the unloaded power supply is drawing so much on it's input? A brief tutorial on how this type of supply is supposed to work would be very helpful... Regards, Dave -- dave04a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.parse.com/~ddunfield/museum/index.html