CV Transformer - Capacitor on separate winding ?

[Randy McLaughlin]

From: "Dave Dunfield" <dave04a at dunfield.com>
Sent: Monday, May 02, 2005 8:34 PM


> 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

My favorite chassis, it is a constant-voltage you need to keep the capacitor 
connected.  CVT's are found on two S100 manufacturers - first TEI and later 
CompuPro.  On systems like Cromemco's the power-supply runs high and the 
more cards you add (more load) the closer to 8v you get on the 8v line 
(starting at up to 11v), with CVT power-supplies it should stay near 8v 
independent of load.

Do check that the other capacitors (filter) are not shorted.


Randy
www.s100-manuals.com