Re: 577 AC collector supply not reading zero on display

Chuck Harris

I am pretty sure that chassis ground is not part of the
variac circuit. So measurements to ground are suspect.

Try measuring to power line neutral.

Variac's are funny beasts.

If you think for a couple of seconds about the wiper on
the variac, you will realize that it *has* to short adjacent
turns. Absolutely no choice. Shorting a turn on a transformer
is serious business, as it shoots the current up as high as
it can go, and will burn out the turn....

So, what to do? General Radio came to a solution, they used
a brush that was high enough bulk resistance that having it
short a few turns wouldn't damage the transformer, or the brush.
The equivalent circuit of the brush is sort of like this:

Turn 1 <----R----+
Turn 2 <----R----+-----> Output from brush
Turn 3 <----R----+

If the stop on the variac is set so that the brush is parked
over the turns on the variac, the output voltage will never
get closer than one turn from zero.

The proper placement of the stop is so the brush is parked on
a metallic area that is just to the side of the windings.

If your variac is 11V when the knob is in the zero position,
you need to adjust the physical stop so that the brush is not
parked over the windings.

I can't think of any other way the variac could put out voltage
when it is in the zero position.

Also, I'm not believing the schematic I have for the 577's
collector supply. Mine shows C101/R101 as a snubber network
that connects from the wiper of the variac to ground.. there
is L101, but that is immaterial to my problem. My schematic
shows C101 as having a polarity, which cannot be. It has to
be a 3uf non-polar oil, or film capacitor.

-Chuck Harris

DW wrote:

I tried a few things in hopes to resolve the collector supply which seems to be partially on.

I cleaned the contact wiper points as they seemed slightly dirty on the auto transformer and wiper with fine sand paper then wiped it with some cotton swabs tipped in 92% alcohol to clean the surface.

I turned on the instrument and the results are still the same.

I put one probe on ground and the other on R101, which I believe is in circuit with the wiper of the auto transformer T101, I measured the AC voltage. Turned full on it measures line voltage and turned all the way off about 11 VAC, I believe I definitely found where my undesired leakage power is coming from in the collector supply.

Can these auto transformers go bad?

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