[Heathkit] troubleshoot IT-11 capacitor checker

[Mike McCarty]

Michael Tortorella wrote:
> Hi all,
> 
> I am having trouble getting an IT-11 to work.  Symptom: eye tube stays open
> under all conditions with nothing attached to the test points (from page 24
> of the manual: "For positions Rx1 through Cx1, the eye tube should be closed
> with the balance control in any position but maximum clockwise.  The tube
> should be open when the balance control is maximum clockwise.")  If I attach

Ok, so clockwise should be "0", while ccw is "inf". The condition
described is with nothing attached.

> anything (and I have tried various resistors and capacitors) to the test
> points, the eye tube closes and stays closed except when the balance control
> if fully CCW.  I have changed out the paper capacitors and the tubes.  The
> B+ looks normal.  Can anyone offer a hint as to what might be going on?

Yes, the bridge is unbalanced. You say the pot is wired correctly.
That means that no matter how small a DUT you place in there, the
bridge sees it as bigger than the reference part. So, the reference
part is somehow not being connected to the bridge, and the bridge
is comparing the external cap to the few pF of stray capacitance in
the internal wiring is my guess.

Check the switching, to make sure that the reference components
actually get connected to the bridge. There may be a wiring error,
dirty switch contacts, components installed to incorrect switch
lugs, etc.

Mike
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