Help on IM-5228 VTVM

[Richard Post]

>
>>This unit has decided that the meter shall always be pinned in one
>>direction or the other depending on the setting of the function switch
>>and independent of any attempt to zero set. The 12AU7 is good. Any
>>ideas? I have no schematic, but tons of test equipment.
>>
>>Bob
>
Bob,

All of the vtvm's have circuits that are very similar if not identical.  I
have attached the IM-18 Heath schematic.  The meter measures differences in
the two halves of the 12AU7 across the cathodes.  A dirty switch, leaky
capacitors in the grid circuits, bad / dirty pots, or changed resistors can
cause the symptoms you describe.  Those grid resistors are in the
multi-megohm range.  Simply put, the two triodes stages are not balanced
when the meter won't center.  The 6AL5 only comes into play when measuring
AC.

Keep digging in there.  Let us know what cures the problem.

Rich

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