IM-18 problem

[Ed Kotkiewicz]

"Harley L. Miller" wrote:

> I have an IM-18 VTVM with an attitude problem. When it is switched on,
> the meter needle pegs to the right or left, depending on the polarity
> setting. It does this on all voltage functions. The adjustment pots
> have no effect on the problem. I have changed out the tubes with known
> good ones, to no effect. I have cleaned the switches with DeOxit5, to
> no effect. Any ideas?
>
> Harley L. Miller     hmiller at sound.net

EK> Harley, when the 12AU7 and a alignment won't bring things back to true
you need to look at the voltage on either side of the transformer
secondary.  It has a cap and diode in that ckt and the voltages given on the
schematic are pretty close to being on.  If the negative voltage goes beyond
spec, look at the voltage divider ckt that makes up the cathode of the
12AU7.  My bet it the negative is high and out of spec.  From there, look at
those composite resistors.

Good luck.

Ed

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