Hello Larry
Believe that this is one of the machines owned by  Frank K0BRA, who will reply
directly to you, explaining why one actually needs an old toothbrush to hold a
particular switch in place...
We used both (the unit and the toothbrush) during our Nags Head expedition two
week-ends ago, and were able to hear with it the Indian (submarine commo)
station on 16 KHz.  It can be calibrated in absolute values, hence its
usefulness.
73
Andre'
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Larry Kayser wrote:

I have a chance to pick up a Wandel & Goltermann AT-611 Selective Level
Meter covers from DC to green light.  Does anyone have any experience with
this unit on LF?