Cause of intermittent indication on early Avominors

[Phil G4SPZ]

I've encountered this fault twice now in quick succession, and having scratched my head and wasted much time tracing it I thought I'd share it with you. The symptoms are erratic and/or intermittent pointer deflection.

This is caused by the top (front) bearing locknut on the movement coming slightly loose. This nut should hold the front panel zero adjuster in fairly tight friction contact with the front of the frame. The circuit through the moving coil relies on good electrical contact at this point. Early movements (1935-36) have nothing but a fibre washer between locknut and zero adjuster, but in later production this was replaced by a tiny wavy phosphor bronze spring washer.

Careful tightening of the locknut plus a tiny spot of contact cleaner cures the problem completely. The same fault affects both the DC-only Avominor and the Universal model which share a similar movement.

Phil

[threeseven]

Now that could save considerable frustration! Many thanks for the tip Phil!

[Phil G4SPZ]

Oh, yes, I wasted quite a bit of time on those two! Twiddling the front-panel zero adjuster while displaying a reading should quickly identify whether this is the cause of the intermittency or not.