Zenith Roundie Craigslist Score!

[old_coot88]

Originally Posted by DavGoodlin

Watch out for the horizontal efficiency coil near the damper tube. The plastic coil form can break at any time and the turns unravel. My initial fix is to thread some small ty-raps through the chassis to hold it steady in the short term. I might address this in a future post if I find something a bit more professional.

Yes, with the number of newly-acquired Zenith roundies being discussed on here recently, I was hoping someone would address this issue in depth.
The horiz. efficiency (sometimes called 'horiz linearity') coil is a super-critical component that if it fails, renders the whole set unusable.
The failure mode is the coil form itself, which crystalizes with age and breaks, releasing the windings from their sharply-tuned resonance point. With the resonant point lost, the cathode current in the H output soars 'waay out of spec, which will quickly destroy the tube and possibly damage the flyback.

There's an early version of this coil which is OK. The coil form is brown phenolic and does not disintegrate with age. Later versions ARE a problem. The coil form is easily identifiable as an off-white, semi-translucent plastic (nylon?) material. This is the stuff that disintegrates. Anybody with a newly-acquired Zenith roundie should take a look and see which version is used.

Normally the coil is tuned to a sharp 'dip' point, using a milliameter in series with the cathode of the H out tube.

If the set has the 'bad' version of the efficiency coil, there are also three coils on the convergence board using the same material, and are prone to the disintegration problem, but don't render the set unusable.