[Central-Electronics] Repair of a CE 600L

[Lynn Fisk & Georgia Keysor]

Gentlemen, I asked the help of several of you regarding my 600L which suddenly developed a problem with the load/mismatch circuit.  As soon as I powered up the high voltage, the mismatch light came on and I was out of luck.

Thanks to all who offered some help to me, off line and on the air.  In the end, it turned out that I had no failure of any of the components in that circuit, except for a failing contact on the telephone type relay which must draw in and hold when the high voltage is engaged.

After getting the amp out of the case, I noticed that when I fired up the high voltage, the relay was pulling in, then dropping out almost instantly.  And I observed that whenever I pushed in the mismatch reset button that the relay would pull in but only so long as I held the button in. This button grounds the cathode of the 12AT7.  This cathode must be grounded for the circuit to work.  One of the contacts of the telephone relay provides this ground which keeps the tube functioning after the initial "flash" which occurs when the high voltage is engaged.

A simple cleaning of that relay contact fixed the problem.

I hope my experience will be helpful to others.  7, K5LYN.