CTC9A
05-10-2017, 10:33 PM
I'm helping a friend get his late 1966 Magnavox going and we have run into an elusive problem. He has recapped all electrolytics, most everything on the deflection board, replaced the boost rectifier, all tubes in horiz section. It now produces ample HV if the 6BK4 regulator is out but drops to about 18KV if in. It looks like the high B+ is lacking at about 385 instead of 410 and 381 instead of 405. When you check the grid of the 6BK4 the HV comes up about 3 KV. Voltage is about right on the grid of the 6BK4 (considering the HV pot is full so at max ground) but looks low on the cathode (the B+) so could this be biasing the tube to over conduct? What could be pulling down the B+? When it is running full, it has good focus, etc.... when it is shunting, it still only draws about 192 ma on HO but the fly will get hot if left over 15 min or so. Also, focus is poor and brightness dim when shunting down. Unregulated HV now will produce between 24 and 30 KV (before was only about 24 max) And when shunt regulator is in, it holds a very steady 18 KV regardless of brightness or contrast. Any advice appreciated!