View Full Version : Magnavox 40T584S Chassis U92011BB

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!

Electronic M
05-11-2017, 08:29 AM
Have you checked the high value resistors on the reg tube? If it uses two 1.5M resistors they need to be a matched pair.

You may want to look into upping that B+, either by removing load from it or perhaps connecting the set to a variac and cranking the line voltage to the set a bit above what is coming out of the outlets. Remember conduction is determined mostly by the G-K voltage difference.

Also if you have a service manual for that set it likely will tell you how to measure reg current off of resistors in the circuit...That info may help.

05-12-2017, 03:15 PM
But there must be a reason the B+ is low. I think the boost is okay on the G but the B+ on the cathode is low. Another thought. Originally it had two 160 Mfd caps off the secondary between the rectifier diode and ground with the other side of secondary tied to junction of the two caps. He changed these to 180 mfd. But I wouldn't think that would make this much difference. It only has half wave rectifier.