Sparton 16A211 Width Problem

[Penthode]

Originally Posted by Steve McVoy

OK, my turn with a 15 inch color set issue. I've been trying to find a width problem with this set with no luck. It lacks about an inch on each side. Width and Hor. Linearity work properly. I've checked all the tubes (by substitution) and all caps are new.

I have the proper waveforms and p-p voltages at W17 and W18 (see the schematic). Power supply busses are within 5 volts. On the 6BG6s, screen voltage is right, but the cathode voltage is 12 vs 18 on the schematic. Grid voltage is -25, but I noticed that setting of the hor. drive changes it from about -15 to -30. I can just barely get 20 kv on the CRT with the HV control full up. Boost is 550 vs 620 on the schematic. Damper plate voltage is right.

I had a recent problem trying to get adequate width with a set using a 6BG6. I use an AVO Mark III transconductance tube tester and a bad tube will give only just adequate transconductance. The checker showed that the plate current was a bit low indicating cathode emission was down. Keep an eye that the plate current under test is correct and not low. I would try a few 6BG6's by substitution.

I suspect the Sparton's 6BG6 emission is low since the cathode voltage you indicated is low.

Question: Why did Sparton use a 6BG6 which such an old design tube in a color TV? I would consider substituting a 6CD6 as I believe it has a beefier cathode? (My guess after only just visually comparing tubes).