Dove right in! Help with RCA 21-D-8628

[JohnCT]

Originally Posted by BandDirector

What is B+, and, what do I need to worry about?

In your schematic, you'll see the 5U4 rectifier tube.



That, along with the filtering capacitors, is what turns the AC voltage from the transformer secondary to the DC voltage that the TV circuits need. The 281V point on pin 2 of the 5U4 is called B+ voltage.

The term itself is a throwback to the very early radios which were powered by batteries. Radios had "A" batteries, "B" batteries, and "C" batteries.

Why it's important is that any excessive load or short on the secondary of the transformer can cause it to fail. Why your transformer failed is something we don't know right now - it could have been a bad 5U4 or a short or loading down of the 281V B+ line. It also could have just failed on it's own by developing a shorted turn or there could have been a prolonged load on it.

Monitoring the current in the B+ line and monitoring the current on the AC input side is important if we install a new transformer and want to be sure that there is no other problem in the TV chassis that would overheat or otherwise damage the new transformer.

John