B+ voltage testing

[kardenm]

Can someone explain B+ voltage to me, what it is, how to test it, what it means?

I keep seeing that test come up in problems on this forum and on repair guides but I'm a relative novice so don't know much about it YET.

Trying to learn as much as possible. Thanks.

[lilshawn]

the B+ is what the monitor uses to generate various voltages around the board but mainly the high voltage for the flyback transformer. it's generally thought that testing the B+ (since its the base for allot of different systems) and seeing if it's low or higher than what it's supposed to be, is a good indicator of what's going on.

testing is dangerous, if you don't know what you are doing. b varies from monitor to monitor, but the voltage could be as high as 160 volts DC. (could go way higher if there is a problem.)

usually, if it's supposed to be say 150v DC, if it's low (130) it could be something is bogging the voltage down (bad caps etc). if it's WAY down (90 or less) then there might be a shorted component someplace. if it's way high (170+) maybe something is "open" and is causing the B+ to "run away" because of no load attached to it.

when testing the B+ i use alligator clips on the meter leads and ensure nothing is touching before powering on the monitor. You don't want to try and hold the probes on and chance slipping off and shorting out the chassis.

[kardenm]

Thanks lilshawn: 

Where on the chasis do you typically test for this voltage?

Is it AC or DC?

[SirPeale]

It depends on the chassis where you measure it from, but the black lead always goes to the monitor frame.

On a G07 it's measured from the tab on R901, closest to the tube.  On an Sanyo EZ it's measured from TP91 (which is a pain to get to).

And it's DC.

[smartbomb2084]

     Doesn't a shorted component cause INCREASED voltage and an open component cause DECREASED voltage? When a monitor's switching power supply shuts down it is because of a short causing massive amounts of current to flow through the monitor. If the B+ is higher than normal in a linear power supply it is almost always due to a short somewhere. A fuse blows because power has increased due to a short. When it blows it is now open and power cannot flow. Am I wrong here?

[SirPeale]

A shorted component creates increased draw, taking voltage away from other things on the board.  An open one shunts current away from parts that would need it, creating different pathways and the voltage can go up.

And it all depends on what's shorted; some things just cause a malfunction, other things cause so much draw that the fuse blows.

[kardenm]

Where would I measure B+ on a WG7302 and what should it be?

Thanks for the input.

[SirPeale]

According to the Randy Fromm flowchart, it should be taken on the output side of coil L802 at the junction of C818.  It should be ~123VDC.

[kardenm]

Thanks SirPeale:  Where did you find the Randy Fromm flow chart?

[SirPeale]

I have a copy of the Big Blue Book.

[kardenm]

Aahh! I see, I see. Thanks again.