Electrohome G07-CB0 B+ adjustment ??

[fa001]

I am trying to repair an Electrohome G07-CB0 monitor.  The manual says to use R909 to adjust the B+ voltage to 120V. However adjusting R909 makes no difference to the B+ value. And the B+ reading is 153V. The manual also states that if the B+ voltage is more that 138.5 V that the high voltage hold down circuit will kick in.

So the question is if R909 is not making the adjustment, what else can I adjust?

[Dennis]

You evidently have regulator failure.  No amount of adjustment will do anything if the regulator has a problem.

Added Note:

There is a chance that if you have a loading problem (lack of horizontal drive), this type regulator will not regulate right.  What does the monitor do when you fire it up.  Do you actually have a raster and if so, is it too wide and/or too tall?  If it does come up, does it shut right back down?  If you have a scope that can handle the voltages on the collector of the HO xistor, is the waveform clean or does it have multiple "bumps"?  This information will help others in the forum to assist you better.

[Ken Layton]

You measure the B+ at one end of the giant white resistor on the left hand side of the chassis. One end of the resistor shows the regulated output and the other end is unregulated. Be sure you have the meter connected to the correct end of the resistor.

The B+ adjusting pot sets the output of the power supply section. If it won't adjust then you have a problem in the power supply section that warrants further inspection.

[fa001]

The monitor does nothing when powered on. No light and no glow in the neck. That is why I was checking the power at B+.
I measuerd the B+ at connector B1. But at Kens suggestion I moved to the large resister.  Same result. Changing the adjustment makes no change in the B+ voltage.
So a bad regulator sounds like a good possibility. How can one do an in-circuit test of the regulator?

[Ken Layton]

Has the monitor been capkitted? Is the small soldered-in fuse blown?

[Kevin Mullins]

Be sure to Ohm the pot itself to be certain that it isn't bad.
Make sure that it covers it's full range.

I've found myself chasing a shutdown problem once and it turned out to be the B+ pot the whole time and it was even still factory sealed, so I always check that now as well as the usual components.

[Dennis]

Did you scope the drive to and after the HO xistor?  With no drive, the regulator won't load right.