I have a G08 monitor that blows the fuses at locations F400 and F401. I found a burnt diode at location D407, replaced it and the fuses still pop. I did check the 2N6259 transistors, and no shorts there. Anybody have any suggestions on where to look next?
D407 was bad, but D406 was not? Both are part of a simpler doubler circuit. The only thing this output feeds is the 100V regulator on the HV board. You can see if this is shorted by removing the 10 pin connector on the HV cage. If one or both fuses still blow, you have another problem.
Actually, I think you do because a short in the 100V regulator should only cause F400 to blow.
If you're sure the 6259's are good, unplug their connectors from the paddle boards. There is no harm in running the monitor with them or the yoke unplugged for testing.
There are several driver transistors in each deflection channel with tall copper heat sinks. You've got +/-60VDC on these pairs. If they wind up touching, they will blow the fuses. If they have touched in the past, these transistors may be shot also.
Some of these chassis use the upper tab on these TO220 transistors as a signal path. One of the legs may be clipped and covered with silicon to prevent a short. Leaving only two legs to support them in addition to their tall size makes them prone to getting bent and shorting out somewhere.
If the power transistors have failed in the past, they may have blown some resistors on the little paddle boards. Just replacing the power transistors won't remove the problem.
Overall help:
There is a hack you can do to the main transformer that will lower the voltage supplied to the monitor that will let it run with less stress. It has worked nicely for me several times.
Look it up on the web- it simply involves moving the AC input lines from two inputs on the transformer to two different lugs. Quick disconnect spade connectors. No tools needed.
These monitors can be challenging to work on, but are straightforward in their operation. It is quite rewarding to get a zapped one going again.
Good luck-
Kerry