I have 2 HP8563E both bought with dead YTO. and both YTO has aged BJT that has insufficient gain to keep the oscillation happen. The new original YTO is not obtainable, so I modified the YTO driver loops and installed two different type YTO (omini / advantek)? and both of them works without problem.
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Also check the power supply. HP YTO (EYO) in 85xx has output level control loop. The loop will over voltage the BJT and finally destroy it if 5V (for output FET) is not present for some extend period.
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I¡¯m also work on the new oscillator board for the EYO. With some modern tiny SMD device the rebuild YTO performance can even better than the old thin film circuits.?
