HP8563E A15射频板上的100MHz VCO故障

亲爱的，我在HP 8563E频谱分析仪上遇到了问题。
A15射频板上的100MHz VCO不会振荡（在VCO的输出端进行测量，带有TEST位置的带子）。
我确信VCO有故障，PLL环路必须工作。
电源轨（+/- 15V）很好。
我检查了有源元件（没有去焊），但没有一个看起来有问题。
调整C750电容器不会带来任何影响。
我很高兴知道是否有人已经经历过（并解决了）这样的故障，或者VCO的某个部件是否容易出现故障......我的备件中几乎所有的VCO有源元件（100MHz xtal除外），但是
在射频板上脱焊之前，我很乐意接受你的建议。
感谢您的阅读和帮助。
干杯，多米尼克B.



     以下为原文

  Dear all,

I have an issue on a HP 8563E Spectrum Analyzer. The 100MHz VCO on the A15 rf Board doesn't oscillate (measurement made at output of the VCO with strap on TEST position). I am sure the VCO is faulty, the PLL loop must work.
Power rails (+/- 15V) are fine.
I have checked the active components (without de-soldering) and none of them seem faulty.
Adjustment of C750 capacitor doesn't bring anything.
I would be happy to know if somebody has already experienced (and solved) such failure or if  a component of the VCO is prone to fail ...
I have almost all the VCO active components in spare parts (except the 100MHz xtal) , but before de-soldering on the rf Board, I would be very happy to have your advice.
Thanks for reading and for your help.
Cheers,
DominiqueB