Help to repair oscilloscope Philips PM3207 (Algemeen)

door Onno-I , 04-11-2020, 08:20 (1481 dagen geleden) @ Maurice
Gewijzigd door Onno-I, 04-11-2020, 08:27

Hi Maurice,
I'd like to disagree with you on the voltage over V800. It is not reverse-biased constantly.
Its anode is connected through R804 to the (relatively) positive output of D801, conducting to feed the bias current to V801, controlled by D801.
When V801 is oscillating, pulses from the feedback winding of T801 will make the connection point V800 C802 T801 go positive until V800 starts to conduct. I believe a diode is needed here to prevent T801 from being dampened by R804, but it must be a zener to prevent the voltage on this point to become too high.
So there will be a signal across V800 between peak values -0,7 and +2,8V, or maybe normally not even reaching +2,8. You cannot measure this with a DVM.
Carlos has measured 0,7 V across V800, which means it is conducting (or most of the time, there might be a train of narrow pulses) and V801 is not oscillating.