So I worked through the flow chart:
Is there 180V on pin 5 of P900? No, I was reading 100V
C901 and C902 are brand new 100uF 50V caps.
Check for a leaky C905 - not sure how to do this to be honest, but it was a brand new cap.
Check or replace Q900, Q901, Q902, Q903:
- Q900 - BU409 - tested and ok
- Q901 - MPSA06 - replaced with new from kit
- Q902 - MPSA06 - replaced with new from kit
- Q903 - 2N3904 - replaced with new from kit
Re-checked for 180V on pin 5 of P900 and still reading 100V
Double checked Q900, Q901, Q902, Q903 - All ok.
No burnt resistors
- R903 - replaced with new from kit
- R904 - tested and ok (470 ohm 1/2 watt)
- R905 - tested and ok (560 ohm 1/2 watt)
- R906 - tested and ok (6.8k 1/2 watt)
- R907 - replaced with new from kit
Replaced ZD902 with new from kit
After all that there was no change and still reading 100V from pin 5 of P900.
SO! What is strange when checking for shorts on the transistors to the chassis, I noticed that was getting a short / continuity reading when testing Q900 which is the BU409 transistor on the front of the HV Cage. There was no short on any other transistor, so when I took the cage out I noticed a small plastic washer that was lodged in another component. From checking back to my original pictures I could see that this should be sitting on the inside of the HV cage to stop the screw and nut from making contact with the cage!
I removed the screw, put the plastic washer in place and then tightened it back up and when checking found the short had gone.
I then powered up and.....nothing, no picture at all (whereas there had been when the transistor was shorting) and the voltage on P900 pin 5 is now reading near nothing (-0.5mV), the spot killer is off, the LV2000 lights are both on and there's chatter but just nothing coming through. I thought I might have damaged the BU409, but tested it and it seemed fine, but replaced it with a spare anyway and having the same issue.
It's got to be right that the plastic washer insulates Q900 right? Any thoughts on next steps?
