11-07-2013, 08:56 PM
So, to the mysterious disappearing trace. I've had a frustrating few sessions with this one.
Initial diagnosis was easy - the EHT Generator was fading out. The Oscillator Anode voltage should be over 400v p-p and it gradually dropped to about 200v p-p. At that stage the Rectifier Heaters weren't lit and the PDA, and presumably the other voltages, died, but why? Switched off to move the Probes about, switched on again and it didn't come back. Ah, well, at least the fault is solid, which always makes it easier to find, doesn't it? Had to go away for a couple of hours and when switched on again it all came back. The PDA was around 7kV and for the next hour it was solid. End of Part 1.
Went down the Garden this morning to do the usual things which includes switching on the Waterfall Pump. I actually switched the 'scope on - I'm using the socket switch for safety - and didn't notice for some 7 hours. When I noticed there was a moment of panic in case something disastrous had gone wrong, bearing in mind that the Fan isn't running, but it was sitting there quietly working with a nice bright trace - odd. Switched off as I had to go out, and anyway I figured I'd let it cool down. Came back this evening, switched on and it's solid as a rock. Well over 400v p-p on the Anode and over 7kV on the PDA. Even the Pot works, although it doesn't change the PDA by very much.
In all honesty I haven't a clue. Perhaps it's simply drying out completely. I'm still not convinced that it's right yet, but I think it will do for the minute so I can get on with the rest. I've bought a couple of wire-ended 680pF 10kV Caps just in case they're the problem.
Any suggestions gratefully received, but I can't guarantee immediate action.
Alan
Initial diagnosis was easy - the EHT Generator was fading out. The Oscillator Anode voltage should be over 400v p-p and it gradually dropped to about 200v p-p. At that stage the Rectifier Heaters weren't lit and the PDA, and presumably the other voltages, died, but why? Switched off to move the Probes about, switched on again and it didn't come back. Ah, well, at least the fault is solid, which always makes it easier to find, doesn't it? Had to go away for a couple of hours and when switched on again it all came back. The PDA was around 7kV and for the next hour it was solid. End of Part 1.
Went down the Garden this morning to do the usual things which includes switching on the Waterfall Pump. I actually switched the 'scope on - I'm using the socket switch for safety - and didn't notice for some 7 hours. When I noticed there was a moment of panic in case something disastrous had gone wrong, bearing in mind that the Fan isn't running, but it was sitting there quietly working with a nice bright trace - odd. Switched off as I had to go out, and anyway I figured I'd let it cool down. Came back this evening, switched on and it's solid as a rock. Well over 400v p-p on the Anode and over 7kV on the PDA. Even the Pot works, although it doesn't change the PDA by very much.
In all honesty I haven't a clue. Perhaps it's simply drying out completely. I'm still not convinced that it's right yet, but I think it will do for the minute so I can get on with the rest. I've bought a couple of wire-ended 680pF 10kV Caps just in case they're the problem.
Any suggestions gratefully received, but I can't guarantee immediate action.
Alan
