LabGuy's World: PhiloCam -
Image Dissector Camera Project, Part 2

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The DC power supply and X/Y magnetic scan amplifier - 20161230

       At this point in the project, the the magnetic scanning amplifier was the next logical step. Instantly, of course, this was going to need its own power supply and the power supply project was born as a parallel stream. I worked back and forth on these for the past several weeks. Both projects were completed successfully. The scan amp story will be documented on the appropriate [magnetic scan amp project page]. The power supply and scan amps are documented in their own multipart videos, presented below for your convenience. Warning! They are long and boring to the uninitiated. Over an hour each of unadulterated home work bench electronic construction porn! Lots of hand tool construction. Following that, we return to the camera project proper.




VIDEO: Dual Scan Video Amplifier (multi-part playlist)- 20161226

       Here is the YouTube video playlist of the deflection amplifer project. Enjoy the construction of a protoboard two channel wideband analog amplifer. Observe the organic approach I take to the construction of my projects. This is a completed project.




VIDEO: Let's build a power supply (multi-part playlist)- 20161226

       Here is the YouTube video playlist of the power supply for the deflection amplifer project. This is a completed project.




VIDEO: The PhiloCam Project (multi-part playlist)- 20170101

       Here is the YouTube video playlist of the image dissector camera project project. This picks up the story at the point where I moved all of the open air wired circuits, from the wooden breadboard, into a couple of very nice aluminum enclosures. Lots of hand tool small work bench porn for the initiated! Refer to these videos outside of this article for reference on various construction techniques.



The high voltage DC power supply - 20170102

       Moving the high voltage supply into the aluminum enclosure has been challenging. But, perseverence has paid off. The unit is fully assembled and ready for testing today. Let's get started. Please refer to the electrical schematic on the right side of the drawing shown above.

       Immediately, the bottom end of VR1, which should be -1,500V is only -500V. The 12V power input is pulling 600mA and should be about 300mA or so. The one megohm (R10) and half megohm (R11), two watt, dropping resistors were roasting hot. The top of VR2, the drift tube connection, should be -900V is not even -100V. The normal current for this resistor string is supposed to be 1mA. Something is draining twice that from the power supply and very little of the current is getting as far as R1. Where is all of that current going? The search was on!

       I first checked the feedthrough capacitors by removing the two with the highest voltages on them. No change, so the leakage current was not occurring in the feedthrough capacitors.

       Further testing has revealed that VR1, the 500K potentiometer, has gone bad. It was not rated for high voltage. Its internal phenolic wafer material has broken down and is shunting most of the current to ground. The circuit tested OK outside of the box. Of course it did. The potentiometer's metal body was not grounded! There was no where for the leakage current to go, so all of the divider voltages appeared normal. Good thing I didn't touch VR1 when it was floating outside the case! Dumb luck at its finest.

       Will have to purchase a new potentiometer tomorrow. In fact, just to be safe, I'll upgrade all three of them. Being a holiday, January second, all the local surplus stores are closed today.


       This is an on going project. Don't forget to bookmark and check back occasionally for updates.


REFERENCES:

       1. PDF file [ITT F4012 Image Dissector Tube ] from around the late 1960s?

       2. PDF file [LM1458] dual operational amplifier in 8 pin DIP package

       3. PDF file [D43C1] PNP power transistor in TO202 package

       4. PDF file [LH0101] 60W high power opamp


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        Created: November 29,2016 Last updated: January 2,2017