CRT Magnetic Deflection Driver DesignJune 27, 2015 5:32 pm Projects
I’ve uploaded the design files for my CRT deflection yoke driver board. This works for CRTs that use magnetic deflection. For a complete design, you will need the following boards:
- ScopePower – +1KV power supply
- ScopeVideoOnly – Video amplifier, focus chain, and nothing else (the ScopeDefl electrostatic design combines this amplifier with the electrostatic deflection amplifiers)
- ScopeVideo – +60V video amp bias supply
Gerbers are in the repository but you can get them directly from OshPark by clicking the links for each board above.
This board, unlike the others I’ve developed so far, requires both +12V and -12V. My projects typically use an Artesyn NFS40-7608J but it is now obsolete and a bit expensive, so you probably should use something else for power.
There are places on the board (C6/R9, C10/R20) for coil compensation components. You can figure out the values that you need with a little experimentation.
The board is designed for a vector-style yoke, not the far more common raster scan yoke that has a high inductance vertical deflection coil with lots and lots of turns. It can usually drive the horizontal coil no problem, but you’ll want to modify the vertical winding to reduce the number of turns. Check out my blog post on winding deflection yokes for more information on making your own.
The LM4765 audio amplifier, which drives the deflection coils, will dissipate a lot of heat so you will need to bolt it onto a good-sized heat sink (at least 3″ x 3″ aluminum with fins, not some dinky little TO-220 clip-style heat sink).
Circuit operation is pretty straightforward. CRT electron beam deflection is proportional to the magnetic field which is proportional to the current in the coil, so the LM4765 controls the coil current (measured through R14/R25) instead of the voltage. The current is therefore directly proportional to the input voltage (X or Y). An extra gain stage facilitates the width/height and left/right offset adjustments.