BUILDING THE MJR-7-Mk3 - Photos.
The resistors, presets, zeners and fuseholder are added in the first photo.
The 0.4uH inductor is made using 13 turns of 18swg (1.2mm diameter) enamelled copper wire. The inside diameter is 1cm, which is the diameter of an AAA (LR03) battery, used in the next photo to wind the coil.
The inductor, LEDs and diodes, and two wire links are added next. The LEDs specified have their cathode identified by a shorter lead, but some other types have a flat on the case next to the cathode. There are a few types which do not conform to this standard, but in any case the check mentioned later is a good idea to ensure correct operation of the LED bias chain.
The next three photos show the board with all the capacitors added. Take care to connect the electrolytics in the correct polarity, most have a light coloured band with minus signs down the side next to the negative terminal.
This is a good time to check that the LEDS are working correctly before adding the transistors. The different earth connections can be taken to a single point on the board, as shown in the phote, for this test, and wires with clips used to connect to the power supply. With 60V connected (the red wire and clip is positive) the LEDs should all light, and if their voltages are checked each should drop around 1.75V. There is some variation, my own samples varied from 1.7V to 1.8V, which is not a problem.
Finally the transistors are added. The aluminium heatsink bracket is held in place by 3mm diameter bolts through the power mosfets. The mosfets must be separated from the metal bracket by suitable TO3-P insulating washers (I used mica washers) with heat conducting compound used on both sides of the washer. It is a good idea to drill the back of the bracket first so that it can be bolted to the case and heatsink. The bracket alone is sufficient heatsink for initial testing and setup, and if only low power operation is required the back panel of the case being used may be sufficient for cooling, but generally a finned heatsink will be needed.
The mosfet leads can be curved slightly as shown next to make them more flexible so that they are less likely to be damaged by thermal expansion effects.
There is one final addition which is the high current mosfet links. These wires carry highly nonlinear currents, so they are kept close together to avoid large area current loops which can generate magnetic fields which may be picked up by the sensitive input circuits. This looks a little untidy, but is far better than using wide tracks on the board.
