Still awaiting arrival of the engraving bits. ETA - Aug 23. Dang!
Got the mill power supply mostly built... and changed course again!
I decided I really wanted a variable voltage supply. I remembered an old power supply I had picked up at some hamfest back in the 70's that has been sitting around gathering dust since. It was a bench supply meant to be used while servicing tube-type equipment. It had outputs for a 6.3 volt ac filament voltage, and a plate voltage output that was variable from 0 to 300 volts dc at 100 milliamps max.
Well, that was of no use to me... my tube days are long gone and 100 ma was not enough to run my stepper motors. But it would make a good basis upon which to build the supply I wanted. I had never even opened the box up and was delighted to find inside a vintage, original General Radio Company Variac model 200 B in beautiful shape.
General perfected the Variac back in the 30's, got a patent on it in 1935 and copyrighted the Variac name. This was one of their early production models. It is hardly rare, but is a nice conversation piece among us electronics nerds and is very useful.
Anyway, I decided to gut the box and build in a modern, regulated, variable supply for the cnc mill, but I wanted to keep the external appearance vintage. So out came the old paper capacitors, the big selenium rectifiers, etc. I hated chopping all that up as it really was a handsomely built unit inside, very professionally done. There was a single spdt switch on the front panel which would cause the D'Arsonval meter to read either output voltage or amperage. I installed a stepdown power transformer after the Variac to provide isolation and to lower the voltage for the rectifier/regulator board I made with the mill. It has large traces and I was able to mill it with the bit I have. With the Variac all the way up, the rectifier delivers 27 volts dc to the driver board.
The board just has a full-wave bridge rectifier, a smoothing capacitor, an LM7805 5 volt regulator for the driver board, and a glass 4 amp fuse. I kept the old wiring harness I chopped out and reused the wire. They made really nice wire back then. The frosted, bayonet-mount power indicator bulb was replaced by a white led and current limiting resistor. To do this I took a bayonet-mount bulb and cracked and removed the glass with pliers, then soldered the led to the filament leads and gobbed a bit of gorilla glue to hold it in place. It shines out through the original, red plastic dome lens. I found a couple of terminal blocks in the junk box and put them to work on the board too.
I had to open up the meter and remove the internal shunt. It was only made to handle 100 ma and was as thin as thread. I gave the meter a good cleaning while it was apart and re-glued the glass to the meter face because it had turned loose over the years. I made an external shunt that calibrated the meter to use the top 0 to 500 volts scale to indicate amps; thus, 100 volts on the scale means 1 amp. Similarly, I calibrated the lower scale that used to be 0 to 100 ma to be 0 to 100 volts, so that the 25 mark means 25 volts.
On the left side of the picture, there used to be some wierd connector going through the case side. I removed it and was left with a 3/4" square hole. So I cut a piece of ABS plastic, mounted it on the inside and drilled it out to hold an bayonet ac fuse assembly that fuses the power cord as soon as it enters the box. The power cord was old and frayed. It was an electrocution / fire waiting to happen. It got replaced by the power cord from my microwave that died last fall.
The supply is now connected to the mill and is working flawlessly. But there was not room to install the light dimmer for the spindle speed control and it would have ruined the appearance anyway, so it will be in its own enclosure.
With that done, I'm building an enclosure for the linear actuator for the heliostat and while waiting for glue to dry I'm boring people by posting this stuff on the forum.
Don't miss the next exciting episode!