Saturday, 28 January 2012

Inside the EQ2 motor controller

I decided to open the control box for my SkyWatcher telescope's EQ2 mount multi-speed motor. Other than curiosity and to see how you might fine tune the speed, my main goal was to check the 6V power supply polarity in order to connect a mains adapter.

Inside the Control Box

The instructions say the tracking speed is factory set using a variable resistor inside the control box - which is actually quite easy to open.
Multi-speed EQ2 motor control box, opened

The trick is to carefully remove the four push buttons. They have a clear plastic cap which comes off with slight pressure, then the white plastic has a clip arrangement - lifting from one corner worked for me. Then there are the four main screws (shorter ones at the top, longer ones at the bottom - remember this when you reassemble it), and you also need to remove the top screw holding the power plug as well.

Close-up of daughter-board for push buttons and LED
Main PCB in control box
There are two screws holding the four push button daughter board, which you can remove and lift up to see the main printed circuit board (PCB). You can see the main chip is an Amtel AT89C2051 - specifically mine says Amtel 0849 AT89C2051-24PU. Here are some photos posted on The Sky At Night forum by drummerp showing the inside of his EQ2 control unit, which uses an Amtel 0524 AT89C2051-24Pi controller chip. The only other difference I noticed was the colour of the four push switches (mine are black, his were yellow).

I'm presuming the variable resistor is the small square unit with a blob of glue on top of it - to hold it in position after factory calibration. It looks like you'd have to melt or otherwise remove the glue, run the motor with the control box open, and use a screwdriver to fine tune the speed. Very much like how you were expected to fine tune the speed of the single speed EQ2 motor, although that gets an exposed control knob.

Highlighting variable resistor and power connections 
Power input to the control box

I wanted to determine the polarity of the EQ2 multi-speed motor's 6V power supply, normally supplied by four D type batteries (the big ones used in flashlights/torches). This would have been easy with some basic electronics gear, but not having that to hand, I read the power connection labels on the PCB instead. The red wire connects the inner pin on the female power plug to VCC which is short for common collector voltage, aka the positive supply voltage terminal. The black wire connects the outer spring of the female plug to GND, or ground, which in this context means the negative supply.


Once I'd determined the power supply polarity (center/tip positive, confirmed in this review of the Skywatcher SK1309EQ2 by Robin Scagell), I was able to connect a mains adapter to the motor in place of the battery pack.

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