Vixen DMD-3 motor controller

[Pixies]

This is about a repair rather than pure DIY. So if this is the wrong forum, my apologies.

I recently purchased a pair of MT1 motors, gears, etc. plus a malfunctioning DMD-3 controller. For the Vixen Super Polaris mount I'm trying to restore. I thought I'd see how these things worked and whether the mount was worth trying to set up with auto-tracking eventually. But first, I just want to see if I can fix the controller and use it for simple tracking and control.

I stuck a long pointer on the shaft of a motor, connected the controller and tried to work out what was going wrong. It turns out the basic sidereal tracking on the RA channel is flakey. I timed full rotations of the shaft and they weren't consistent. 11-13 minutes, roughly. With the pointer, I can see that the shaft rotation occasionally sticks or shudders, too. Also, if I try and slew (x2 or x16) and change direction (hemisphere) it doesn't always change immediately.

The Dec motion appears to be fine. I swapped motors over too, to confirm that this isn't motor-related - and it isn't. I also stuck my multi-meter intro the stepper motor sockets and can confirm that all 4 lines of both RA and DEC are working. So it's not a cable/connector problem,

Now, I'm no electronics engineer, but I used to be a bit handy with a soldering iron in my youth, so took the controller apart. I had a shot at working out the rather simple circuit board and here is a quick sketch of what I think is a basic schematic, NOT a circuit diagram:

 

So, this is how I see it working - bottom up.

The crystal and the mc14060bcp (binary counter) supply a regulated clock signal to the RA and DEC channels. These take input from the x2 and x16 slew speed switch - adjusting the output clock frequency I assume.

The first stop are the Toshiba tc40103bp ICs (down counters), which use frequency division to produce the clock signals for the stepping controllers. On the RA channel, it also takes input from the x2 and x16 slew speed switch.

Then on to the stepping controllers:

The DEC channel has a single stepping motor driver IC - ucn4202a. This includes output transistors internally with an output sink current of 500mA . The output from this goes straight to the DEC stepper motor. However, the RA channel uses a different stepping motor driver IC - pmm8713. This has only an output sink current of 35mA, so its output goes on to a Darlington Sink Driver -  TD62103P , with  output sink current of 500mA, then on to the RA stepper motor.

(Anyone suggest why there is this difference? I assume it's related to the fact that the RA channel has a constant output for sidereal tracking, but not sure why the 2 channels don't just use the same component models?)

 

Anyway. Judging that the fault affects 2 areas:

1. the sidereal tracking signal and 
2. hemisphere (direction) changes

looking at the diagram, that seems to indicate a possible single component failure - the RA  stepping motor driver IC - pmm8713. I assume that a failure with the Darlington Sink Driver would mean one of the 4 RA stepper output lines would be failing, which doesn't appear to be the case.

 

So my first attempt will be to replace the pmm8713 chip with a new one. If that doesn't fix it, then it's back to the drawing board. 

Anyone have experience with a similar fault, or properly understand these types of controllers able to comment? If I'm barking up the wrong tree, it will be good to know before I get the soldering iron out.

 

[Pixies]

It was the Darlington Sink Driver -  TD62103P.

I replaced it with a modern ULN2003A (£3 for 3) and now it works fine. The shudderring with the stepper motor has gone and it makes a full rotation every 10 minutes.

 

Just letting you know in case anyone has the same problem. I have 2 spare ULN2003A ICs, if anyone wants one!