Wednesday, May 30, 2007

My LXD55: fixing the electronics

My RA motor unit got burnt. I don't know the exact reason for that, I've three guess:

1. I powered my notebook using the same SLA battery and on some moment, the notebook draw too large a current making the voltage unstable for the LXD55.
2. I found a coil around a magnet like stuff inside the motor unit, and some wire on the coil got its plastic cover partially damaged.
3. Sudden power disconnection during a small accident

To diagnose the exact problem, I swapped the control circuit of the RA/DEC motor, and now the RA motor will move. So, the problem would be on the small PCB of the RA motor.

I heard that we can get replacement from Meade, and we can also send the whole mount back for warranty but one cannot just send the motor unit back. Sending this huge mass across the Pacific Ccean twice is not a cheap game, obviously not a viable way for me.

So, I end up trying to fix the problem myself. On disassembly, I found a chip got burnt. Electronic guru from local and Internet told me that it should be a Dual P Channel MOSFET, which is from Siliconix/Vishay with model number Si4947ADY. My friend told me that this chip, together with a 4936 forms the driver for the motors. So, I end up ordered 3 sets of these chips in the hope to fix it myself.

There is a big concern before attempting to fix it. Without knowing why the chip got fried, I'm going to fry another chip again very easily. Secondly, maybe some other components are burnt too, but they looked nice at the surface. Those were the risks.

Another concern is about the cost. I don't expect to sink too much money on this mount. I just want to see how cheap mounts work out.

Finally, there are alternatives. I found that quite many people are burning their LXD55 in different parts, that means I can get replacement parts second hand. Some people marked up quite a lot for the components, and some people sell them very reasonably. Maybe these are better path. I would probably buying some of these as well, since it is a not so reliable/durable system.

The bottom line is that, LXD55 is not a cheap mount at all, since it might not live very long. A Takahashi looked more expensive at the surface, but it delivers more performance, and much more durable, and it could end up cheaper.

So, finally, what did I do?

On waiting for the replacement chips to come, my friend has found that our local deaer has the RA motor unit in stock at a moderate price. So, I got one through him and replaced the whole unit. Of course, it works out very nicely!

A few days later, the replacement chip which I had order from the US arrived my home. So, I proceed to replace the faulty chip by myself. Surface mounted chips are very hard to solder, and since I didn't touch the soldering iron for more than 7 years. I consulted a skillful friend and he told me to buy a pump to suck away those excessive tin. With his advice, I spent half an hour and got it done. I tested the repaired unit and found that it works like before. Now, I have a backup RA motor unit!

I feel proud to do it, since now I know the whole LXD55 from the mechanical point of view after the tuning of the RA and DEC bodies, I also know it to a certain degree in the electronic view point!

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