Don't stack anything on HDD-200!

A warning to the wise: don't put anything on top of your HDD-200 and don't put any weight on the top cover.

A couple of days ago I was troubleshooting interference patterns that had appeared on my TV, and had to lean over the HDD-200 to get at some cables. In doing so, I put my hand on the HDD-200 top cover and pressed down a bit. I heard a pop, saw a bright light, and the HDD-200 went dead.

This happened because there is a transistor heat sink on the power supply board that is only about a quarter inch below the top cover, which is rather flexible. Although the transistor itself is isolated from the heat sink, the heat sink is at circuit ground and this is not the same as AC neutral or ground potential. No doubt, the circuit ground is below AC ground potential in order to provide negative supply voltages to the main circuit board. The chassis is at AC ground, so when the heat sink touched the top cover, the negative supply got shorted to ground and pop went the fuse.

DON'T EVEN THINK ABOUT THE FOLLOWING UNLESS YOU KNOW WHAT YOU ARE DOING. I removed the top cover (two torx wrenches required.) There was a tiny tell-tale burn mark on the heat sink and inside of the top cover (as well as divots in the metal.) I replaced the slo-blo fuse, which is not easy as it is an axial lead unit vertically soldered to the power supply board and covered with heat shrink tubing. I temporarily replaced the fuse with an inline fuse holder to test the unit. The replacement fuse popped too, indicating further damage to the HDD-200. I removed the power supply board, replaced the fuse again, and it popped again. This was a hopeful sign, as it meant the power supply board was damaged and the main board might be OK.

I got lucky: the problem turned out to be two shorted diodes in the main bridge rectifier. Pretty easy to detect and easy to replace (I just happened to have two diodes in my junk box that slightly exceeded specs of the blown diodes.) If the main transformer or one of the rare Taiwanese semiconductors had shorted, I would have been out of luck. After replacing the two diodes, the fuse did not blow. I reinstalled the power supply board and the HDD-200 came back to life. It works fine now (*whew*.)

Clearly, the physical design of the unit could use some improvement. There are several unprotected heat sinks that are too close to the relatively flexible top cover. Of course, Motorola will never fix this, so do not put any equipment on top of the HDD-200 and don't press down on the top cover while the unit is on.

The power supply board could probably have been designed better, too. I think the fuse should blow before the diodes. There's a surge protector in the circuit that limits inrush current to 2.5A, so they might have been able to use a slightly larger fast-blo fuse and slightly beefier diodes in the rectifier. Maybe the transformer would have had to be larger as well. Of course, all that would cost a few pennies more. But heck, I'm not an engineer, so what do I know?

BTW, the interference patterns were caused by an unterminated video cable attached to my distribution amplifier.

By: Dick Green
F rom the 4DTV Forum