Saturday, July 25, 2020

Found the problem with the power-down of my 3174

RESULTS OF MY INVESTIGATION INTO THE POWER-DOWN BEHAVIOR

I removed the signal cable and checked the ground side of the edge connector, the IDC connector and the Gotek. Those were all correct. I tried removing the signal cable entirely but the machine still refused to power up when the Gotek was connected to the power cable, but was just fine otherwise.

I then thought to check the voltages on the Molex power connector. This four pin connector had a red wire, two black wires in the middle and an orange wire. PC connectors for floppies are wired with a red wire, two black in the middle and a yellow on the end. In the PC red is for +5V and yellow is +12V. 

On the damned 3174, red is for +12V and orange is for +5V. If I had a proper keyed adapter cable with a male Molex and female mini power connector, all would have been well. Unfortunately, since I was using jumpers temporarily I had no keying to protect me and no idea that the color codes would be misleadingly close to the rest of the world's coloring for power leads. 

I had fed +12V to the Gotek power input. Obviously that blew out something and it now appears as too much of a short for the 3174 power supply, causing the shutdown. The connector on the Gotek presents 18 ohms, which would cause a draw of 278ma on the +5V rail. That is a consumption of almost 1.4W, way too high and indicative of catastrophic failure inside the Gotek.

I will open the unit and hope that I can find a failed component that would have protected the rest of the box from damage, but I fear that is unlikely. More likely, the over-voltage cause several components to fail and it won't be worth the effort to repair. 

Another Gotek is ordered and on its way. Drat.


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