I have been able to revive the 20-year old autopilot on my boat mostly by replacing electrolytic capacitors. I bought failed main boards and control heads (user interface) on eBay and have been getting some to work so I'll have spares. After replacing the caps, one formerly dead control head works fine once it gets started. But starting it may take several minutes. I'm just a hobbyist but I've built a few circuits and isolated the problem on this control head to a 34063A in a SOIC-8 package used as a step-down converter (25 to 5 volts). I'm using this datasheet.
On a good working control head, I get the following voltages on the 34063A's pins from 1 to 8: 25.44, 5.09, 0.73, 0.0 (GND), 1.25, 25.44, 25.44, 25.44. On the problem control head's 34063A, pin2 the switch emitter will slowly rise from about 3.1 volts, pin3 the timing capacitor slowly descends from about 2.5 volts, and pin5 the comparator inverting input slowly increases from about 0.8 volts. After about 2-3 minutes when pin2 gets up to 5 volts the control head works properly and the voltages on all pins closely match the working unit. I've let it run continuously for 48 hours without a problem.
So, how do I fix the problem? Based on the symptoms, could it be the 34063A itself, the inductor connected to pin2, or one of the required caps? The resistance of the inductor matches the one on the good board. I'm not sure how to find the supporting caps shown in the datasheet's schematic for a step-down converter configuration. I'd appreciate any ideas or suggestions. Thanks.
On a good working control head, I get the following voltages on the 34063A's pins from 1 to 8: 25.44, 5.09, 0.73, 0.0 (GND), 1.25, 25.44, 25.44, 25.44. On the problem control head's 34063A, pin2 the switch emitter will slowly rise from about 3.1 volts, pin3 the timing capacitor slowly descends from about 2.5 volts, and pin5 the comparator inverting input slowly increases from about 0.8 volts. After about 2-3 minutes when pin2 gets up to 5 volts the control head works properly and the voltages on all pins closely match the working unit. I've let it run continuously for 48 hours without a problem.
So, how do I fix the problem? Based on the symptoms, could it be the 34063A itself, the inductor connected to pin2, or one of the required caps? The resistance of the inductor matches the one on the good board. I'm not sure how to find the supporting caps shown in the datasheet's schematic for a step-down converter configuration. I'd appreciate any ideas or suggestions. Thanks.
