Roff
Well-Known Member
I built this little set-reset latch out of some 2N7000's. I turned it on, and the LED it was driving came on, but instead of 10mA as designed, it was glowing dimly with about 1mA through it. I looked for solder splashes - none. I started disconnecting wires. After disconnecting both the drains that were connected to the LED, it still glowed. I disconnected the gate of the cross-coupled device, and the LED went off. I checked the gate-source resistance. It was not high, like it should be, so I replaced that FET. Same problem. I checked the wiring again carefully, and it was fine. I checked the gate-source resistance of the FET I had just installed. Same problem. I checked the gate-source resistance of an unused part. It was fine.
I had an ah-hah moment. My soldering iron must be blowing gates. I checked the tip, and it had 24V rms on it! The iron is an old Weller TC202 with the Curie point pencil. The tip is grounded by design, but I discovered that the receptacles in the 3-wire connector where the pencil plugs into the power unit were stretched to the point where they were intermittent. I closed them up a little bit with an icepick, which solved the problem, at least temporarily.
I wound up having to replace all 3 MOSFETs. The little flip-flop works now.
I had an ah-hah moment. My soldering iron must be blowing gates. I checked the tip, and it had 24V rms on it! The iron is an old Weller TC202 with the Curie point pencil. The tip is grounded by design, but I discovered that the receptacles in the 3-wire connector where the pencil plugs into the power unit were stretched to the point where they were intermittent. I closed them up a little bit with an icepick, which solved the problem, at least temporarily.
I wound up having to replace all 3 MOSFETs. The little flip-flop works now.