After replacing a transistor in an HP-33E's dead DC-DC converter, the unit was still not functional. Close examination of the transformer revealed that one of the fine wires in it has been cut. This apparently disabled the switching power supply so its output voltage always remained near zero. I examined the schematics for the circuit and it seemed that the windings that got disconnected were not critical for the unit's operation, so I attempted to short circuit them using a piece of wire. This partially restored operation of the power supply; however, when the calculator was turned on, the converter's output voltage dropped to around 4V DC (in contrast with the 6.6V DC in a healthy unit.) This gave me an idea: instead of a short, I connected the two endpoints of the cut winding using a 3.3μH choke. The calculator came to life with good output voltage.
Bad transformer in HP-33E DC-DC converter circuit
