The new parts arrived to repair the Topward TPS-4000 power supply I had diagnosed in a previous post.

The Motorola MJ2955 was replaced with a MOSPEC branded part as originals are in short supply and popular with audio amplifier enthusiasts.

The 2SA1015GR was replaced with a BC557C as I have many of these.

Being the inquisitive type, I decided to cut open the old power transistor to see what was inside.

Using a digital microscope I managed to capture significant damage on the silicon.

Completed power supply.

The Topward TPS-4000 is a linear bench power supply with two independent adjustable outputs and a third fixed 5V output. It is this 5V output that has failed.

Original schematic (incorrect)

The original schematic seems to be missing the pass transistor and has the other transistors connected in an odd manner which suggested something might not be right.

Reverse engineered schematic (correct)

The transformer/bridge rectifier/filter capacitors have omitted for clarity.

LTspice Simulation

Q2, BD912, would have originally been a 2SA699 transistor.

Failed components – Motorola MJ2955 pass transistor and 2SA1015 sense transistor.

The MJ2955 has a short circuit between the collector & emitter and the 2SA1015 has some odd junction behavior when tested under forward & reverse bias.

I’m waiting for a replacement MJ2955 and the 2SA1015 will be replaced with whatever jellybean transistor I have at hand.