03/05/2026
It's been a very busy start to the year with a lot of uninteresting business as usual type stuff. I thought this David and Goliath fault on a vintage PSU was worth posting. This is what a professional 20A linear PSU looks like, an uncomfortable one person lift and not much better for two people over relativity short distances! The unit presented with no regulation, output over Voltage for selected coarse range at full smoothed rectified Voltage. Removing the plug in PCB containing the regulator circuit resulted in no output, eliminating the pass transistors. A quick check in circuit showed resistive (not short circuit) junctions B-E B-C on VT1, out of circuit checks confirmed this. Both VT1 and VT2 replaced with BC558. So the mighty Farnell B30/20 was defeated by the lowly 2N3702, but not for long! The specifications for this unit are quit impressive, 70's design, built early 80's still going strong.
