Retro-Mod Your Lab: The 1981 "Indestructible" Bench Power Supply

If you're serious about your electronics workbench, you know that relying on dry batteries is a recipe for frustration and voltage drops.. In March 1981, A.R. Winstanley published the ultimate "first item of test gear" for the serious constructor: a high-stability Bench Power Supply that still holds up as a masterclass in DIY engineering..

The Core Tech: Built Like a Tank

The secret to this build's longevity is the LM317K integrated circuit regulator in the beefy TO-3 metal "can" package.. These chips aren't just regulators; they are practically bulletproof, featuring built-in thermal overload protection and safe-area compensation that makes them "nearly indestructible.".

Key Specs:

Variable DC Output: 1.25V to 25V..
Fixed AC Tappings: 3V, 6V, 12V, 15V, 20V, 24V, and 30V for raw power needs..
Current Limiting: Precision control to protect your sensitive projects from magic smoke..
Stability Pro-Level: Uses a transistorized "pre-regulator" (TR1 and TR2) to kill ripple and ensure the ICs never exceed their maximum differential voltage..

The Architecture: Block Diagram

The design flows from a multi-tapped Douglas MT79AT transformer through a bridge rectifier into the dual-stage regulation system..

[Fig 3. Block diagram showing the system flow from AC input to regulated DC output]
The block diagram outlines the path from the mains transformer to the unregulated rectifiers, the current limiter stage (IC1), and the final voltage regulator (IC2). .

The Circuit: Precision Engineering

The schematic features a sophisticated layout where IC1 handles the current limiting and IC2 manages the voltage regulation.. Dual potentiometers allow for both coarse and fine voltage adjustment, giving you surgical precision over your output..

[Fig 4. Complete circuit diagram of the Bench Power Supply]
The schematic details the full wiring, including the 1N5401 rectifier diodes, BZX61 Zener diodes for the pre-regulator, and the LM317K regulator stages. .

Pro-Tips for the Modern Builder

If you’re scouting parts to recreat this vintage beast, keep these 1981 "hard truths" in mind:

Chassis Alert: You must use an aluminium chassis.. A steel case will mess with the magnetic fields of your analog meters and give you trash readings..
Heat Management: The LM317s and power transistors need finned heatsinks and proper isolation using mica washers and silicon grease..
The "Safety Check": Before you flip the switch, run a resistance check between the IC cases and the chassis—it should be infinite.. If you have a short, you're going to have a bad time..

The Wiring Layout

The final assembly requires meticulous interwiring between the 100x100mm glass-fibre PCB, the front-panel meters, and the rear-mounted heatsinks..

[Fig 6. Wiring diagram of the complete unit]
A detailed wiring guide showing the connections between the transformer secondary taps, the PCB, and the front panel controls. .

Verdict

Whether you're restoring a vintage unit or building a "retro-mod" version with modern components, the Winstanley Power Supply remains the gold standard for DIY test gear..