Henelec PA25 power amplifier and MU442 power supply

Over on the Vintage Radio Forum, someone asked recently about the Henelec PA25 power amplifier module. The PA25 was produced by Henry’s Radio of London in around 1970. Since I have a pair of them, including the associated MU442 power unit, I’ve documented them here.

Here are a couple of general views. The MU442 has sockets for mains in, signal input (3-pin DIN) and speaker outputs (DIN again). There are also a pair of power supply fuses. Connections to the PA25 modules are via wiring harnesses with edge connectors.




Here’s what one of the modules looks like. It’s a sandwich of a PCB and heatsink.




This is what’s on the PCB…



…and on the heatsink…



…and the back of the PCB.



The heatsink has a pretty label on it:



This is the MU442.



Inside, there’s a mains transformer with 2x 22VAC secondary windings, four BYZ13N rectifier diodes in stud-mounted packages, and a pair of smoothing capacitors.



I drew out the schematic diagram of the circuit. It’s made with silicon transistors and is pretty conventional, I think,



Here’s a PDF file of it: PA25_Schematic


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