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