Whilst restoring a BC348 I discovered it contained a fairly lethal power supply. It used an ancient transformer capable of supplying the necessary voltages and current but the electrolytic capacitors were well past their sell-by date.
Most valve receivers contain a correctly rated transformer and maybe a choke but older circuits are usually quite rudimentary. Rectifier valves and old capacitors often result in a solution that generates excessive heat whilst providing minimal regulation. These usually contribute negatively to both energy use and frequency stability (unless you want to heat the shack). Most receivers include some form of separate regulation for the VFO and oscillators but performance usually improves when a stable supply is used.
This design is based upon a high voltage power supply using an LR8N3 to stabilise the voltage with some added components to rectify, smooth and regulate a 220v dc supply capable of a maximum 100ma.
This regulator is ideal for retrofitting in most legacy valve receivers being capable of providing a regulated supply between 50 and 250v DC.
After building a prototype I found that I needed to change the resistor values to provide easier voltage adjustment at 220v, these are shown in the schematic below.
The final design is single-board solution incorporating the following specifications and features –
• Input @200 to 240v AC
• Output 50v to 240V DC adjustable (default setting 220V)
• Link which can be removed to fit a choke
• Output current @80ma (most 5 valve receivers consume @ 68ma)
• Three stage capacitor smoothing
• 100ma fused output
• Input and output Straight 2 Pin Screw PCB Terminal Block Connectors (Blue AC input, Red DC output)
• M3 stand-off mounts
• 1M ohm bleed resistor to safely discharge the unit on power down.
Build
All the components fit on a double-sided PCB 65mm x 42mm.
Here is a finished unit installed on the BC348 dynamotor plate. As you can see the unit takes up very little space with the transformer and choke using most of the space.
Here are some images of the completed unit
Testing
Once assembled each unit is tested and adjusted under load to the 220V default voltage.
I use a dab of green nail polish on the pot once adjusted and fit a QC sticker which ensures I remember which units I have calibrated ready for dispatch.
Final Thoughts
I fitted the new regulator into my BC348 receiver in 2024 and to date its worked well. The radio consumes less power and is quite stable. Several people have also used this regulator with positive results. The heatsink on the unit doesn’t dissipate much heat but if your use this unit I would ensure the PCB is orientated horizontally to ensure the free movement of air.
The original circuit came from a power supply design and if you have a 240v output ac transformer this regulator combined with a suitable volt meter could easily be assembled as the basis for a variable high voltage DC supply?
If you are interested in this project, please contact me through this website.
73 Rex G8UBJ