BA23 Power Supply (H7684-A / H7864-B) Replacement

This 100% new power supply, fondly named H7864-C, is a 1:1 replacement for the Astec AA12131 (DEC p/n H7864-A, H7864-B) power supplies.

It is physically compatible and will fit in any BA23 housing, and will work with any PDP11, MicroVAX (KA630/650/655) or RTVax.
Electrically it meets or surpasses all of the original power supply requirements.


A 1:1 copy of the original power supply metal shell was designed and manufactured, It is yellow zinc plated, so it matches the original power supply color while still being environmentally friendly (the original supply appears to be Cadmium plated steel). It was slighty modified to provide support for the new PCB and the two internal power supplies.

TDK-Lambda 200W power supplies are used, p/n LS200-5 and  LS200-12. LS200-5L and LS200-12L can also be used, it makes little difference (the L suffix indicates an open frame, no suffix indicates an enclosed power supply which includes a small fan).

A custom PCB holds the DC power connectors, as well as options for generating the LTC (QBUS signal BEVENTL, 50/60Hz line clock) and the DC_OK and POWER_OK signals (BDCOK, BPOK). The PCB can be assembled as one of the following options:

  • All-digital option: microcontroller generates (fakes) all the above signals
  • Mixed signal option: microcontroller samples the AC input signal and generates signals according to presence and frequency of the AC sinewave, or
  • Fully analog option – LTC generated directly from the input AC sinewave, and DC_OK and POWER_OK signals generated by a SN74AC14 IC. No microcontroller.

For all practical uses that I can think of, the all-digital option is the cheapest and most reliable.


All the original power supplies’ features are met or exceeded:

  • 85-264VAC input, no need for manual selector
  • Remote on/off
  • 5A or 10A overload switch – though I doubt it will ever trigger
  • Two fan connections, with 10V or 12V  fan drive voltage, jumper-selectable
  • 5V @ 40A, 12V @ 16.7A capable
  • DC Voltage fine adjustment + remote sense
  • Overcurrent protection, overvoltage protection
  • Full generation of BPOK, BDCOK, and LTC signals



All project files have been placed on github.