Bias/Control Board Overview


The H7104 Bias/Control Board (DEC # 54-13382) accomodates a lot of interwoven functions:

Voltages and Master Clock:

  • +11VA auxiliary (bias) voltage rectification and smoothing
  • Generation of 5.1VA reference voltage
  • Master Clock
  • Generation of regulated +12VA and -12VA
  • Provision of regulated +5VA

All "VA" voltages are for internal use ("auxiliary") only. I will call all internal voltages "VA" even if not the case in the schematics.

There will be "VB" voltages later on, too. These are voltages backed up by H7112 battery backup (if existent).

Handling of signals:

  • +5V DC OK
  • Switch Current Sense
  • +5V Current Sense
  • Relay Drive
  • Crowbar (Overvoltage) Sense
  • +5%/-5% Margin switch
  • Power Switch Fail
  • AC Low

You can easily see why this board is called "Control Board", too.



Check the H7104 Bias/Control Board for any obvious damage and blown fuse. If everthing seems o.k. re-insert it into the Motherboard.


Testing the unregulated +11VA DC chain

The +11VA DC voltage has got a nominal value of 11V, but may be considerably higher when under light load only.

  • Apply mains to primary of T1 (Motherboard) only (e.g. at connector P1 of AC harness pins 1 and 11)
  • Measure unregulated +11VA DC (across C11), expexted values: about 13.8V DC (higer voltage because of low load) and about 620mV ripple
  • Measure +5.1VA reference voltage (across C15)
  • Scope Master Clock Signal (at J1, pin 8 or at Motherboard J1, pin 14 or J2, pin 11), measured signal shape:


Master Clock Signal: 50μs cycle time - 38μ/12μs ratio

vertical (misaligned): 5V/cm - horizontal: 10μs/cm

overshooting of unknown origin


  • Scope Clock Signal (at collector of Q2, accessible between anode of D9 and earth)
  • Measure DC LOW signal (at J1, pin 7)

Making the plug-in boards accessible

When the Bias/Control Board is back in place (plugged into the motherboard) most of the "points of interest" are very badly accessible or not accessible at all. After some trying (with the dangerous voltage of 300V DC in mind) I decided to build an adapter which rises the board under test up above the PSUs top. The parts you need are:

  • 1 x  2x12 board edge connector 3.96 mm pitch
  • 1 x 2x24 board edge connector 3.96 mm pitch
  • 1 x 12 strip board with edge contacts 3.96 mm pitch of about 150 mm height
  • 1 x 24 strip board with edge contacts 3.96 mm pitch of about 150 mm height


The problem is: Parts with 3.96 mm (= 0.156 inches) pitch are obviously out of date :-((

I found suitable strip boards, but only 50 mm in height, so I will probably solder three together to get one of about 150 mm.

There are even edge connectors available, but at an exorbitant price ... . I will look out for cheaper ones first - even used ones will do.


Finally I found the edge connectors I need at for a very good price and best of all: free shipping worldwide! I still have to persuade a friend of mine to make the boards for me as I've decided against using the short buyable strip boards.

... still under construction ...


Next:  Not yet available, but come again soon for more :-)) - Previous: Step 2.1: +5V PSU Motherboard testing

5V BIAS-CONTROL.pdf -- H7104 Bias/Control Board Schematic