Step 2.1: +5V PSU Motherboard testing

Parent Category: Stories Category: VAX 11/750: The Resurrection of a Comet Written by UlliH
Motherboard (All part references apply to motherboard schematic drawing DEC # 5412550 unless stated otherwise)

The first action is to remove all plug-in boards (5V Bias/Control and +/-15V Regulator, one slot is empty in the 5V PSU); what is left is the motherboard (DEC # 54-12550), the Bottom Panel Assy (AC input, DEC # 70-15856) and the Output Assy (5V transformer, rectifier, smoothing and output, DEC # 70-16151-01). After this strip down you've got the pure raw DC section available, that is the mains bridge rectifier (two dual diodes D1, D2), the two big smoothing capacitors (2 * 4500μF) and their bleeding resistors (R3, R4 6kΩ each). Apart from the raw DC voltage monitor (E1 and accompanying circuitry) there is nothing else connected (respectively conducting: Q1, Q2, Q3, Q4) now. When there's no 875 Power Controller (as in my case), you've got to build a mains cable first. I decided to connect mine to the AC Harness (DEC # 70-16153), which normally connects both PSUs to the Power Controller, as I need this home-made cable anyway because of the missing 875. The following table shows the connections from connector P1 of the AC harness to the individual consumers (T1 Bias Transformer / Main AC Input of both H7104-C and H7104-D):

Component 875 Power Controller
H7104-C (+2.5V PSU) H7104-D (+5V PSU)
Assembly ---
Connector Bracket Motherboard Function Connector Bracket Motherboard Function
Connector P1
T1 Neutral 230V
2 --- --- --- 3
Phase via R1
3 --- --- --- 4
4 2
1 T1 Neutral 230V --- --- ---
5 4
Phase via R1 --- --- ---
6 --- ---
13 6 Neutral 115V
7 14 2 T1 Neutral 115V 14 2 T1 Neutral 115V
8 --- ---
17 7 Neutral 115V
9 13
6 Neutral 115V --- --- ---
10 15
7 Neutral 115V --- --- ---
11 11
3 T1 Phase 11
3 T1 Phase
12 ---
12 5 Phase 115V
Neutral 230V
13 12 5 Phase 115V
Neutral 230V
14 ---
--- 5 4
15 5 4 Phase
--- ---

I bought some MATE-N-LOK standard crimp pins and a 15 pin MATE-N-LOK housing. Both are still available without any problems. Well proven things obviously outlive a long time (sometimes). I crimped two pins to a wire each (using the crimping tool I already own) without inserting the pins into the housing. I use these two wires for testing purpose flexibly; e.g. plugged into sockets 12 and 14 of plug P1 of the AC harness (I've got 230V mains here in Germany) to test the mains bridge rectifier of the +5V PSU. Later I will build my own mains harness with all pins neatly fitted into the 15 pin housing.

1. Low AC input voltage test

  • Connect about 20V AC to mains input
  • Measure AC current after capacitors are saturated. Expected value: about 4 mA (about 2.5mA through bleeding resistors R3 and R4 and about 1.5mA through R12 and R13 of the voltage monitor circuit)
  • Measure DC output at J3, between pins 4 (+)  and 1 (-). Expected value: about 28V DC
  • Measure DC at both bleeding resistors (R3, R4). Expected values: about 14V DC each
  • Remove 20V AC from mains input and measure time for the output voltage to drop to 37% (about 10V DC). Expected value: about 15 seconds

Everything is fine so far :-))

2. Reform Capacitors

  • Connect about 20V AC to mains input for about an hour
  • Control the AC current stated above
  • Connect mains to mains input with a 230V 60W light bulb in series for about an hour
  • Measure DC at both bleeding resistors (R3, R4). Expected values: about 135V DC each
  • Measure AC voltage across light bulb. Expected value: Less then about 20 V AC

The 230V/60W light bulb is a very convenient series resistor for this purpose. It has got only about 67Ohms when cold (with low current), this rises to about 880Ohms when under full load (about 260mA). So it limits current to a very reasonable value in case of a short without troubling too much under normal idle conditions. Before the PSU starts "real" work the bulb has to be removed of course.

All four affected resistors (R3, R4, R12, R13) get noteworthy hot when the PSU is connected to mains power.

Still everything within limits :-))


3. Test Auxiliary Voltage Transformer T1

  • Connect mains to primary side of T1 (e.g. at P1, pins 1 and 11 for 230V AC mains)
  • Measure AC output at J3 pins 7 and 8. Expected value: about 15V AC
That's o.k, too, so the motherboard sections tested so far are all fine :-))

Download this file (5412550 Motherboard.pdf)5412550 Motherboard.pdf[H7104 Motherboard Schematic ]1049 kB