After powering on the PDP-8/L main box five time sucessfully, something wrong happened: it was not possible any more to load bits into the memory. At this point, the voltages were checked:
| PDP-8/L | |||
| pins | mesured values | expected values | use |
| AA2 | 4.9 V | 5 V | logic |
| AN2 | 3.7 V | 7 V | panel lights |
| AB2 | -15,1 V | -15 V | keys and switches |
| BS2 | -30.2 V | -30 V | memory supply - |
| BM2 | +32 V | -6 V | memory supply + (varies) |
Voltage mesured on charge, on pins corresponding to the G785 Flipchip connector, indicating that the power supply must be fixed. Expected values are given in the PDP-8/L Maintenance Manual Vol 1, p5-4, Table 5-3, Power Supply Specification.
For comparaison, voltages on the expansion box were:
| BA08A | |||
| pins | mesured values | expected values | use |
| AA2 | -0.3 V | 5 V | logic |
| AN2 | +9.8 V | 7 V | panel lights |
| AB2 | -0 V | -15 V | keys and switches |
| BS2 | -35.8 V | -30 V | memory supply - |
| BM2 | +0 V | -6 V | memory supply + (varies) |
During the sixth test, smoke came out of one of the transformator !!!
Testing the PDP-8/L bypassing the power supply was carried out using five external power supplies, providing five different voltages:
All wires were disconnected from the G785 connector excepting the green, the brown and the black wire (common). The green and the black wire connect the power transistors (placed on an external heat sink) to the G826 Flipchip (Regulator control). Unlike shown in the picture, the grey and orange wire (panel power) must also be disconnected from the G785 as well as the grey and blue wire (panel lock).
The G785 connector is connected to five DC power supplies. The voltages and amperages are set as above.
Troubleshooting the logical function of the G826 regulator Flipchip.It turned
finally to be OK.
Eventually, the responsible for the power supply failure was found: one of
the four diodes of one of the two bridges was short.
Testing the power supply of the BA08A extension box. Two flipchips are left in the box : the power supply connector (flipchip on the right hand side) and the regulator (flipchip on the left hand side). A load is provided by a 12 V 25 watt halogen bulb. Voltage turns to be OK (5.02 V).
