Power Supply Tests and Upgrades
The machine was in good clean and complete condition on receipt, so work could immediately commence on power supply tests and safety upgrades.
Mains Power Safety
Mains Safety Revisions
Chassis Revisions
The 600 and 700 machine power supplies are designed with traces for the mains transformer primary taps on the same board and adjacent to the low voltage power supply circuits. Even in 120v land this seems a bit cavalier. DoPECC reworks the mains wiring on all 600 and 700 machines in the interest of safety:
- all mains wiring is removed from the low voltage PCB
- transformer primary tap connections are made separately and are dressed away from all other wiring
- mains switches and fuses are dressed and insulated to minimise risk of contact
Low Voltage Test and Verification
The 600 and 700 power supplies are not prone to age-related failures. Filter capacitors generally age well, it’s DoPECC’s practice to remove, reform and test these capacitors before considering replacement. In the present case, all electrolytic capacitors passed tests for capacitance and leakage after ten minutes of reforming. This has generally been the case and DoPECCs Wang machines have most of their original electrolytic capacitors.
One 100uf/25v electrolytic on the regulator board was obviously corroded and bulged, testing showed no capacitance and this was naturally replaced. Other components on the regulator board appeared to be in good condition.
The low voltage supplies were brought up with light loads and found to be:
- +5V ran at 11v with less than 50mV of ripple
- +17 ran at +19v with less than 50mV of ripple
- -17 ran at -19v with less than 50mV of ripple.
In all cases the ripple waveforms matched the rectification scheme in use, confirming that all rectifier diodes were conducting as expexcted.
The +5V supply seemed very high but was only verly lightly loaded. It will be worth retesting this under full load, if it remains so high this will lead to uncomfortably large dissipation in the pass transisitor of the 5V regulator.
Voltage Clamps
Volt Clamps
Volt Clamps
Volt Clamps
The calculator has over 200 TTL ICs, some of which are unusual types that are now very difficult to find. TTL chips have an absolute maximum supply voltage of 7V and it is clear from the measurements above that a short-circuit of the pass transistor in the 5V regulator would be very dangerous to all of the TTL chips.
Likewise, the MK4008 RAM chips are now rare and these two are vulnerable to destruction if the total voltage between their +5 and -13 rails happens to exceed 20V.
In order to mitigate both these risks, voltage clamp boards are installed:
- 5V rail is clamped at 6V
- -13V rail is clamped at -14V
The clamp boards could be fitted neatly into the space beside the printer, this allowed the Power-OK and Power-Fault LEDs for each supply to be seen through the case opening for the printer.
Power Rail Testing
The voltage regulator PCB could now be installed for the first test of the power supplies. Using dummy loads on the +5V and -13V rails:
- +5v = readily adjusted to +5.0
- -13v = readily adjusted to -13.0
- +12v = +12.46
- -12v = -12.39
This gives confidence to install the logic boards and attempt a first full power-up.
ROM Emulator Fitted
Wang 600 with ROM emulator ROM emulator mounted in machine
The DoPECC ROM Emulator was fitted at this stage in preparation for first power-up.