Test, Reform and Volt Clamps
The Wang 700 power supply is pretty simple and supplies four voltages:
- +5 for the TTL logic, just over 350 ICs
- +12 for core drive and tape amplifiers
- -5 for the SN7525 core sense amplifiers, these seem to be very rare ICs
- -12 for tape amplifiers
The primary side of the power supply was not designed with safety first in mind, there are many exposed mains terminals and the power supply PCB freely mixes traces at mains voltage among those carrying the low voltages above.
Safety Modifications
The mains wiring was renewed, exposed terminals were shielded, and so far as possible mains was removed from the PCB.
Removing all mains from the PCB was judged to involve too much modification to the machine so an acrylic shield was mounted over the PCB to reduce the chance of accidental contact with the remaining traces that carry mains potential.
Filter Capacitors
The filter caps were removed and tested for capacity and leakage. All were found to be in-spec and were then placed on a reformer at rated voltage for an hour or so. After confirming performance and minimal leakage the original capacitors were returned to service.
Volt Clamps
Wang 700A with Volt Clamps
The low volt supplies all have linear regulators with associated pass transistors. Such supplies are at risk of over-voltage if the pass transistor(s) were to fail short circuit, this can be catastrophic if there are hundreds of at-risk parts downstream (as in the case of the +5 supply) or if there are very rare parts such as the SN7475s.
Volt clamps were applied to three rails which were judged to be high-risk:
- +5 due to many hundreds of TTL chips at risk
- +12 due to risk of overloading core memory wiring
- -5 due to risk to SN7574s which may be unobtanium.
After some rounds of PCB Tetris, reasonable places were found for the three clamp boards.
Power-up and Test
Wang 700A with Volt Clamps
First power up was successful, the fifty year old voltage regulators were still in-spec after fifty years!
5v power Ripple on 5v rail
The +5V rail showed a clean 50Hz ripple waveform, well within acceptable limits.