Volt Clamps, ROM Emulator
Having determined that the microcode wire ROM is not servicable, the 720 will now be fitted with a ROM emulator.
Volt Clamps
Volt Clamps
The large number of TTL ICs in this machine, including rare oddities such as the 7525 core sense IC, are all at risk from a short-circuit failure of the series-pass transistor in the 5v supply regulator. To mitigate this risk, a volt clamp is fitted to the 5v line, if an over-voltage occurs this will clamp the 5v rail and blow the supply fuse, protecting the TTL logic.
Examining the other power rails, -5v and +12v are also series-regulated and feed rare ICs, so these rails are also fitted with clamps.
The clamp boards can be fitted quite nicely, two fit beside the ROM emulator and the third in front of the display driver boards.
ROM Emulator
ROM Emulator
The ROM emulator also fits nicely at the rear of the card stack, it is mounted to minimise interference with the cooling air flow and the ROM image select switch is conveniently accessible with a long tool passed between the fan blades.
Mains Wiring Revision
The Wang machines originate in 110v land, but even so the treatment of mains voltages seems somewhat risk-prone. The transformer primary taps are on the same PCB as the low voltage supply and many mains connections are easily touched.
To reduce risk the mains wiring is completely redone:
- remove all mains voltages from the low voltage PCBs
- isolate the primary wiring and primary taps
- add a short IEC input lead for convenience