Chassis and Power Supply
Wang 600 Motherboard
Wang 600 Chassis
One of the first steps in any restoration is to clean all the parts and then to assess, test and verify the power supply.
Cleaning
This machine was generally very clean with no significant dirt or dust internally. One unusual feature was a marked yellowing of the aluminium chassis, this could be scrubbed off to reveal clean aluminium beneath. It was unclear what this may have been, perhaps cigarette smoke residue?
The machine is built on a large PCB motherboard, this was also pretty clean and required only a light wash.
Mains Power Hazard
Wang 600 Mains Voltage Hazard
When examining the original mains wiring it was found that one poorly-dressed wire had been crushed between chassis parts during assembly and the insulation had been eroded to expose the mains conductor. This tends to confirm Wang’s less-than-ideal design of mains wiring.Power Supply
Wang 600 Revised Wiring
Wang calculators generally have very simple power supplies based on a multi-primary transformer which allows easy adjustment for 120v or 240v operation. The transformer typically has multiple secondaries to provide logic levels and higher voltages for display tubes.
The 600 series continued Wang’s practice of putting PCB traces for the primary and secondary sides of the transformer on the motherboard PCB. This results in an uncomfortable element of risk when working on these machines and DoPECC prefers to remove the primary connections from the PCB and make all mains connections in a separate, specifically insulated area.
The layout and wiring has been revised to:
- insulate all mains parts,
- remove mains wiring from the PCB and shift to a dedicated terminal strip
- add a connector to the transformer so that the motherboard is easier to remove and service
Low Voltage Testing
The low voltage supplies are relatively simple: rectifiers, filter capacitors and linear regulators.
The diodes all tested OK in-circuit.
Removing the motherboard required disconnection of the pass transistors for the +5 and -13 supplies, so it was easy to put these on the curve tracer and verify their transistor action.
Filter Capacitors
It is DoPECCs practice to inspect filter capacitors and if they look OK to lift a leg (or remove) and electrically reform them before testing capacity, leakage, ESR and loss angle at 100Hz. In most cases the reformed capacitors test OK, perhaps suggesting that manufacturers sourced high-quality parts for these advanced and expensive machines.
All the filter capacitors in this machine looked OK, reformed to very low leakage and then tested OK, so no replacements were made.
Volt Clamps
This machine has two high-risk power rails:
- +5v that supplies many hundreds of DTL and TTL ICs
- -13v that supplies rare static RAM and MOS ROM ICs Both are at risk from short circuit failure of their respective series-pass regulator transistors.
To mitigate this risk, volt clamps are fitted to each of these rails. The +5 rail is clamped at +6 and the -13 rail at -13.5.
Power Supply Testing
After the work above, the power supplies were brought up with all logic boards removed and dummy loads applied to the rails. After warm-up, results were:
- +5 rail: +5.1v with 30mV of ripple
- -13 rail: -13.2 with 40mV of ripple
- +12 rail: +12.0 with 40mV of ripple
- -12 rail: -12.45 with 40mV of ripple
This was judged to be quite satisfactory for proceeding to a first full power-up.