By Keith P.
In early July of 2019, the power supply of one of our rarest and iconic machines, started to fail. This is the IMLAC PDS-1 originally produced from 1970 to 1972. Despite the efforts of our staff to troubleshoot and replace components, we were soon left with a completely failed power supply.
Typical of these situations, we set about to do an engineering evaluation toward designing a form, fit, and functional replacement. The photos below show the power supply system in its’ original form.
IMLAC Power Supply – Power Input, Rectifier, and Filtering (left chassis)
IMLAC Power Supply – Regulator Chassis (right chassis)
Accessing the schematics, we found what voltages and currents the power supply system had to provide. Next, using the power supply components we have available for this purpose, we had to design a system which fits in the chassis and interfaces with the control and power signals the computer needs to run.
Surprise ! The Power Supply Generates a Non-DC timing Signal
The schematic below shows a section which we couldn’t figure out initially. At first glance, the collection of four diodes on the left looks like a bridge rectifier. On closer examination, the anodes and cathodes are not hooked up like a bridge rectifier. What we have here instead is a frequency doubler which is used to generates a 120 hz signal from the 60 hz power line that is used as a periodic interrupt for the video display logic. Not at all expected.
Original IMLAC schematic showing 120 hz sync signal generator (frequency doubler)
Below is our replication of this circuit. We used a miniature 120 VAC to 10 VAC transformer.
Schematic for 120 hz sync signal generator
After the surprise above, we set about removing the original power supply components and installing a new configurable supply along with the necessary modifications to the internal wiring harness.
Below are two images showing the right and left power supply chassis as modified.
IMLAC Power Supply – After Modification the Power Input, Rectifier, and Filtering are no longer needed (left chassis)
The regulator hardware has been replaced by a configurable power supply(on the right) and the 120 hz signal generator on the left ( orange color board with caution label ).
We completed and tested the above modifications in about 1 1/2 weeks. A week after the unit was put back in service, a third power supply ( located in the control console ) also failed. We replaced it with another configurable supply as shown below.
Control Console rear showing replacement supply. This powers the control console LEDs.
The system has now been running since late July without incident.
This is a good example of the type of work we have been performing for the last 15 years.