Power Supply

Ensuring the power supply is stable and operating close to or within factory specifications is the first place to start.  While the former owner had begun work on the PSU, it’s always best to ‘assume nothing’ and start from scratch.  

My machine is 110v and so a transformer was provided by the original owner to step our 230v mains voltage down to the 110v requirement.  The transformer meant that the chassis was floating free from ground which is fine from an operational point of view, but did cause me a problem when using a grounded oscilloscope as it reflected the 50hz mains cycle across the dc voltage signal.  

Grounding the computer chassis solved this problem.

Before applying power however I decided to reform the large electrolytic capacitors using a bench power supply and slowly raising both voltage and available current.  While this took some time it was a worthwhile process, as all capacitors are now functioning and performing well.

I replaced the transformer ‘tuning’ capacitor you can see in the background as it was leaking and contained PCBs. I also cleaned the POWER OK relay contacts but otherwise the PSU was in good shape.  It is a simple diode bridge with a filtered rectified supply, pre-dating switching power supplies. 

Ripple current was higher than I would have liked, and marginally above specifications, but it has not caused any problems.

Adjusting the memory read/write and sense voltages is accomplished by tuning pots located on two of three PCB’s located inside and to the right of the PSU chassis.  However, when the PSU is fixed correctly in place with its rear locating screws it is impossible to access the voltage tuning pots on the PCB’s as they are hidden under the chassis frame.  The solution is to remove the four retaining screws at the rear of the PSU and then push on the right side of the PSU moving it slightly to the left, thereby revealing the pots.  

Once set up the PSU has been stable and trouble free. Fine tuning of the R/W and Inhibit currents required further adjustment of the PSU voltage pots.  The currents specific to each machine are written on the Core Memory box.