Imsai 8080 power supply problem

[valley8080]

I received a working IMSAI 8080 recently but after unpacking it found it did not work. Checked main board voltages, even removed all boards, including front panel, and found voltages on the main board were +3v, +12v, and -23v. Power supply is a PS28 with both 95K caps in place. Nothing visually wrong in the PS section. Scope traces at the transformer secondary may show some clipping. Looks like there is a DC bias of around -7v (add this to each voltage and they are close to the published spec). House wiring is grounded (checked this). Scope traces at MB looks smooth (just off). Any ideas?

 

[MikeS]

I received a working IMSAI 8080 recently but after unpacking it found it did not work. Checked main board voltages, even removed all boards, including front panel, and found voltages on the main board were +3v, +12v, and -23v. Power supply is a PS28 with both 95K caps in place. Nothing visually wrong in the PS section. Scope traces at the transformer secondary may show some clipping. Looks like there is a DC bias of around -7v (add this to each voltage and they are close to the published spec). House wiring is grounded (checked this). Scope traces at MB looks smooth (just off). Any ideas?

House wiring ground shouldn't be relevant but I'd sure check the ground connections within the PS; what do you measure across each of the caps?

 

[Dwight Elvey]

Hi
Sounds like you do not have a ground connection in the right
place. I suspect that the power supplies ground is not connected
to the point you were using as ground.
Dwight

 

[Erik]

Is there an update to the status of this box? Have you made any progress?