Black Box Amiga, repairing switching power supply

[dluck]

Hi,
I'm bringing up one of the original Black Box Amigas from 1984. I can get it to wake up and send the bootup messages to the serial port. However the switching power supply resets every few seconds. I currently have the switching power supply hooked up to some loading resisters so as to not stress the main board out. Unfortunately I do not have schematics for it. It is a +5,+12,-12 supply. It is made by California DC, Newbury Park, CA. Model LR5301-45.
Measuring the +5V, I see it starts at around 4.85 and then gradually rises to 5.1 at which time it resets. So the overvoltage protection seems to work. I suspect something wrong with the feedback and generation of the PWM signal. But not having schematics make it hard to follow the signal. I've verified that the initial 120AC->DC high voltage conversion and filtering are fine and stable. I've checked all the caps with my Cap Wizard. I've also checked the output +5,+-12 caps out of circuit and they are fine.
Anyone have schematics? I can post a picture of the power supply if that would help.
This is for an exhibit being planned for June out here in SFBA.

thanks all.
dale

 

[Hugo Holden]

The fact is without the schematic these supplies can be awkward to repair.

I faced exactly the same issue with a 5155 supply, no manual. But if you are determined you can do what I did and copy the schematic out from the supply and draw up the pcb/component topography and make your own manual for the supply. You would have to love your computer and the power supply to be prepared to do it.

When I did this for the IBM-5155 supply, I found for example that they monitored all the power rails, not just for over voltage. They created a "voltage window" detector, so the supply would shut down unless all of its outputs were in a specified voltage window. On top of that they monitored the supply's global output current and mixed that into the shutdown system.

I'm not sure if your supply is anything like that, but it gives some indication of the complexity of some SMPS's, of the voltage error detection systems and the real requirement to have the schematic for any serious attempt at fault finding, especially if you don't "get lucky" if it turns out its not one of the usual suspects like faulty electrolytics or diodes & transistors that are easily found on random testing.

So if you cannot find the schematic, there is always this option, albeit time consuming.

 

[dluck]

I do have a spare identical power supply that does work. But for now comparing the two and trying to track down the problem is not worth the time. I've already ordered a modern replacement +5V,+-12V switcher which is smaller than the original 36 year old supply. I'm tracking down a noise problem on the video sync signal output at the same time trying to remember how we used romwack. Currently I have the serial port plugged into an rs232 to usb and running putty to talk to this old guy with my laptop. Need to find a couple VT100 terminals to add to this yet. One for the serial port on the Amiga and one for the Sage IV.