October 28, 2024

Sun 3/80 -- power supply

I have a power supply from a sun sparc system of some kind. The sparc system got recycled, but I kept the power supply because it had the proper connector to mate with the 3/80.

Today I got that power supply and plugged it in without the motherboard connected. Nothing happens. In particular the fan does not spin. I was hoping to measure voltages before connecting it, but that is not going to happen.

So I get brave (perhaps too brave), connect it to my sun 3/80 and flip the power switch. The fan spins! However I hear a crackle and see a flash of flame from the 3/80 board. I quickly switch it off. A capacitor near the PSU connector burst (C17118). So, I will need to evaluate a number of things. It is possible that the capacitor itself is the only problem. Sometimes old tantalum capacitors "depolarize" and need to "reform" after a long period without seeing power. This is what we hope for. This board has not been powered on for at least 10 years.

The other possibility is that Sun did crazy things with their power supply pinout and this sparc system supply, despite having the right connector, may have an inappropriate pinout.

Check the pinout

Looking down on the motherboard with the click tab to the right, the PSU signals look like this: 
	Grey  Brown
	Blue  Blue
	Black Black
	Black Black
	Red   Red
	Red   Red
My take on this is: 
Red = 5 volts
Black = Ground
Blue = +12 volts
Brown = -12 volts
Grey = power on
The Grey signal measures 1K to ground and 1K to +5 on the motherboard. I opened up the PSU and traced this signal. It goes to the collector of a transistor. The emitter of this transistor is ground. This suggests that this is a signal coming from the PSU to indicate power is good and probably drives a reset circuit on the mainboard.

While I am inside the power supply, I give it a general inspection, looking in particular for leaky electrolytic capacitors, but everything looks just fine (somewhat surprising for a 25+ year old power supply).

So, why didn't the PSU start up before I connected it? Often times switching power supplies will not run without a load. I am not keen on reconnecting it to my sun 3/80 to use it as a handy load. However, I really would like to get it to start up and measure some voltages. By the way, my PSU is marked: 

Sun model CR-81, Part Number 300-1038-04
DC +5  12 Amps
DC +12 2 Amps
DC -12 0.1 Amps
So, I dig around and find a 3 ohm 25 watt resistor. This will pull 1.7 amps. I use clip leads to connect it to +5 and ground on the supply. It fires right up with this load! I am a bit surprised (but pleased) that this works.
I measure: 
4.99 volts
12.00 volts
-12.13 volts
0.012 volts on the "power good" line.
All this looks perfectly good. I let is sit and heat up the resistor for a few minutes, then call it good.

Why did we blow a capacitor?

So why did C17118 explode? This appears to be from +12 to ground. It is worth noting that the main use of +12 on this board was to power a pair of disk drives -- which I never intend to do. My guess is that this capacitor (marked 336 16K) was simply old and lazy and after 20 years of sitting idle, wasn't enthused about being recruited again for service.

It takes only a minute or so to heat up a soldering iron and remove the parts of the exploded capacitor. I am not going to worry a lot about replacing it. Given that I don't plan to use disk drives, the only thing likely to use +12 are the RS232 drivers. There is no network interface on the board that might want +12 power.

You can read (as I just did) online about exploding tantalum capacitors. They are used much less often these days because of their tendency to do this. Capacitor technology now offers ceramic and electrolytic capacitors that are now used where tantalums used to be. Especially on a supply rail where there is plenty of energy available when something goes wrong, dramatic things happen.

Give it a second chance

A new day has dawned. With the debris from the exploded capacitor removed, I am ready to try it again. I connect the 3/80 to the PSU, power it up and watch for fire and smoke. Nothing.

No LED's on the board. I can use the hard drive power connectors as a handy place to measure supply voltages, and I see 5.03 and 12.2 volts. I let the board just run for a few minutes to "burn in".