On 10/01/2017 04:38 PM, Seth Morabito via cctalk wrote:
All,
I have two DECstation 5000/133s here with dead power supplies. I've done
some basic troubleshooting on both, and I've recapped the other, but
they're still dead dead dead.
I don't see any obviously blown components, and the fuses are fine, but
several of the caps I replaced WERE leaky. I inspected for bad traces
but found none.
So far it looks like the high voltage side is totally fine, but the low
voltage side is completely dead. No activity. Switching transistors SEEM
fine, but I haven't pulled them to check them out of circuit.
Most of these off-line switcher supplies have some kind of
start-up circuit that charges a small cap in the control
circuit.
When it reaches some voltage, the switcher chip starts up,
and then an extra winding supplies power to the
control circuit. So, the start-up circuit involves a
high-resistance resistor and a small, low-voltage cap (16 -
25 V rating).
If either of those fails, the switcher will never start up.
Jon
