DECstation 5000/133 power supplies dead (H7826)

Jon Elson elson at pico-systems.com
Sun Oct 1 17:22:40 CDT 2017


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


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