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TonyDuell · 24th Mar 2021, 7:04 pm

I'm putting this under 'vintage computers' because the supply in question comes out of a vintage computer peripheral and becasue I doubt you'd come across such supplies in other types of vintage technology. But anyway...

I have a Weir HSS100/4 power supply that has failed in a big way... The sequence of events :

I power it up, no load, with a lamp limiter. My experience is that SMPSUs are fine on lamp limiters provided they are supplying a lot less power than the rating of the lamp, so the input voltage is not seriously reduced. It comes up fine, but the +5V output is a little high.

Switch out the lamp limiter and start loading the output. The +5V rail RISES as I load it. This is something I've seen before and generally means high ESR/open capacitors on that output. So I replace the 2 appropriate ones.

+5V rail now seems fine off-load and when supplying 1A (no lamp limiter in the circuit) But the thing seems to be oscillating at an audible frequency, I can hear it.

Connect the 'real' load, 4 boards of logic/microprocessor, etc. It can't supply the current (it is the original supply from the unit), the 5V output is pulled down to 2.5V or so. No lamp limiter in the input circuit of course.

While I am thinking what to do, there is a small 'pop' and the outputs drop to zero.

Turns out the 2A fuse in series with the mains input has blown.

Check over the supply, the chopper transistor, a BUW12A, is shorted all ways round.

My first thought is that the mains smoothing capacitors have dried up, the chopper was thus running with a low input voltage and working too hard.

All rectifiers test OK, by the way.

I shall replace said capacitors and the chopper transistor, but is there anything else I should be suspicious of?

24th Mar 2021, 7:04 pm	#1
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,215	Weir HSS100/4 PSU blowup I'm putting this under 'vintage computers' because the supply in question comes out of a vintage computer peripheral and becasue I doubt you'd come across such supplies in other types of vintage technology. But anyway... I have a Weir HSS100/4 power supply that has failed in a big way... The sequence of events : I power it up, no load, with a lamp limiter. My experience is that SMPSUs are fine on lamp limiters provided they are supplying a lot less power than the rating of the lamp, so the input voltage is not seriously reduced. It comes up fine, but the +5V output is a little high. Switch out the lamp limiter and start loading the output. The +5V rail RISES as I load it. This is something I've seen before and generally means high ESR/open capacitors on that output. So I replace the 2 appropriate ones. +5V rail now seems fine off-load and when supplying 1A (no lamp limiter in the circuit) But the thing seems to be oscillating at an audible frequency, I can hear it. Connect the 'real' load, 4 boards of logic/microprocessor, etc. It can't supply the current (it is the original supply from the unit), the 5V output is pulled down to 2.5V or so. No lamp limiter in the input circuit of course. While I am thinking what to do, there is a small 'pop' and the outputs drop to zero. Turns out the 2A fuse in series with the mains input has blown. Check over the supply, the chopper transistor, a BUW12A, is shorted all ways round. My first thought is that the mains smoothing capacitors have dried up, the chopper was thus running with a low input voltage and working too hard. All rectifiers test OK, by the way. I shall replace said capacitors and the chopper transistor, but is there anything else I should be suspicious of?