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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 10th Apr 2021, 4:19 pm   #21
TonyDuell
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Default Re: Weir HSS100/4 PSU blowup

I've found a BC337 in the oscillator circuit (takes a feedback signal from the chopper transformer) that is shorted b-c and open b-e. Alas I don't have any in stock, and while something like a 2N3904 would probably work here, I'd rather eliminate all 'probablies'. So I'll have to order some. All the other transistors test correctly as pairs of diode junctions and have sensible hfe readings on my multimeter. All diodes check correctly too. Of course they might break down at a higher current, but let's not worry about that yet.

The good news is that the chopper transformer 'rings' very nicely on my tester (built to a design in 'Television' magazine). So I don't think shorted turns there are the problem.
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Old 10th Apr 2021, 6:27 pm   #22
ortek_service
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Default Re: Weir HSS100/4 PSU blowup

Hopefully that might sort it, if it was one you hadn't previously checked.
I did quite like BC337's, using them a bit with their quite-high 0.5A rating (even if the power rating wasn't too high), to replace many others / used in my own circuits.
I think BC337's were in Maplin (Many years before they closed down, when shops held stock of components!) But I may haver got mine from Cricklewood or TV spares places like Grandata etc.
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Old 1st May 2021, 1:24 pm   #23
TonyDuell
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Default Re: Weir HSS100/4 PSU blowup

The latest on this saga.

Earlier in the week I had a day when I could be sure of being in all the next two days so I put in an order to RS for replacement transistors and fuses.

I've fitted a BC337 in place of the defective one and powered up on the lamp limiter. The 5V rail came up and the bulb was dark (brief flash at switch-on as the capacitors charged of course). I can still hear the supply oscillating but it sounds a lot happier.

After shorting out the lamp limiter I tried loading the 5V output. It will supply over 3A (1.5 Ohm resistor) and doesn't seem bothered. My meter shows a minmal drop in output from 5.12V to 5.09V which seems fine.

I'll try it back in the unit soon.
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Old 1st May 2021, 4:11 pm   #24
ortek_service
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Default Re: Weir HSS100/4 PSU blowup

Good to hear that it looks like you've managed to fix this odd design - without too many failed components & cost of these. And that its main transformer doesn't appear to have failed.
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Old 1st May 2021, 4:21 pm   #25
TonyDuell
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Default Re: Weir HSS100/4 PSU blowup

I've now reassembled the PSU casing and refitted it. It seems as though it can now power the logic boards, I get almost exactly 5V at a spare slot on the backplane, which seems reasonable (there is no remote voltage sensing in this PSU, there will be some voltage drop along the wiring to the backplane, etc, not to mention the regulation of the PSU itself)

I need to do some more tests on the unit but at the moment it seems as though I have, indeed, cured a fault.
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