Two Isotech IPS603 power supplies were being thrown out at work because they were faulty. That seemed like a sad end to what are quite nice power supplies. They are still currently being manufactured and since they both had different faults I thought it would be worth a bit of mix & matching to see if one could be salvaged with little effort.
One of them was blowing the mains fuse instantly and the other had a flickery non-functional display. I tried swapping boards over and that didn’t work. Evidently the faults are both in the power board at the back.
I thought I would tackle the blowing fuse one first as an easier proposition. The first guess has to be that the primary semiconductors were blown so I desoldered and removed the heatsink assembly closest to the mains input. The in-circuit resistance readings were suspect but on removing the heatsink the transistors just measured as non-directional shorts. The NPN bipolar transistors (as identified by the 2SC3320 part numbers) were readily replaced for a few pounds and that power supply burst into life.
Now it can be that a passive component fails and takes out the semiconductors. If that were the case the semiconductors would blow again, either immediately or fairly soon. But in any case replacing the passives is beyond my interest level!
Having one working power supply and one faulty one is nice when you don’t have a circuit diagram. I used the good one to feed the bad via a connector labelled “power”. The bad one then had a good display and seemed happy.
The voltages on the good power connector were
Yellow +5V
Black 0V (ref)
Orange +5V
Brown -15V
Red +15V
The faulty one was
Yellow 0V
Black 0V (ref)
Orange +4V
Brown -13V
Red +13V
Incidentally, I found the 0V colour by spotting a 74HC373, looking up the data sheet power connections and buzzing pin 10 to the black wire on the connector.
It looks like the faulty one is parametrically faulty, so perhaps a feedback path is broken, giving an incorrect voltage on all the rails. But I am not a service engineer and without a circuit diagram I am a bit stumped for now.
Legal disclaimer: if you get a shock off working on such a power supply don’t blame me. The internal voltages are likely to be high for a while after turning the power off and working on mains equipment with the covers off is always potentially risky.
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