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Old 3rd Jul 2022, 3:29 pm   #2
The General
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Join Date: Apr 2007
Location: Milton Keynes, Buckinghamshire, UK.
Posts: 329
Default Re: Advance bench PSU revived from the dead

As I suspected, the PCB contains two separate circuits, a switching regulator followed by a linear one. The unconnected terminal pins were on the switching regulator side of the PCB. There's also a current transformer associated with the switching regulator, ooh, need to get that wired in the right way round... Also, it was apparent that the voltage & current controls on the front panel were the wrong way round. They can't be swapped as the multiturn voltage pot won't physically fit in the other hole. The wires were neatly dressed with them as found too... That panel must be from another Advance PSU, dammit!

Now for the fun part, determining which harness wires connect to the unconnected PCB terminal pins. Fortunately, the wires had been left in their original length when they were soldered to the edge connector, so by checking their physical length & with a bit of educated guesswork, I worked out which wire goes where. All the tacked on additional capacitors were also removed.
Prudence dictated that this part of the circuit should be tested separately & so I connected up another current limited bench supply to the switching regulator & carefully applied power. Well it worked, so throwing caution to the wind it was time to apply mains power to the unit & test it fully. Again, it worked. Good stuff. With the original component values as found in the voltage & current regulation circuits the maximum output was about 35V on the '60V' range & 18V on the '30V' range. Max current limit was 3.6A on the 18V range & 2A on the 30V range. Checking the transformer core size indicates a rating of about 150VA which equates to 4A AC output & somewhat less DC so this is somewhere in the ballpark.

The bridge rectifier was only rated at 2A so I changed it for a 6A one. The meter was sticking, it was quite easy to dismantle & I found some small fragments sticking to the magnet & blocking the coil. There was enough access room to pull a tiny sliver of cloth through the gap which nicely removed the annoying bits. The voltage range was accurate enough but the current range was wrong so a tweak to a resistor value sorted it.
A good soak test with some wirewounds cooking away on my bench was the final bit of testing along with scoping out the circuit to make sure there was no spurious unpleasantness to ruin one's day.

Actual final bit was cleaning the outer panels with a damp soapy sponge & relabelling the front panel controls. I consider this a repair rather than a restoration so the white labels are OK even though they look a bit naff.
I think it turned out OK overall for something that looked initially like a parts unit....
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