Daiwa PS-220 came to me used, and I expected it to be in working condition. When I powered it, the light came on in the power switch, but no DC. A quick check showed the bridge rectifier was OPEN on all four diodes. Strange, but my other DAIWA needed the bridge rectifier changed also. I used a much higher rated (40amperes) bridge rectifier.
Still no voltage on the output. Hmm.
The two power transistors, and driver transistor were also open.
I don't know how they all went at once, but they did.
Given the circumstances, and that I did not quite understand the details of the circuit, I ordered replacements for all semiconductors that I did not have in stock.
Today, I replaced them. It took several hours and getting the power transistors to line up with the holes in the heatsink was quite a challenge and took half of the time. The diodes on the board all checked OK, but I just changed the op amp and pre-driver NPN transistor.
One challenge is that the wires on the circuit board are solid core as they used wire wrap to connect everything up. A couple of them broke in the effort. I also found a cold solder joint on the voltage adjust pot and I suspect it had been like that from the start of the unit's life.
Selecting replacement parts was a bit of a trick as nothing original is available any longer.
In no particular order, here is what I used, all from Digikey:
Regulator Final Driver: TIP41A NPN
Pre driver, 2N3904 generic NPN
Final regulators, NJW21194G
These power supplies always THUD when you turn them on due to Inductive resistance, so I added a Metal Oxide Varistor, Ametherm, MS22 20005. No more thudding. See my other article about the change to the inrush current when using MOV in series.
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