Wednesday, 30 December 2020

Daiwa PS-220 Power Supply Repair

 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.  

PAL 500 Power Amplifier