Switching power supplies are great, unless you connect them to an HF rig. Just like any other piece of electronics, they can break down. Due to the complexity of the switching circuitry, defects are often more difficult to find. This Alinco was even more of a challenge, as I could not find the schematics anywhere. All I could do is look at it, and draw a basic schematic diagram myself, so I could figure out the construction and the most likely suspects.
This PSU, brought here by another HAM, blew its fuse continuously, and it took a few more fuses before the owner determined that the problem was bigger than he initially hoped for. These were the likely suspects:
- Rectifiers (all diodes and bridges proved to be OK)
- 2x 26N50 power MOSFETS (both dead as a dodo)
- 1x 2SC5353 (NPN transistor, also dead)
- bad connections or short circuit problems (none in this case)
When FETS blow it is often clearly visible – a common nickname for a FET is ‘Fire Emitting Transistor’… In this case there were no visible signs of fire or explosions, so I used test equipment instead. Both MOSFETS were completely shorted internally. You can test this by using a simple multimeter, and without removing the components.
Tip 1: When testing within a circuit, make sure you use a multimeter with at least 10MΩ/V DC,
Tip 2: Make sure nothing is live, and no single capacitor holds any charge,
Tip 3: If you want to know how to test a MOSFET with an ordinary multimeter, go here.
The 2SC5353 had to be removed before I could test it properly. No signs of life. The outcome proved to be completely in line with previous repairs where MOSFETS were involved. If one goes, the other goes too, and they will drag any driver with them into the grave.
No local shop here could sell me the two MOSFETS, nor the NPN transistor. Grrrr. Replacement components were eventually ordered on eBay and sent here from Singapore. The biggest hurdle though was removing the defective parts. The Alinco DM-330MVE is built according to RoHS standards, which means that lead-free solder is used, and my simple 25Watt soldering iron was not up to that task.
Tip 4: lead-free solder has a much higher melting point. Even with the proper solder/desolder equipment you have to be very careful, yet work as fast as you can. Due to the higher temperatures involved, it is easy to destroy the printed circuit board.
Thanks to Guus PA2FAB, who had an unused soldering station in his shack, I could finish the repair. He even doesn’t want it back. What can I say? Thanks a lot, mate!