After opening the case nothing burned was found, except for the rests of the fuse: the glass got broken into million of tiny glass pieces. But except that, no further damage where visible.
Testing with the ohmmeter I discovered everything after the diode bridge was in shortcircuit. After some minutes, I figured out the two main switching transistors were damaged.
This power supply uses a pair of 2SK3469 power mosfet transistors to do the switching. The 2SK3469 transistor has a TO-220 plastic case, so they was mounted directly to the heatsink, without thermal grease or insulating material. After desoldering them, I found both transistors were in total shortcircuit: from any pin to any other pin they had zero ohms, even from the gate to drain and source! Something really catastrophic happened with the transistors.
Desoldering the transistors, I noticed the PCB has a double layout for these transistors. There was the TO-220 holes, but there was other holes, suggesting a larger transistor could be used. The extra holes seems to indicate a TO-247 transistor could be used also.
Then I remembered I had an spare PC ATX power supply, so I opened it to take a look. This ATX switching power supply had a pair of 2SK2698 power mosfet transistors, in TO-247 package. What a nice coincidence!
After studying both transistors datasheets I concluded the larger 2SK2698 could be a substitute of the smaller 2SK3469, so I decided to unsolder the 2SK2698 transistors from the ATX PC power supply and install them into the Telecom AV-830. It should work...
The 2SK2698 transistors fitted into the heatsink
Interestingly the new transistors had a small ferrite bead on the gate pin, and I left them in position: It should prevent high frequency oscillations. Because the new transistor cases are not 100% plastic, I installed insulating sheets and added a bit of thermal grease.
To prevent another explosion, I tested the power supply connecting in series with the mains a 40W light bulb ( a classic one, not a low energy one! ) After turning on the power supply, the bulb briefly turned on, and then the power supply was outputting 13.8 volts. All went nicely, so I removed the bulb and connected it directly to the mains, and tested it with a some amps load. Everything worked just as expected.
Now the only issue I have to solve with this power supply is the inrush current. It is so large that I have replaced several times the on/off switch. Any ideas?