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Picture of '62 amplifier board

Shown above is a closeup view of the PC board in 62's power amplifier prior to repair. The large black lead near the center is the negative lead from the power supply. It must deliver about 20 amps to the board.

Clint, KA7OEI, speculates on the series of events that led to the complete failure:

  • Sometime between 11/05/03 and 12/30/03, one of the two final transistors in the '62 amplifier failed. By 12/30/03, the output power was likely in the 20-25 watt area instead of the normal 130 watts.

  • The 100 watt amplifier is actually two 40 watt amplifiers in parallel (the transistors used are rated at 40 watts each at 175 MHz and are easily capable of more than this at 2 meter frequencies.) Their outputs are combined, yielding the full output power. Inevitably, the two amplifiers aren't going to put out exactly the same amount of power, and equalizing resistors are placed between the two amplifiers to help "power sharing" and to soak up some of the mismatch. With one failed amplifier, the pair's balance was upset, causing the resistors to have to dissipate much more than their power ratings.

  • With the resistors having to dissipate way too much power, they burned open. This, coupled with the imbalance, probably put additional stress on the remaining device - which was now poorly matched with the load. Additionally, the "dead" amplifier section was no longer properly matched to the driver stage - something that could both present a mismatch to the driver and/or cause additional/excess drive power to appear on the remaining stage.

  • The mismatch probably excessively stressed both the driver and the final stages. It is likely that the remaining final stage failed due to these stresses and was followed in relative quick succession by the driver stage - which after failure of the last final, likely experienced a catastrophic mismatch.