[Kenwood] TS-930S failure

[Dr. Gerald N. Johnson, electrical engineer]

Certain tuning conditions in the tuner could result in a series resonant
but unloaded circuit that would look like a short. Other conditions
would be fairly low impedance or high impedance as seen at the tuner.
What the PA transistor see compared to what the tuner provides (when
mismatched) at its input is significantly affected by the length of coax
between tuner and radio and the phase lengths of the low pass filter and
the PS matching network.

Older RF transistors were less tolerant of mistuned loads, though I've
had a couple TS-120 and a TS-130 that I tune by laying a brick on the
key at full power and then I adjust the tuner for maximum output. So far
the SWR protection and the transistors have withstood that abuse every
time.

Transistors run at higher voltage like the 28 volts of the TS-930, could
easily be closer to their absolute voltage limit at the peak of the RF
cycle than transistors run from a 12 volt supply.

Either scenario is possible in the radio failure, e.g. PS first or PA
first.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.