Wednesday, December 26, 2012

Destructive Testing of PA-47

Late last night before leaving my Shop, I inadvertently put a 12.6 Volt supply plug onto the 1 Watt PA, which was designed for only 9 Volts (see previous post). The results was a toasted 2N7000, but that is NOT really a problem, as I have replacement parts (and boards).
PA-47-1 and PS-47-5

Later in bed, and while trying to fall asleep, I got to thinking; Why not do some Deliberate Destructive Testing while recording circuit parameters? I could think of several scenarios that would be fun to try.

And besides, how often do you get to deliberately abuse two of your favorite RF Power Amplifiers?

Some Abuse Comes to Mind:
  • Put an over-voltage on the supply connector (similar to the mistake above).
  • Run the PA at the over-voltage condition while cooling the FET with Freon.
  • Create maximum VSWR at the PA output with a shorted and open coax (this may have to wait for another day) 

For the tests, several FETs were prepped from randomly selected Volunteers. Each was outfitted with an attached HeatSink. The HeatSink is a 1 inch square piece aluminium flashing, super-glued to the FET.
Preparation for Expected Failures
Initial Normal Use - Base Line Tests

Input to the amplifiers is provided by my Propeller microprocessor system, which supplies about 12mWatts into the amps (see previous posts).

PA-47-1, A one Watt PA, with Normal DC Supply Voltage
  • Power Off - HeatSink at 75.7F
  • 9.1 Volts DC, RF input Off, 41 ma, HeatSink at 82.4F
  • 8.8 Volts DC, RF input On, 250 ma, HeatSink at 103.6F => 20 Volts PP at 50 Ohm Load => 1 Watt
PA-47-5, A Five Watt PA, with Normal DC Supply Voltage
  • Power Off - HeatSink at 73.4F
  • Fan Off, 12.1 Volts DC, RF input Off, 90 ma, HeatSink at 82.9F
  • Fan On, 12.1 Volts DC, RF input Off, 270 ma, HeatSink at 73.8F
  • Fan On, 11.5 Volts DC, RF input On, 1170 ma, HeatSink at 102.9F => 42 Volts PP at 50 Ohm Load => 4.4 Watts

PA Input (lower) and Output (upper)
Output is 4.4 Watts
5 Volts Per Div

Destructive Tests

PA-47-1, A one Watt PA, with Over Voltage Supply - Rated at 12.5V at 1200 ma
  • Power Off - HeatSink at 75.7F
  • 12.1 Volts DC, RF input Off, 80 ma, HeatSink at 84,9F
  • 11.8 Volts DC, RF input On, 710 ma, HeatSink at -25.3F with Freon => 30 Volts PP at 50 Ohm Load
  • 11.8 Volts DC, RF input On, 520 ma, HeatSink at 148.6F without Freon => 25 Volts PP at 50 Ohm Load
  • Output Transformer was Hot to touch.
  • Only the FET Failed 40 seconds after Freon was removal, afterwards maintained 145F while power was applied - FET was toast!

PA-47-5, A five Watt PA, with Over Voltage Supply - Rated at 31.5V at 3170ma
  • Power Off - HeatSink at 73.4F
  • Fan connected to a separate 12 Volts DC Supply
  • Fan On, 30.19 Volts DC, RF input Off - The FET and the two drive (Sziklai) transistors Failed within 3 seconds of DC voltage being applied.
End of Destructive Testing

The Dead Soldiers

Several FETs were used to collect this data, only a condensed synopsis is provided above.


The PA-47 Circuit boards survived much better than I would have had imagined, components failed long before the PCB traces. For Higher Power PA's more design aspects need to be understood and considered.

The two PA circuits have be restored to there former condition, awaiting further tests or use.

Destructive Testing - Epilogue :-)

For the above described Destructive Tests, all intentionally destroyed FETs were randomly selected Volunteers (RIP). They are well preserved and available for additional photos and future forensic examinations.

Although mildly inconvenienced, no Electrons were Captured, Created or Destroyed as part of these experiments, they were ALL returned to their point of origin.

Almost all (well, maybe some) of the released Magic Blue Smoke was captured or contained for return to the part manufacture, if desired.

All transferred RF energy was absorbed by the 50 Ohm Load, most of it was emitted as therms and dissipated into the surrounding area. Only a small fraction of RF energy was transferred to a local Receiver where is was used to monitor the testing progress.

Only Ozone Depletion-Free R-134 Freon was used for the cryogenics phase of these tests.

With some microscopic surgery, all Causalities were fixed, repaired, or replaced. Both boards have fully recovered and will be returned to original service.

All of this was in the name of; Science, Ham Radio, and/or just plain FUN :-)


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