Will,
The maximum current that the 50watt amp input should need would be 7ma.? The current would also go down as the voltage goes down due to a lack of drive capability.? This supports your measurements.?
The QDX drive puts the QSK transmit/receive MOSFETs in a marginal region.? Turning off enough to allow the output to bypass the two diodes and start feedback would then wipe out the finals.? A high voltage from a bad SWR would put a strain on the part of the circuit and could have been the reason that Q5 failed.? I would also check Q3.? Both are required for proper switching between transmit and receive.
If you do not use the QDX PTT to drive an amp higher than the 7ma under the full 5volt requirement, installing the 100ohm resistor in parallel should not hurt.? A better solution is to install a P Channel MOSFET to act as a buffer between the 5volt PTT high and the 7ma drive needed by the 50watt PA PTT input.? I would use ones like the AO3407 used for the reverse voltage protection circuit of the QDX or a BSS84 as used in the QDX PPT output.? This allows the QDX to be used in other amplifier circuits without the issue of a short on the PTT line causing a failure in the QDX.
I suspect that the issue would not come up under normal conditions.? I have used my rev 3 QDX to trigger a 40meter 50watt amp running on 13.8 volts without issue.? The high voltages produced in a miss matched circuit could be the main issue causing the failure.? Q5 failure indicates that RF fed back into the T/R circuit.??
Above are observations and suggestions.? They are offered free without any guarantees on my part.? You should check for errors.? Please let me know if there are any.
73
Evan
AC9TU
