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Midland 70-066 PA Modifications:
Large Caps – Midland 70-066 above:
Add 100pF between B and E of Q502 on the component side of the board as per the photo.
Add 1000pF between B and E of Q503 on the component side of the board as per the photo.
Adding the capacitors B-E on the driver and PA transistor is required to stabilize the amplifier. It is
necessary to replace the coils in the LPF to get the second harmonic output of the transmitter down to an
acceptable level.
Having made the modifications, reassemble the PA and apply power. Terminate the output in a good
dummy load with a power meter – not an antenna! Set RV502 fully clockwise for maximum power out.
Key the transmitter and adjust for maximum power starting from the output end and working back
towards the synthesizer. Don't forget CV102 on the synthesizer board. After modification and tuning it is
normally possible to get 50W out of the transmitter with no sign of instability, do not run it at this power
for long – there is not enough heat sink on the PA. When you have maximum power out adjust RV502 for
no more than 30 Watts out. If you wish to run 50 watts, then attach a 12 Volt fan to the rear heat sink.
Allow the fan to blow across the rear fins.
Midland 70-066 low Band Conversion to 6 Meters: Transmitter PA Part 4 Ver 2.2 21Nov09 Page 7
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