:MTR3 b Power control

I wonder if some of you have reduced the output from this tranciever in to 100, 200 or 500mW on 9v battery.
Woukd like to have control of output and conserve my 9v battery. To regulate PA voltage would be waste of energy.

73 de LA1KHA Kjell

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You could try to tweak the key shaping circuitry so that the maximum amplitude and hence current is limited.

You could either try to increase the resistor R23 or insert an additional, low resistance, high wattage resistor between Q5 and L18.

The respective part of the schematic is attached.

I have not fully investigated this, so others are kindly asked to chime in.

You will have to check whether the timing of the key shaping will still be fine after the mod; also the matching to the LPF might have to be checked (not sure).

If either approach works, it will be the simplest possiblr direction, because it will limit the Tx current without needing to adjust the rest of the supplies.

Adjusting R23 will be a bit tricky, as the optimal window for the current for Q5 will be small (and part-specific).

Hope that helps!

73 de Martin, DK3IT

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Does the rig have the PWM ‘Tune’ mode? If so maybe you can tweak the software to enable it for normal Tx as well.

In another project, I tried to implement PWM-based control of PA power, and my experience is, in a nutshell: It does not work well. You may create unwanted mixing products and you will likely modify the keying shape.

you could also try to insert a trimmer resistor (maybe 47-100 k) before the base of Q2 and adjust that to the desired rf power level.

the downside of that approach is that the Hfe of both transistors will vary and you will combine the variance from both.

when adding a resistor at the gate of Q5, you may observe that the output power may rise during long transmissions, as its Hfe is temperature-dependent and will rise with the temperature. should not matter at 100mw, but may at 500mW.

also, as said, the rise and fall times of the key envelope may change, so you may need to adjust the capacitors of the RC circuit around Q5.

73 de Martin, DK3IT

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The group ATSprint at groups.io was the one that was running when the MTR series was released by Steve. It is possible that it has some material or discussion about power level changes.

(When yahoo closed yahoogroups at the end of 2019 Steve started a new group on groups.io called kd1jv-designs, but the content of the old group was saved and placed into the ATSprint group. I had a role in that process. )

73 Andrew VK1DA/VK2DA

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