Coil uH

[Qiaozhi]

To increase the tx pulse width, I change the Value of R7 while monitoring on TP1 ?

Attach the 300uh coil, and measure across R3 (3R3) with a scope, and note the maximum voltage. Let's say it measures as 7.4V. That means your maximum coil current just prior to TX-off is 7.4 / 3.3 = 2.2A.
Then fit the 500uH coil, and (once again) measure across R3. Adjust R7 until you get a maximum voltage of 7.4V. That will mean your maximum coil current is the same as with the 300uH coil. You may also need to measure the average current being drawn by the detector. If it seems too high, you'll need to adjust R8 to lower the TX pulse rate.
I would replace both R7 and R8 with preset pots so that you can fiddle around to your heart's content.

[Tibuck19]

Attach the 300uh coil, and measure across R3 (3R3) with a scope, and note the maximum voltage. Let's say it measures as 7.4V. That means your maximum coil current just prior to TX-off is 7.4 / 3.3 = 2.2A.
Then fit the 500uH coil, and (once again) measure across R3. Adjust R7 until you get a maximum voltage of 7.4V. That will mean your maximum coil current is the same as with the 300uH coil. You may also need to measure the average current being drawn by the detector. If it seems too high, you'll need to adjust R8 to lower the TX pulse rate.
I would replace both R7 and R8 with preset pots so that you can fiddle around to your heart's content.

Thank you kind Sir, I will adjust as you suggested, I had already replace R7 with a 5K trimpot ( but I think I will need more then a 5K for R7 to get the voltage needed ?) and R-8 with a 50K trimpot when I first assembled the board. Honestly I would rather fiddle less with the board and more on digging holes hahaha