[Voxophone]

Another update.

I tried warming the coil up again, for longer this time. Again I saw an increase in Q which temporarily improved performance somewhat in the set, but again it didn’t last. Interestingly, the Q of the other two coils seemed to rise somewhat when the assembly was warm. This is the opposite effect to what I would’ve expected as wire resistance should surely increase with temperature. Can anyone think of a reason for this?

Afterwards I pretty much wrote off the original coil and focused on fitting the 220uH one from the signal generator in its place. Luckily it’s an exact fit into the middle of the hollow former. I tied the ends of the old coil together with a shorting link in case it might have some effect on the circuit. Not sure if this was really necessary but it seemed like a good idea.

With the replacement coil in position I can get the response to peak. It’s not as good as the other wavebands (0.7 Vpp out at 1 MHz for 1 Vpp in), but it’s still much better and more consistent than the original. Strangely the peak is reached with the trimmer screwed all the way out. Screwing it in reduces the output. I’m thinking this could mean too much capacitance in the circuit? Perhaps a series padder to reduce the trimmer capacitance might help?

Regardless, the set sounds much better now and seems perfectly listenable. I’ll probably do a bit more experimenting to see if I can improve it any further, then fix the new coil in position more permanently.

Thanks to everyone who helped me in figuring this one out. That’s 2/3 sets with weak MW problems sorted!

Cheers

Liam