Gear cutting for RF Tuner

[BillC]

I am now working on the coil unit of the RF preselector, it is very fiddly to work on . Taking hours . The images show the general layout . The low band has fixed inductance in series with the tuning inductance, when switching to the mid band that fixed inductance is shorted to ground which also places the bottom end of the tuning inductance to ground thus allowing the mid band to tune over its' range.

 

[VK3YNV]

Hi Bill,
Trying to figure it out, do the tuning slugs come in from the end that's facing down? What material is used for the coil formers?

 

[BillC]

Hi Ray , the slugs move in and out from the upper side which is also the high impedance end of the winding ,the slugs must move in the high impedance end for the tuning to work properly. The coil formers seem to be shellaced / varnished paper cut into strips and rolled in a spiral , two spirals rolled one on the other in oposition to form a tube ,then varnished and cured dry. I think my formers are ex. car radio surplus stock.The brass rod sticking up from the top of the plate in the first image is a guide for the slug rack. thanks for your interest, now back to hay cutting. Cheers.

 

[BillC]

Whilst testing my tuner amplifier I could hear lots of noise being generated into the monitoring receiver . I removed the transistor from the output stage but noise returned as soon as dc. was connected .It was being coupled via the load resistor and the output coupling capacitor from the dc. voltage line to the monitoring receiver. How noisy is your power supply ? The images show a test setup and my solution to the problem . Am I on the right track with this ?

 

[BillC]

 

[BillC]

More on the B+ rail noise . I built a simple filter on the B+ rail right at the amplifier . The filter uses a .1mf capacitor either side of a 1 mh. rfc . A PI filter configuration , in series with the B+ feed to the amplifier stage . I also placed a diode from B+ to ground to conduct the relay coil transients. No noise from the power supply leads now. The pre selector works well now and tunes from 200 khz to 3mhz [mid band] and 3mhz to 30mhz [hi band]. I am still working on the low band coils and switching system.

 

[VK3KAL]

Yes, mighty fine work Bill.

 

[BillC]

Thanks Neil, a bit of a challenge to work it all out . But a good learning process too . Good to see you both at the break out // .