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Old 14th Nov 2005, 12:44 am   #12
adibrook
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Join Date: Dec 2004
Location: Weymouth, Dorset, UK.
Posts: 377
Default Re: My attempt at a TRF

Thanks! That will be very usefull!

Today i went down the local 1 shop and bought a pack of 2 window alarms. They are very simple, contained in a little case about the size of a lighter. They have a magnetic reed switch, oscillator and a piezo speaker.

I ripped the PCB and stuff out, and soldered the tiny ceramic speaker leads to the battery terminals (with batteries removed). Then i soldered doorbell wire to thesame battery terminals, and poked them out of a hole where the power switch used to be (with a knot to keep them there). I soldered them to the battery terminals to anchor them, since the cables from the speaker itself were very thin.

Anyway, what i ended up with is a high impeadence Xtal earpiece. With the case reassembled it's probbaly quiet safe too.

I tested it with a little DAF96 amp built on a proto-bord with 6 9V's as a power supply, and it seemed to work ok.

Anyway, now comes the cool part. I connected a 100K resistor across the blown O/P transformer, and the connected my Xtal speaker across that.

I actually herd two station! Both were french, and wever very very weak, and were accompanied by wierd whistling and stuff, but the main thign is it worked!

It's in no way perfect yet, but it IS a workign radio reciever.

I think it may have been very weak because of my coil. It was a bogroll coil, but the ariel primery and secondery were wound with about 5mm gap in between. I think people suggested winding them one on the other, wheras i wound them next to each other.

Also the bogroll tube was abit deformed.

I think maybe an RF preamp would be usefull here. I could stick in a 6SN7, with one side being an RF preamp and one side being an AF preamp. (i'v decided only to use octals in this radio...next one will be all noval).
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