Thursday, 19 May 2011

Dippers

A couple of weeks back I won a Heathkit GD-1u Grid Dip Oscillator in the weekly raffle at Warrington A.R.C.


Yes - of course there are the usual raffle prizes - bottles of booze, packs of Kit-Kat - everything you'd expect from the best club (I am wearing my flame-proof suit). But, despite the temptations of beer and biscuits, I prefer to pick up the nice pieces of hardware occasionally on offer - and this dipper certainly is nice!

OK - there was a down-side...

The dipper came without any coils and the meter glass had collapsed onto the movement. A quick disassembly and some suitable glue soon sorted the meter and my friend Albert, g3zhe (who would be called my 'Elmer' if we lived on the other side of the pond) has loaned me his set of coils to copy. Here's one of them...


The coil set came in the nice original fitted box...


First thing I needed to do, having borrowed the cols, was test the dipper. I made up a resonant L-C network using a coil from my g3wpo FET dipper and a 1nF capacitor...


A quick estimate of the coil dimensions and a visit to an on-line coil impedance calculator suggested an inductance of 12.03 micro Henrys, giving an associated resonant frequency of 3.53 MHz.

Here's a video (YES - Shack Nasties is entering the video age!!) showing me tuning the dipper after placing the test resonant network next to the "Band A" coil chosen to place the Heathkit GDO in the correct band...

video

Here's the frequency dial setting associated with the dip...


The reading of 3.75MHz isn't a bad result given the tolerance of the capacitance and the crude estimate of inductance. To "double check", I tuned the g1wpo dipper, which gave the following result...


AMAZING! 3.75MHz (Scale "B" at the bottom of the photo) - these things actually work!

In fact, the old Heathkit dipper seems rather more sensitive than the g1wpo - here's a(nother) video clip, showing the Heathkit meter as I move the test resonator up to the coil...

video

The same effect on the g1wpo produces a rather less distinct "dip"...

video

All I have to do now is copy the coils for the Heathkit - no mean task - I'll tell you how I get on.

What a fantastic raffle prize - sure beats a packet of KitKats!

...-.- de m0xpd

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