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B2 Spy Suitcase RX-2
- Created: Wednesday, 21 July 2010 08:38
- Last Updated: Wednesday, 30 October 2013 11:53
- Written by Administrator
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Now the IF/AF part is working, it's time for the next part of the receiver: HF amplifier, Local Oscillator and Mixer. That is located on the second subchassis, so it's time for some more sawing, drilling and soldering.
Again the subchassis is made from double sided printed circuit board, that can be obtained cheap at radio-markets.
Building the second subchassis
The tuning capacitor is not mounted on the platform, but on the side. I used rings to bring the capacitor to the correct position. I had to drill the holes in the rings a bit larger, becaus it was an English capacitor and thus the diameter was not 3mm but slightly more. Fortunately I had two bolts in my Strange Metrics Drawer that fitted...
The platform does not run through to the frontpanel, as you can see. Probably to give the 6x3 2-deck switch some space. At the top and the bottom you see the supports for the strips that will carry the trimmers and coils for the three bands.
Bottom view of the second subchassis.
At the top side of the platform, the copper was removed from the holes for the IF-transformer with a large drill, and a grinder was used on the other side to separate the connections to the transformer. Here also the supports for the trimmers/coils can be seen.
Let's have a look at the schematic for this part:
Schematic of the receiver part. Click HERE for a large version.
On the left are the tuning coils with the trimmers, with both the coupling winding and the resonating winding connected to the bandswitch. That signal is routed to a grid of the valve for HF amplification. On the right you see the part of the circuit that makes the local oscillator. It is a Hartley oscillator which means that the cathode is connected to a tap of the coil. There is no information on the number of turns of the coupling winding and the position of the tap, so that means calculating and experimenting. The valve also acts as a mixer, which means it fulfills 3 functions. The resulting products are filtered by the IF transformer and routed to the IF/AF part.
The capacitors in series with the tuning capacitor in the Local Oscillator are Padders which take care of the synchronisation between receiving and LO-frequencies. But their values were way off according to my calculations. So I used values that are more like what I calculated.
Soldering has started...
Chassis painted, more parts in place
Now some mechanical work. The gear wheels had to be prepared; a scale had to be glued to one of the gear wheels, and a spindle extender had to be glued to the other gear wheel that will be the connection between the knob and the capacitor, and will drive the tuning scale wheel.
Prepared gear wheels. On the left with spindle extender, on the right with tuning scale.
After preparing the gear wheels and the drilling of the hole for reading the scale, the receiver front was painted with non-ferro primer:
One of the challenges was making the coils. Because there is no information on the coils whatsoever, I had to calculate the values from the value of the tuning capacitor and the desired frequency range. Without boring you all with the formula's, take for granted that the value of the coil at the antenna side has to be about 19uH. I bought a bag of Neosid coilsets for 1-5 and 5-15MHz. They consist of a coil former with core, a ferrite bus that covers the windings and a copper housing. The Al-value of the coil for 1-5MHz was given as 13nH. You can use that to calculate the number of turns for a certain inductance using this formula:
L = N2 x Al
and that means about 39 turns are needed. So far the easy part. But the number of turns on the antenna side is a bit of a gamble. I used 6 turns, which means a impedance ratio of about 42. And that results in a 2000 Ohm impedance on the valve side using a 50 Ohm antenna. Whether that will turn out to be correct has to be seen...
The first coil ready for use.
As soon as the paint on the front had dried, I started mounting the controls. First the vernier dial, that had to be taken apart before mounting. This because the hole was made just large enough for the knob's base. But then the knob itself does not fit and so I had to take it apart. The areal input is made from a pertinax banana-plug receptable that I found in a dump store.
First items mounted..
The white stains above the knob is glue which I used to fasten a hair from my paintbrush that will function as reference for reading the scale and is glued across the looking hole.
Finally the scale was printed and glued to the disk that in turn was glued to one of the gear wheels. Because the gear ratio is 40:45, the tuning capacitor's range of 0 - 180 degrees is extended to 202,5o on the scale. That's why just over half the circle is used, which increases readability.
Bandswitch and scale with gear wheels in place.
Now is the time to mount the two subchassis to the frontpanel, using 5mm spacers. In spite of all layers of paint, everything fitted. The assembly really looks like the original:
Subchassis mounted. Look at the first coil mounted to the upper strip with trimmers.
I found some bakelite rectangular knobs on Ebay and those are used now for the receiver. The tuning scale is not very clear on the picture; at the lower right corner you can see the hole for the future power cable.
The result so far.
Also the antenna receptable is clearly visible now, and the connector for the headphones. The finger plates are of course still missing, but now it is time to connect everything together and see if it works.
I use a nice little program called FrontDesigner to design the finger plates. Anyone who wants the sources: drop me a line... Accuracy is needed, because the finger plates have to cover the holes very precise. To check that, I printed them on paper and cut them with a pair of scissors, just to see if I did not make any mistakes. Besides I put the required texts on the knobs. The result:
I ordered photopositive lacquer from ELV which I will use to make brass sheet light-sensitive, similar to making PCB's. The idea is to use some corrosive chemicals that will blacken uncovered parts of the brass, and when removing the lacquer, leaves the shiny parts. I don't own the necessary parts for making PCB's, but Hugo PA2HW does... And again he offered to help me.