Junkbox 4 (Battery superhet)

[Variometer]

A few weeks ago, I felt like building a battery superhet. With no particular thoughts for the circuit, I first built the baseboard/chassis with all the major components on it. I wanted to build it like a "battleship", hence all the brass nuts & bolts etc. Home-made tagboards with the tags placed where I felt they should be. It took about two weeks to make the framework.
Two days ago, I wired up the detector and output stage using "bog standard" circuitry. I tested these two stages by connecting a crystal set to the grid of V3. It worked quite well, but with dreadfull selectivity (as I had expected).
Next I wired up V1 and V2, again using standard circuitry. By the time the wiring was complete, I was getting a bit impatient to see if it worked, but at that stage, had given no thought to the coils. I soldered three 100uH RF chokes in, one for the aerial coil and two in series for the centre tapped oscillator coil (more in hope than expectation). On plugging it in, a high pitched whistle filled the room Looking underneath, I quickly saw that I had missed out the resistor that should have linked G2, V1, to the HT. On putting it in, the set (much to my surprise) worked quite well, with a bit of instability that I cured by tweaking the IF trimmers a bit.
After I have given some though to more sensible coils, I have high hopes that it will go from good to excellent.
Valve line up - DK96 - DF96 - DAF96 - DL96. HT 54 Volts. HT current 3.5mA.
Bob

[Ed_Dinning]

Hi Bob, a very nice piece of work. What are your thoughts on the coils and wavebands?
Homemade should be OK for MW/LW, but commercial ones are normally better on SW. There are plenty of wearite P's, dencos available, but at a price.

Perhaps even a frame aerial. I may even have a scrap battery chassis available

Best regards Ed

[Peter.N.]

Very neat construction, a lot better than the manufacturers of the era.

Peter

[Variometer]

Thanks for replies.
I built it with a flexibility to experiment. That is why I left the coils to the last. There are two solder tags for the aerial coil and three for the oscillator coil, so I can put them in and take them out easily without messing up the rest of the wiring. I never bother with LW at all! For MW, I was going to experiment with chokes to just prove it works. But it works anyway, without any experimentation (much to my surprise). I was intending it for SW, so I will try winding my own to start with. Easy enough as there are not many turns. May even fit it out for plug in coils.
The IF transformers are also mounted so that they can be taken out without too much fuss. Each is held on a brass plate with four holes, the holding nuts being above the chassis. Underneath, each IF has two solder tags mounted on each side on the main chassis. To remove them, I just unsolder the leads and take the nuts off. I hope to get round to making my own IF transformers and this will allow me to test them.
Rather than mess about with the delicate IF slugs, I have mounted four small trimmers on top for adjustments in addition to four fixed 47pF caps underneath the chassis. I can therefore adjust each side of the IFs between about 47pF and 120pF.
I haven't done anything on it today as I felt like a rest from it. Couple of weeks doing the hardware and two days wiring it up.
The chassis is aluminium, the top being smoothed down and painted grey. Underneath, it has been left bare. The wood base section is French polished. I may yet make a new black acrylic front panel of greater height.
Bob

[MALC SCOTT]

A very nice job, well done

[Ed_Dinning]

Hi Bob, if you are experimenting with SW coils let me know and I can let you have some Litz wire, it does make a difference at HF. I'll also be at Golbourne so I may see you there.

Ed

[Variometer]

Thanks for replies. I think I will stick with solid copper SW coils though. I have never seen SW coils made from litz. I had considered using litz for MW coils in the past, but abandoned it because it is so expensive these days and small RF chokes seem to work just as well. I really prefer to work with valves/components that are still cheap and easy to find. The only item I purchased specially for this receiver was the wire for the battery leads.
The radio bug only bites me occasionally and it is months since my last one, a two-valve chassis TRF (Attached).
Bob

[PJC58-Hythe]

Hi Bob, A nice well built little set, well done! Hope you manage to get the right combination of coils / RFC's to complete it. Be nice to see some pictures etc. of the finished item.

Peter

[Mike Brett]

Hi Bob
Very nice construction, I really like the use of black acrylic for the panels. I usually use pcb board for my radios, can I ask where you get the acrylic from?
Mike

[Variometer]

Hi Mike,
There is a bottomless pit of black (or other colour) acrylic sheet on Ebay. It comes in a number of thicknesses, but I prefer 3mm. I buy A4 size, but larger sizes are available. There is a protective coating on both sizes that is peeled off before use, leaving a high gloss surface.
Bob

[JHGibson]

Hi Bob,
Nice work. You can easily make cheap hi Q medium and shortwave coils by winding the solid copper wire on the clear plastic (polycarbonate?) cylindrical bottles that pills come in from the chemist. They mount by drilling a hole in the base for a brass screw, no steel here, it will affect the Q.
John.

[Variometer]

John,
That is what I plan. Quite thick copper wire wound round a plastic former.
I haven't done anything yet, but will let you all know how I get on.
Bob

[Variometer]

I finally got round to trying to covert it to short waves. The coils are all RF chokes in fuse holders, but putting lower value chokes in was not successful!
As a temporary and easy experiment, I:
Replaced the aerial choke with a 10K resistor.
Disconnected the fixed vanes of both halves of the tuning capacitor.
Connected the signal generator between chassis and the top of the empty oscillator coil fuse holder that goes to G1 on the frequency changer valve (Heptode DK96) via a 100pF capacitor.
I was then pleased to find that it works great on medium and short waves using the signal generator as a local oscillator. Tuning via the signal generator frequency control.

An interesting experiment.
Bob