The Short-Wave Adapter

Radio Record, Volume V, Issue 24, 24 December 1931, Page 15

The Short-Wave Adapter

=_-__- Hints from a Constructor (To the Hditor.) HYVER since the "Superheterodyne Short-wave Converter" was deseribed, constructors have written in xreporting failure. So far as I know mine was the only report of success. Having built the converter four times, perhaps a few notes may be of use to those not so fortunate. I have on each occasion built a battery version adapting the circuit to "Anode Bend," first Det, "Grid Leak First Det." (Grid Leak with reaction on First Det.), and "Untuned Virst Det." Let constructors take heart, also take down theirs from the attics where doubtless many have stored them in disgust, remove the cobwebs, blow out the dust, and tune ’em in within half an hour. All that is required is a slight alteration to the oscillator coils. Select the 25-metre oscillator coil first, and take off the winding that couples the "srid to plate’? Id. Then put it on again, putting the bottom end of winding to plate pin of valve base and the ton to the cathode pin of valve base.

That is just the opposite to the illustration in the "Guide" and "Record." In the schematic drawing, if one accepts the top of the coils a» being nearest to the top of the page, it is O.K. However, the coils are rather difficult to understand, and most constructors would work from Iigs. 4 and 5 in the "Guide." This is the only way to get the oscillator in this particular cireuit. Mowever, use a good choke. Be sure and start with the 25-metre coil. There are two gond reasons, one being there are plenty of Morse stations offering, and they will kick ap a din, too, even if the broadcast receiver will not oscillate. Having reached this stage, start on the next coil (you want about six coils if you use .0001 condensers), this will bring you up to say 32 metres. When you reach the coil that tunes in the 80-100-metre band, you will probably encounter fresh trouble, for when the dials are brought into resonance on this band, an awful howl may result. This howl may possibly take place also on the 40-metre band, but this is unlikely. I£ you strike this trouble take out the oscillator valve; you may then tune in Morse and telephony, but not too well. If you do tune in stations it

is proof positive that the first detector is oscillating, and must be stopped. Cutting down the B voltage proves satisfactory in some circuits, but in others it stops the first detector from oscillating only after the oscillator. stops. Dimming the filaments in the battery version is not entirely satisfactory. I have found the simplest measure to adopt 1S to take off the connection from the fixed plates of Cl. Leave the biggest aerial coil in its socket, and everything else, including aerial and earth in position. You then have an adaption of untuned first detector. This in turn will stop the first detector from oscillating, and give single dial control, also signals appear to be as loud as when the first detector is tuned. However, interesting comparisons can be made. Further Points. {2 is impossible to give particulars of number of turns on "grid" winding of oscillator circuit owing to the different capabilities of valves. There is a difference of as much as 50 degrees on the dial between an English valve and a 201A. Unless the power pack has plenty of reserve a separate eliminator is necessary. , Tune the "intermediate stages" where the broadcast receiver gives most amplification and keep the dial in

that position; use the same type of valve as oscillator always, and the stations will tune in at the same dial readings. The coils will have to be wound, bearing in mind that you will have to try to cover the wavelengths, ‘so about six coils with .0001, or three with, say, 0002. Personally, I use two double- | spaced .0001 condensers in parallel totune the’ oscillator stage. The pre-selector circuit L6, C4 is unnecessary where the first stage of the intermediate amplifier is tuned. Use. choke coupling here, As for sensitivity, the adaptor, when coupled to a receiver of medium power, will perform almost as well without an aerial and earth (and by this I do not mean 2 feet of wire for an aerial, but ‘no aerial’), as a three-valve shortwave receiver will do on, say, 60 feet. ORSE stations that are not audible on screen grid receivers, such as the 1930 receiver described in the "Guide," when the receiver is brought out of oscillation, are with the super het., received at R9, with the broadcast receiver not oscillating. ; Power lines are most troublesom even on the lower bands. Weak caryiers can be brought up to a roar, but of course that does not make the signals readable. It is on those stations that before were received faintly, but eave readable, that the supersonic adap-

tor proves its worth:

W. A.

Riddell