FIDELITY RECEPTION By E. KALEYELD, PaoXE

QUALITY has always been a point we tried to achieve in our broadcast set, and when a new set had been constructed (What! Again?) the XYL and yours truly nodded to each other and said, "This is just as it ought to be"....until the unhappy day when we again visited our friend with the f100 radiogram.

Usually our set would be on the workbench the next day, the XYL devoid of symphonies, and yours truly of jazz, for some days, and the same procedure described above started all over again. We once tried a TRF receiver, and it was much better than the superhet BCL box, so that gave us an indication of what direction to look for the solution. Several weird and wonderful switching arrangements were tried, more or less successful, only more less than more!

One upset the calibrations of the dial, the other intro- duced severe IF instability, until at long last we believe we have found it. As usual, simplicity itself.

With the selectivity wide open, reproduction of the stronger stations was a revelation. Even our friend with the big radiogram admits that, which really means something. Then, one flick of the switch, and the set is back to a normal superhet, without impairing the sensitivity and selectivity it normally has.

The scheme is very simple indeed, and we only wonder why it has not been exten- sively used before. There is so little to it, that any BC superhet can be converted to a high quality receiver in one hour, at a cost of a few shillings.

Well, enough now about the conception. We won't bore you with the advantages; they are apparent, and all we need say is, "Try for yourself". The idea is to switch out the first IF trans- former, and use resistance coupling instead, leaving the second transformer in place to drive the detector diode. By so doing, the selectivity curve widens, thus increasing the fidelity. The tuning is not affected, the curve remaining reasonably symmetrical. Of course, the gain goes down, but the AVC system will take care of most of that, while the rest can be compensated for by the manual gain control. Nearly always there is plenty of gain to spare.

On our receiver, which is the standard frequency changer--IF--double diode triode-audio pentode, we can receive with a not-too-good antenna (the best place being given to the transmitting aerial!) about twenty stations at very good strength in the wide-selectivity position.

If you look at the circuit in Fig. 1, you will see that with the switch in the position as drawn, we have a normal IF circuit. The 25k resistor with its associated 0.05 F capacitor acts as a decoupling network to the primary of the IF transformer, whilst the 100k resistor with its bypass capacitor of 0.05 F acts as an additional filter in the AVC line.

The interstage coupling, therefore, is quite normal, and the receiver has the usual characteristics of selectivity and gain. In the alternative position of the switch, however, the contacts short out the two IF windings, and disconnect the bypass capacitors, leaving the resistors as anode load and grid leak respectively, coupling between them being provided by the 0.0001 pF mica (plus possible stray capacitances to be exact).

The whole circuit is exactly the same as for a resistance-capacitance coupled audio stage, the only difference being that we are handling RF instead of AF. Which reminds us, RF gets much more easily out of hand than AF does, so keep the leads short and direct, and mount the switch as close to the IF transformer as possible. This is often easily accomplished, as the first IF transformer is mostly at the back of the set, which is a nice, inconspicuous spot for the switch. The latter, incidentally, may be either of the toggle or wafer type (DPDT).

It is most important to see that the leads do not come anywhere near the anode circuit of the IF valve, as serious instability might result. The two resistors should be mounted close to the transformer, with their ends supported if necessary, while the 100 pF coupling capacitor can be fitted directly underneath the base of the IFT. Once again, do keep the leads short and direct. and keep them away from the IF anode!

If it is a home-made set, you yourself know, of course, what circuits you have included, but often the cheap type of manufactured receiver (and home-built ones, too) have nothing but a capacitor and a variable resistor from anode to ground, just right to cut off those high frequencies. If you only knew the trouble studio technicians go to, so as to preserve as much treble as possible, you would agree it is decidedly unfair of you to waste them in resistance/capacitance network like that.

So if your set is of the above-mentioned variety, have a go at the simple negative feedback arrangement shown in Fig. 2. This involves a resistor, and a variable capacitor of the mica type for the sake of compactness. The capacitor limits the feedback at the very ibw frequencies and provides a very nice bass lift, without affecting the higher frequencies.