Before You Start!

You really will need to know if replacing the RF/IF Amp 3SK74's with BF981's has made any difference!

Before Upgrade - Check existing Signal-to-Noise Ratio:

Connect a small RF test signal, using a simple 10Mhz Xtal Oscillator via an attenuator resistor/network directly to the SW A aerial socket.

Tune to this signal using AM wide, and note the reading on the S-Meter. I used one value of resistor that just about gave an S1 reading, and another one which gave S9 +40dB.

You can also use the RF ATT switch to obtain additional readings etc. Make a subjective "assessment" of the backgound Hiss (receiver internally generated noise) with no signal input at all, compared with the tuned-in test signal, along with S-Meter readings. The more HF spot frequencies and signal levels the better, to get the full picture. If you can connect an Oscilloscope to the "RECORD" front socket, you can watch/measure the results.

After Fitting BF981's - Repeat tests and compare results.

My results after fitting BF981's:

1. Lower level/Finer grain/white noise sounding Rx internal hiss.
2. Background atmospheric noise/hiss now audible on 28Mhz (SSB Filter) peaking up/down using AT1000 ATU & loft mounted 1/2 size G5RV.
3. When tuning across 40M & 20M bands, broadcast bands etc, AGC now appears to do something! i.e Tuning into strong signal, down goes background hiss/noise and then on removing input signal as AGC/S-Meter reading dies, up comes backgound hiss (atmospheric hiss if tuning off signal to clear spot!)
4. Fairly constant 30mv audio output for off-air signals S1 to end-stop on S-Meter.
5. Increased overall gain by about two S-meter points, but no signs of increased/worsened overloading on 40M band at night.
6. Sensitivity is now 0.25uV SSB and 1uV AM for S+N/N 10dB or more !

• Back - Now You have a go!

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