On 9/20/05 2:54 PM, in article , "Antonio
Vernucci" wrote:

If no other fix comes along ,do this ;connect a series resonant circuit
to ground in the if path .use a small coil form and a variable cap
,tune it with grid dipper before you install it.. This circuit should
be a sharp trap and can easily be tuned to the interfering frequency to
remove the culprit..This may not be top notch engineering,but, it
always works for me..We used to call it a suckout trap... GL W4PQW

Hi Randy,

sorry for my late reply, but I have been out of town.

Thanks for the advice, and you suggestion is just what I am going to do next.

As a matter of fact, by making some measurements, it was easy to determine
what the problem is.

As expected, on 80 and 40 meters I measured the conversion oscillator
frequency to be 1,650 kHz (i.e. the IF value) higher than the receive
frequency, .

Conversely, on 10, 15 and 20 meters, the frequency meter indicated that the
oscillator fundamental frequency runs at HALF the figure one would expect. For
instance, when the receiver dial is at 14.000 kHz, the oscillator runs at
7,825 kHz and the converter tube then works on its second harmonic at 15,650
kHz (equal to 14,000 + 1,650). Measuring the oscillator waveform period with
an oscilloscope, it was easy to confirm that the fundamental is at 7,825 kHz.
The waveform is not sinusoidal and then has a rich harmonics content.

This is just the Hallicrafters design approach, not a problem of my receiver.
Probably they found it easier to build a high-stability oscillator at a lower
frequency and exploit the second harmonic.

But, with the oscillator fundamental at 7,825 kHz, the receiver will receive
both 14,000 kHz and, even better, 9,475 kHz, unless the RF stage provides a
sufficient block for the latter frequency.

Unfortunately, in Europe we have terrific BC signals in the 9.5-MHz range,
that pass through the receiver RF stage tuned coils, independently of the
frequency they are tuned at. Problem is that their ultimate rejection is too
low, and peaking the preselector does not help at all.

73

Tony, I0JX



You should measure the L.O. Signal at the 1st mixer. Low injection voltage
can cause lots of strange intermod problems.

Don