I have recently got a very good single-owner Hallicrafters SX-101A, =
mechanically and electrically intact. The radio is very sensititive on =
all bands and appears to still be well calibrated, despite it has not =
been used for more than 30 years.

A problem I noticed, especially on 14 MHz, is the presence of several =
strong AM broadcast (BC) signals across the band. Interesting to note =
that the receiver preselector control yields no change at all in the BC =
signal strength, whilst it yields a very clear peak in the 14-MHz =
background noise.

The first idea which came to my mind was poor image reception. With an =
IF frequency of 1,650 kHz, the image should be plus/minus 3,300 kHz from =
the actual 14-MHz receive frequency (i.e. in the 10.7 or 17.3 MHz =
ranges) . But, on a separate receiver, I could hear no BC signals at all =
on the expected frequencies.

I then connected a signal generator to the SX-101A and I realized that =
the receiver, when on 14-MHz, is very sensitive to a carrier in the 9.5 =
MHz range.

So, I reconnected the antenna to the SX-101, and, with the aid of a =
separate receiver tuned around 9.5 MHz, I was immediately able to =
identify the BC stations heard on the 14-MHz band with the SX-101A.

I found the following frequency relationhips:

- apparent BC carrier frequencies on SX-101A: 14139, 14189, 14309 kHz
- corresponding BC real carrier frequencies: 9545, 9570, 9630 kHz

The frequency difference between any two 14-MHz carriers is exactly =
twice the real one (i.e. that in the 9.5 MHz range).

I do not believe in intermodulation, because attenuating the input RF =
signal (using an attenuator) cause an equal decrease of the BC signals =
and the real 14-MHz signals (i.e. no improvement of the =
wanted-signal-to-interference ratio).

I believe more in something that has to do with second harmonics, but I =
have not yet been able to derive the equation.

Questions

1) any idea on the mechanism by which BC signals in the 9.5 MHz range =
get through so strong?
2) did you have a similar experience with your SX-101A. Or, in other =
words, is it a design problem or an adjustment problem?

Thanks and 73

Tony, I0JX / K0JX