The next problem is alignment. The local oscillator was out of whack on all three bands. I still haven't gotten the long wave band right. But before I go back and do it properly I figured I'll look at the IF sections first
While I do have two wobbulators meant for this job, both are tube based and need work. So I went with a rather janky approach of using one function generator to generate a symmetrical ramp voltage, which I feed into both the x channel of a scope as well the VCF (voltage controlled frequency) input of another function generator, that is set to the 455 kHz IF frequency, causing it to sweep a symmetrical frequency range around the IF.
And while this method is great for seeing the shape of the filter response, it tells you nothing about the actual frequencies. You have no markers, unlike with a purpose build sweep generator like the Wavetek I showed before. Unfortunately, that one is of no use for AM radios, since it covers a frequency range of around 5 Mhz to 500 MHz.
Using a rather slow sweep frequency just as an example
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And increasing the sweep frequency
The IFs were both off frequency. I tried to adjust them, only to find out, they cannot be adjusted. Where I thought a core slug was lurking was in fact nothing but a concrete like ferrite material. In my attempts to turn what I thought was wax covered slugs, I ended up killing one of the IF coils. Fixed it again.
I ended up moving the IF by 5 kHz to 460 kHz, as this gave me the largest peak within the range afforded by the trim caps
These IF cans were meant to be a set and forget kind of deal, with a frequency set during my manufacturing. Somehow the frequency has drifted, by quite a lot. One of them was at 425 kHz.
This is turning into quite the project.
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Receiving some Bangladeshi(?) station. Around 1420 - 1435 kHz. (LO is at 1899 but I forgot my IF frequency, I've changed it again)
I'm located in Europe.
Ran Shazam on one of the songs and it gave me a Bangladeshi singer.
Edit: measured the IF to (469.6 +- 0.3) kHz, by injecting a signal into the mixer and adjusting for lowest beat frequency
I'm located in Europe.
Ran Shazam on one of the songs and it gave me a Bangladeshi singer.
Edit: measured the IF to (469.6 +- 0.3) kHz, by injecting a signal into the mixer and adjusting for lowest beat frequency