The band noise sounds a LOT different through a 1.8 KHz filter then through a
2.4 KHz filter, which you can quantify as Barry pointed out. However, an even
easier test is to just listen to the noise between the two filter positions. If
they sound the same, something is amiss.
Gary
W0DVN
> On Jan 5, 2019, at 7:25 AM, Barry N1EU <barry.n1eu@gmail.com> wrote:
>
> The filters are all ink stamped on the tops of some of the crystals,
> although the ink fades with age and you may need to examine closely under a
> magnifying glass. For the 1.8KHz filter, you should see the code "48075"
> and possibly "288". For the 2.4KHz filter, you should see the code "48074".
>
> To really do this right, the BFO oscillators and PBT need to be properly
> aligned and the PBT knob in the neutral position. I would just make a 10
> second audio recording of band noise with a computer soundcard connected to
> the audio output and play it back through spectral analysis software to get
> the plot.
>
> 73 Barry N1EU
>
> On Sat, Jan 5, 2019 at 2:38 AM Phil Erickson <phil.erickson@gmail.com>
> wrote:
>
>> Hi all,
>>
>> If this list will indulge a further curiosity:
>>
>> With my VI+ back together, I measured the filter response of the two SSB
>> filters @ 2nd IF : the default one (2.4 kHz nominal) and the one that I
>> thought was 1.8 kHz (supposed to be Ten-Tec #288).
>>
>> Procedure for each filter:
>>
>> - Injected a test tone from signal generator at a known frequency
>> - set the radio for LSB
>> - AGC off
>> - slowly move the tuning dial upwards 100 Hz at a time away from the
>> frequency
>> - record freq offset and approximate tone amplitude in V using a Tek scope
>> (1 Mohm input) clipped to the External Speaker leads
>> - plotted 20*log10(V) as a function of frequency
>>
>> To my surprise, the 1.8 kHz has almost exactly the same response width as
>> the 2.4 kHz. It makes me think that someone accidentally installed a 2.4
>> kHz in that position, as I definitely expected the 1.8 to be narrower.
>>
>> Is there any marking on the standard TT filters that allows you to tell
>> them apart? I couldn't find any, but I didn't lift the filters off the PBT
>> board to see the underside. (I note with some interest as well that Inrad
>> didn't bother to make anything narrower than 2.1 kHz for the 2nd IF. Was
>> SSB somewhat of an afterthought on this radio?)
>>
>> 73
>> Phil W1PJE
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>>
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