Quote:
Originally Posted by dtvmcdonald
I have an SP-600 as well as a National NC-400.
Thus, I believe that the claim that it needed a "special" crystal is correct.
That is, it had not only have a reasonably correct frequency, but had to have exactly the correct characteristics including correct equivalent inductance, series capacitance and parallel capicatnece, and Q.
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Thanks for that; interesting and perhaps a bit worrying.
Key question, when you say to get them working to the spec, to what extent would you have been willing to accept something that worked "quite well", even if not in spec?. In terms of
improving the old HRO 5, especially for use on telephony, almost any modification could be an improvement.
One specific question; can you say anything about the differential capacitor used in the Hammarlund filter? Mistakenly, I initially believed it was quite a large value, but the early manuals don't specify a value, and as it is effectively in parallel with one of the caps on the mixer transformer, I began to realise it could not be very big. A while later, looking at the manual for the SP-600, I saw a value of 3.5-8pF specified in there.
B