SB-401 questions answered!

[Mark Graalman WB8JKR]

  Hi all,

  I just got a note from Jan N8LQR and he said the
Heath master parts list shows it as a .394 uh inductor
which confirms my thoughts that it was a misprint.
 Apparently the original LMO must have spur at
4 times the LMO frequency, I suspect the trap is tuned
to 21.0 Mhz which would be the 4 th harmonic of 5.25 Mhz
the center of the tuning range of the LMO, and since its
a series tuned trap the large 180 pf cap means its a fairly
low Q to cover the range of 5.0 to 5.0 Mhz representing
20-22 Mhz range of the LMO 4 th harmonic. I do however
find it hard to believe that such a spur would get through
the 8 Mhz bandpass filter. Also apparently if the SB-401 is
used with the SB-303 the trap is removed, leading me to
think that whatever the problem was with the 401 LMO, it didn't
exist with the 303 LMO.
 At any rate, might be a good note for all to make in their SB-401
manuals on the correct value of the inductor for the 21 Mhz trap,
and if the trap is missing in any 401 you might pick up, you'll know
why and how to make up a replacement.

 Thanks to everyone that offered input!

73, Mark WB8JKR

Novice 3/71, General 8/72, Advanced 6/80, Extra 8/88
Member Society of Cable Telecommunication Engineers
Member ARRL, DXCC, WAC, WAS, VUCC 2 meters, GJDXCC, OTC, RCC

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