SB-104 again

[John Farrington]

At 01:03 PM 04/09/2001 -0700, you wrote:
>My SB-104A seems to have an awful lot of oscillations (I think they're
>called birdies) at various places on the dial. Seems I've heard about some
>mod or other to fix this. Does anyone know about this?
>Murray


I once had an old copy of QST or Ham Radio that had an article on SB-104
birdies, but can't find it now. As I recall, a big part of it consisted of
placing disk ceramic caps to ground for RF bypass on every one of the many
non-RF terminals (like power connections) on all the under-chassis card
sockets.

There were other articles on improving SB-104 performance in:

- QST, August 1979: "Improving the SB-104A/SB-644A", by Harlan Bercovici W0MYN.

- Ham Radio, April 1981: "Improved receiver performance for the Heathkit
SB-104A"
                           by Richard Tashner N2EO.

- QST, March 1982: "Refining the SB-104", by David Palmer W6PHF, which, among
                     many other things, mentions bypassing every circuit board
                     terminal with a DC function with 0.1 uf ceramic caps;
                     bypassing the distribution points for the 5, 11, and
13.8V DC
                     sources with a parallel combination of a 15 uf/20V
tantalum,
                     0.1 uf disk, and 0.001 uf ceramic caps;
                     replacing the ALC bus and all unshielded Rf and AF signal
                     wires with RG-174/U coax with the shield grounded only at
                     one end.

Someone also mentioned that the 180V power converter module that supplies the
digital display board may cause birdies despite its shielding and internal
filtering.

73 de John KE5ZB

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