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Treetop Circuits

AVC Fix for early SB-51 and SB-75 Boards

With early units, a “dead” period sometimes occurs when switching from AM to CW/SSB if there is a strong signal present. There is a period of silence lasting perhaps a second, then normal operation resumes.

This happens because the SSB time-constant capacitor C11 can become charged to a high value due to the strong signal. Since the amplifier section U1B is blocked in AM mode, this has no effect until the receiver is switched to CW/SSB mode, activating U1B. This creates a strong negative voltage on the AVC line, thus silencing the receiver. C11 then discharges via its normal route through R22, and normal operation resumes.

The fix consists of relocating the cathode of D15 to pin 2 of U1, so that section U1A is blocked. This prevents C11 from charging in AM mode, so the silent period does not occur.

In the SB-51, a similar effect occurs when the AVC switch is turned off (this switch is absent in the 75A-2 and A-3 receivers). This is not affected by the fix.

The fix is incorporated in manufacture on SB-51’s starting at serial number 21, and on SB-75’s starting at serial number 20. It will eventually be incorporated in the board artwork.

To update earlier units, refer to the photo. The conductor from D15 to pin 6 of U1 is cut, and a short wire installed to reconnect D15 to pin 6. A small burr on a rotary tool was used to make the cut shown; the same thing can be accomplished with a utility knife. Take care not to damage the conductor beside the one you’re cutting. It’s a good idea to make two cuts very close to each other and take out a short piece of conductor, to be sure that there’s no whisker of copper bridging the gap.

73
Bob, VE3TOU
May 2011