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Treetop Circuits

Automatic Volume Control Problem in PD3

A problem has been found in the automatic volume control (AVC) circuit. It occurs only on units # 17 through # 47, and affects the AVC action when in SSB mode.

The problem is that the printed circuit board traces which are supposed to go to the left end of C13 (upper middle in the photo) are misplaced and do not connect to the pad. The AVC time constant components C11, R12, C13, and R13 are therefore not properly connected.

This can be corrected by adding a small wire as shown, joining the left ends of R13 and C13 and the right end of R12. There's no problem using regular tin/lead solder for this; it's compatible with the lead-free solder we use in production.

The mistake went undetected because the circuit actually works pretty well (well enough to fool me when I did the final test on the units) without these connections in place. Referring to the schematic in the Owner's Manual, it appears that C10 is charging through D15, providing a sufficiently fast attack time. The discharge path is via R11, D17, and R15.

The fix can be left out indefinitely without causing damage. However, if you wish to experiment with the circuit, you will probably want to put it in before proceeding.

A larger image can be found here: http://www.treetopcircuits.com/pmwiki/uploads/avc-fix-large.jpg

We apologize for any inconvenience.

73
Bob, VE3TOU
January 31, 2010