FT225RD
AGC-modification

AGC-schematic


Beginning

Many FT225-users quickly realize that the AGC is to slow, especially if you are interested in meteorscatter, MS. The AGC can be set into fast- or slow-mode, both modes are to slow for good MS-reception.
SM5CUI, Rune, and I had a brief discussion about the AGC and how slow it is originally and how this could be modified.

Modification

The solution is simple. The AGC-circuit is in the "SSB IF-unit" and the delay is set by one or two capacitors depending on the chosen delay, see schematic. Normally, at fast AGC, both C23 and C22 are connected, ie in series. This means that the resulting capacitance is less than in slow AGC where only C22 is connected. (C23 is then short-circuited to ground.)
If the C22 is removed and replaced by C23, the resulting delay at slow AGC will be nearly as in the original fast mode.
With the AGC-delay modified in this way the AGC will be as fast as it can be without further modifications.


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