Warming effect of driver and pre-driver affects spurs

Lawrence Oberman mentioned an issue he has on a v4 board that looks like RF coupling or leakage when testing CW on 20m for FCC compliance.

He doesn’t see it with SSB but if the rig is placed on transmit for long enough spurs will start to show up .¬† He has only just got Axicom relays to replace the stock relays. Strangely enough he has not seen this behaviour on 15m, 17m or the higher frequency bands, or on the bands below¬† 14MHz.

Others have seen similar phenomena on different bands, some with the spurs appearing on starting TX and then disappearing.

Allison KB1GMX says “Both cases come from the same source, heating in the amplifier usually the pre-driver and driver stages but it can include the finals. These stages are not thermally stable and parts heat up and their bias shifts.”

Reference